L Marelle
email: louismarelle@latmos.ipsl.fr

J.-C Raut

K S Law

O Duclaux

Current and future Arctic aerosols and ozone from remote emissions and emerging local sourcesmodeled source contributions and radiative effects

The Arctic is influenced by air pollution transported from lower latitudes, and increasingly by local sources such as shipping and resource extraction. Local Arctic emissions could increase significantly in the future due to industrialization in a warming Arctic, and further influence Arctic climate.

We use the regional model WRF-Chem to investigate current (2012) and future (2050) sources of Arctic aerosol and ozone pollution and their radiative impacts, focusing on spring and summer emissions from mid-latitude anthropogenic sources, biomass burning, Arctic shipping, and Arctic gas flaring.

Results show that remote anthropogenic and biomass burning emissions are likely to remain the main source of Arctic pollution burdens and of black carbon (BC) deposition over snow, and the main contributors to direct aerosol and ozone radiative effects in the Arctic. However, local Arctic flaring emissions are already a major source of BC in Northwestern Russia, with a direct radiative effect of ∼ 25 mW m -2 , and Arctic shipping is a strong current source of aerosols and ozone during summer in the Nordic Seas. We find that the direct effect of ozone and aerosols from summertime Arctic shipping are respectively negative (due to frequent temperature inversions) and positive (because of the high surface albedo) in our simulations, two new results. With the development of diversion shipping through the Arctic Ocean in summer 2050, Arctic shipping emissions could become the main source of surface aerosol and ozone pollution at the surface, with strong associated indirect effects of -0.8 W m -2 , while flaring would remain an important BC source.

Introduction

The Arctic atmosphere is influenced by air pollution transported from lower latitudes. Anthropogenic emissions from Europe, Asia and North America in the midlatitudes are currently the main source of Arctic pollution, especially in winter and spring when poleward atmospheric transport is efficient and removal processes are weak [START_REF] Barrie | Arctic air pollution: An overview of current knowledge[END_REF][START_REF] Stohl | Characteristics of atmospheric transport into the arctic troposphere[END_REF]. Biomass burning at middle and high latitudes is also an important source of Arctic pollution during late spring and summer [START_REF] Brock | Arctic hazes in summer over greenland and the north american arctic. i: Incidence and origins[END_REF][START_REF] Stohl | Characteristics of atmospheric transport into the arctic troposphere[END_REF][START_REF] Warneke | Biomass burning in siberia and kazakhstan as an important source for haze over the alaskan arctic in april 2008[END_REF][START_REF] Ikeda | Tagged tracer simulations of black carbon in the arctic: transport, source contributions, and budget[END_REF], although its exact contribution is uncertain. These remote emissions are an important source of Arctic aerosols (e.g. black carbon (BC), sulfate) and ozone, which act as short-lived climate forcers that can enhance or mitigate the observed rapid Arctic warming [START_REF] Shindell | Role of tropospheric ozone increases in 20th-century climate change[END_REF][START_REF] Najafi | Attribution of arctic temperature change to greenhouse-gas and aerosol influences[END_REF]AMAP , 2015]. Ozone is a greenhouse gas, and also absorbs short wave radiation directly [e.g., [START_REF] Berntsen | Effects of anthropogenic emissions on tropospheric ozone and its radiative forcing[END_REF]. Aerosols can scatter or absorb short wave radiation [direct effects, Ångström, 1929;[START_REF] Haywood | The effect of anthropogenic sulfate and soot aerosol on the clear sky planetary radiation budget[END_REF]; influence the life cycle of clouds by modifying atmospheric temperature and humidity profiles [semi-direct effects, [START_REF] Hansen | Radiative forcing and climate response[END_REF]; and influence cloud formation, cloud optical properties, cloud height and cloud lifetime through their microphysical interactions with cloud droplets [indirect effects, Twomey, 1977;[START_REF] Albrecht | Aerosols, cloud microphysics, and fractional cloudiness[END_REF]. In addition, absorbing aerosols deposited on snow reduce the surface albedo and lead to earlier snow melt [START_REF] Warren | Analysis of ozone and nitric acid in spring and summer arctic pollution using aircraft, ground-based, satellite observations and mozart-4 model: source attribution and partitioning[END_REF].

Decreasing anthropogenic emissions in Europe, Western Asia and North America are thought to be responsible for the observed decline in black carbon and sulfate aerosol concentrations at several surface sites in the Arctic [START_REF] Hirdman | Long-term trends of black carbon and sulphate aerosol in the arctic: changes in atmospheric transport and source region emissions[END_REF][START_REF] Sharma | 16-year simulation of arctic black carbon: Transport, source contribution, and sensitivity analysis on deposition[END_REF] ©2018 American Geophysical Union. All Rights Reserved.

2013], and these declining trends could continue in the future due to further global reductions in emissions of short-lived climate forcers and their precursors [IPCC , 2013]. At the same time, Arctic warming and the associated decline in sea ice [START_REF] Kirtman | Near-term Climate Change: Projections and Predictability[END_REF] is progressively opening the region to human activity, especially shipping and resource extraction [IPCC , 2014]. Local Arctic pollutant emissions could increase rapidly as a result [START_REF] Corbett | Arctic shipping emissions inventories and future scenarios[END_REF][START_REF] Peters | Future emissions from shipping and petroleum activities in the arctic[END_REF]. It is currently unclear if local Arctic emissions could then start to have a more significant impact on Arctic air pollution, compared to the historically predominant remote sources, and whether local sources could become important drivers of Arctic climate change or have an impact on regional air quality or ecosystems [START_REF] Arnold | Arctic air pollution: Challenges and opportunities for the next decade[END_REF][START_REF] Law | Local arctic air pollution: Sources and impacts[END_REF]. These questions are important for policymakers, who need to understand the trade-offs between air quality and climate when reducing local Arctic emissions of short-lived pollutants and their precursors [START_REF] Schmale | Air pollution: Clean up our skies[END_REF][START_REF] Stohl | Evaluating the climate and air quality impacts of short-lived pollutants[END_REF][START_REF] Sand | Response of arctic temperature to changes in emissions of short-lived climate forcers[END_REF]AMAP , 2015].

However, the answer to these questions is currently unclear, for two main reasons. First, models struggle to represent short-lived pollutants, especially aerosols, in the Arctic [e.g., [START_REF] Koch | Evaluation of black carbon estimations in global aerosol models[END_REF][START_REF] Eckhardt | Current model capabilities for simulating black carbon and sulfate concentrations in the arctic atmosphere: a multimodel evaluation using a comprehensive measurement data set[END_REF][START_REF] Schwarz | Aircraft measurements of black carbon vertical profiles show upper tropospheric variability and stability[END_REF], due to limited model resolution [START_REF] Sato | Unrealistically pristine air in the arctic produced by current global scale models[END_REF][START_REF] Raut | Cross-polar transport and scavenging of siberian aerosols containing black carbon during the 2012 access summer campaign[END_REF], uncertainties in emissions [START_REF] Stohl | Black carbon in the arctic: the underestimated role of gas flaring and residential combustion emissions[END_REF] and uncertainties in processes such as aerosol removal by precipitation and clouds [e.g., [START_REF] Browse | The scavenging processes controlling the seasonal cycle in arctic sulphate and black carbon aerosol[END_REF][START_REF] Mahmood | Seasonality of global and arctic black carbon processes in the arctic monitoring and assessment programme models[END_REF]Qi et al., 2017a;[START_REF] Raut | Cross-polar transport and scavenging of siberian aerosols containing black carbon during the 2012 access summer campaign[END_REF]. Second, the lack of accurate emission inventories also makes it difficult to quantify precisely the impact of local Arctic emission sources. Early inventories by [START_REF] Peters | Future emissions from shipping and petroleum activities in the arctic[END_REF] for Arctic oil and gas extraction, and [START_REF] Corbett | Arctic shipping emissions inventories and future scenarios[END_REF] for Arctic shipping, used in earlier studies [e.g., [START_REF] Ødemark | Short-lived climate forcers from current shipping and petroleum activities in the arctic[END_REF][START_REF] Dalsøren | Environmental impacts of shipping in 2030 with a particular ©2018[END_REF][START_REF] Browse | Impact of future arctic shipping on high-latitude black carbon deposition[END_REF], appear to have ©2018 American Geophysical Union. All Rights Reserved.

underestimated the magnitude of Arctic pollutant emissions, compared to more recent estimates [START_REF] Winther | Emission inventories for ships in the arctic based on satellite sampled AIS data[END_REF][START_REF] Klimont | Global anthropogenic emissions of particulate matter including black carbon[END_REF]. New Arctic emission inventories for flaring associated with oil and gas extraction have already been used in recent studies, highlighting the important contribution from this source to current BC concentrations and BC deposition in the Arctic [START_REF] Stohl | Black carbon in the arctic: the underestimated role of gas flaring and residential combustion emissions[END_REF][START_REF] Xu | Source attribution of arctic black carbon constrained by aircraft and surface measurements[END_REF]Qi et al., 2017a], but with little focus on its potential future impacts.

While global climate models are the ideal tools for estimating future radiative impacts of short-lived and long-lived climate forcers, they often include simplified representations of chemical and aerosol processes including aerosol-cloud interactions [e.g., see [START_REF] Sand | Aerosols at the poles: an aerocom phase ii multi-model evaluation[END_REF]. In addition, the resolutions of global model used to study Arctic aerosols and ozone remain low, typically 2°×2°or lower for coupled chemistry-aerosol-climate models [START_REF] Eckhardt | Current model capabilities for simulating black carbon and sulfate concentrations in the arctic atmosphere: a multimodel evaluation using a comprehensive measurement data set[END_REF][START_REF] Arnold | Biomass burning influence on high-latitude tropospheric ozone and reactive nitrogen in summer 2008: a multi-model analysis based on polmip simulations[END_REF]. Alternatively, regional models only simulate limited domains, which costs less computing power than simulating global domains, freeing computing resources in order to either increase the resolution, use more complex physics, aerosols, or chemistry schemes, or in order to run longer simulations or ensembles to quantify the statistical significance of model results

In this study, we use the regional model WRF-Chem 3.5. 1 [Weather Research and Forecasting model, including chemistry;[START_REF] Grell | Fully coupled "online" chemistry within the WRF model[END_REF][START_REF] Fast | Evolution of ozone, particulates, and aerosol direct radiative forcing in the vicinity of houston using a fully coupled meteorologychemistry-aerosol model[END_REF], which includes explicit calculations of the aerosol size distributions within 16 size bins (8 interstitial and 8 in clouds) for sulfate, nitrate, ammonium, primary and secondary OA, BC, dust and sea salt, as well explicit aerosol activation in clouds and "online" feedback on the meteorology, allowing us to calculate the indirect effect of aerosols directly [details are given in Section 2 and in [START_REF] Marelle | Improvements to the wrf-chem 3.5.1 model for quasihemispheric simulations of aerosols and ozone in the arctic[END_REF]. To our knowledge, the only model included ©2018 American Geophysical Union. All Rights Reserved.

with similar capabilities in the Arctic intercomparisons of [START_REF] Eckhardt | Current model capabilities for simulating black carbon and sulfate concentrations in the arctic atmosphere: a multimodel evaluation using a comprehensive measurement data set[END_REF] or in the assessment report of AMAP [2015] is CESM1-CAM5. However, CESM1-CAM5 uses fewer parameters for the size distribution, does not include nitrate aerosols, and was run in these studies with a low resolution of 1.9°×2.5°and 30 levels. The WRF-Chem model has recently been improved for the Arctic and is now able to capture observed aerosols and ozone in the Arctic [Marelle et al., 2017, details in Section 2.3]. It is run here with recent inventories for current (2012) and future (2050) remote and local Arctic emissions [START_REF] Winther | Emission inventories for ships in the arctic based on satellite sampled AIS data[END_REF][START_REF] Klimont | Global anthropogenic emissions of particulate matter including black carbon[END_REF].

In Section 2, we present the WRF-Chem model, the emission inventories, and the different simulations performed in this study. These simulations are used to compare the relative influences of local (Arctic shipping and gas flaring) and remote (midlatitude anthropogenic emissions, biomass burning) sources of Arctic aerosols and ozone, on surface aerosol concentrations, surface ozone concentrations, and black carbon deposition in the Arctic (Section 3); on the vertical distribution of aerosols and ozone in the Arctic (Section 4); and on the direct and indirect radiative effects of aerosols and ozone in the Arctic (Section 5). Conclusions are presented in Section 6. The focus of this study is to investigate the effect of emissions from these different sources in the present-day and in the future. For this reason, we change emissions but keep the meteorological conditions the same in the present-day and future simulations.

Methods

The impacts of the different emission sources are determined by performing 6-month long (March to August), quasi-hemispheric WRF-Chem 3.5.1 simulations over the Arctic, using emissions for the years 2012 and 2050. The WRF-Chem model is a coupled ©2018 American Geophysical Union. All Rights Reserved.

meteorology-atmospheric chemistry-aerosol mesoscale model based on the model WRF-ARW [Advanced Research WRF;[START_REF] Skamarock | A description of the advanced research wrf version 3[END_REF]. In this study, we use a version of WRF-Chem 3.5.1 modified for quasi-hemispheric Arctic simulations [START_REF] Marelle | Improvements to the wrf-chem 3.5.1 model for quasihemispheric simulations of aerosols and ozone in the arctic[END_REF], including updates to the treatment of surface temperatures over sea ice, trace gas deposition and photolysis over snow, dimethylsulfide (DMS) emissions and DMS gas-phase chemistry, aerosol sedimentation aloft, and also including the effects of sub-grid scale clouds on aerosols and trace gases. The Arctic region is defined in this study as the region from 60°N to 90°N.

Simulation domain and model setup

The simulation domains (polar stereographic projection) are shown in Figure 1. The d1 domain includes regions with emissions potentially transported to the Arctic in under 30 days [START_REF] Stohl | Characteristics of atmospheric transport into the arctic troposphere[END_REF], and the inner d2 domain is focused on the Arctic region. The horizontal resolution is 100 km×100 km, with 50 vertical levels. This resolution is similar to the resolution of state-of-the-art global climate models, although coupled aerosol-chemistryclimate global models included in recent intercomparisons in the Arctic [START_REF] Eckhardt | Current model capabilities for simulating black carbon and sulfate concentrations in the arctic atmosphere: a multimodel evaluation using a comprehensive measurement data set[END_REF]AMAP , 2015] used even lower resolutions of 2°×2°or 3°×3°and 30 vertical levels. We used this relatively low resolution in order to use complex physics and aerosol schemes within WRF-Chem, especialy a 2-moment cloud microphysics scheme with explicit aerosol activation and its feedback on cloud properties, and to add additional ensemble simulations. These choices were made in order to calculate indirect effects due to cloud/aerosol interactions directly within WRF-Chem, without using a parameterization for cloud droplet numbers, and also to assess the statistical significance of our results and separate signal from noise, especially for radiative calculations (Section 2.3). We also note ©2018 American Geophysical Union. All Rights Reserved.

that although there are some models in [START_REF] Eckhardt | Current model capabilities for simulating black carbon and sulfate concentrations in the arctic atmosphere: a multimodel evaluation using a comprehensive measurement data set[END_REF] with comparable or higher resolutions than in the present study, they are "offline" Lagrangian or chemical-transport models unable to calculate the feedbacks of aerosols and chemistry on the meteorology.

The model setup for meteorology, aerosols and chemistry is presented in [START_REF] Marelle | Improvements to the wrf-chem 3.5.1 model for quasihemispheric simulations of aerosols and ozone in the arctic[END_REF]. Briefly, gas-phase chemistry calculations use the SAPRC-99 [Statewide Air Pollution Research Center, 1999 version;[START_REF] Carter | Documentation of the saprc-99 chemical mechanism for voc reactivity assessment[END_REF] mechanism, combined to a version of the 8-bin sectional aerosol scheme MOSAIC [Model for Simulating Aerosol Interactions and Chemistry; [START_REF] Zaveri | Model for simulating aerosol interactions and chemistry (mosaic)[END_REF], which includes secondary organic aerosol (SOA) formation [START_REF] Shrivastava | Modeling organic aerosols in a megacity: comparison of simple and complex representations of the volatility basis set approach[END_REF], and aqueous chemistry in grid-scale [START_REF] Morrison | Impact of cloud microphysics on the development of trailing stratiform precipitation in a simulated squall line: Comparison of one-and two-moment schemes[END_REF] and sub-grid scale [START_REF] Berg | A new wrf-chem treatment for studying regional-scale impacts of cloud processes on aerosol and trace gases in parameterized cumuli[END_REF] ©2018 American Geophysical Union. All Rights Reserved.

The model is run from 15 February 2012 to 1 September 2012, using anthropogenic emissions for 2012 and 2050 (described in Section 2.2). The first 15 days of simulation (15 February 2012 to 29 February 2012) are considered as spin-up and are discarded from the analysis. Model results are thus available for two seasons, spring (March-April-May, MAM), when long-range pollution transport to the Arctic is significant, and summer (June-July-August, JJA), when long-range transport is less efficient, loss by wet deposition more prevalent, and when local emissions could play a larger role, especially shipping due to ice-free conditions. This limited 6-month period is thus well-adapted to reach one of the main objectives of this study, which is to compare the impacts of emerging local Arctic sources and of the more well-known remote sources. It is also the period when the direct radiative effects of Arctic aerosols are the largest [START_REF] Sand | Aerosols at the poles: an aerocom phase ii multi-model evaluation[END_REF].

In this study, we focus on quantifying relative contributions from different anthropogenic emission sources in the present day (2012) and in the future (2050). Therefore, simulations for 2050 are also forced by 2012 sea ice, sea surface temperatures, and meteorology from the NCEP FNL analysis, and use present-day vegetation, as well as 2012 chemical, initial, and boundary conditions from the MOZART-4 model. For tropospheric ozone, we focus on the contribution of non-methane precursor emissions of nitrogen oxides (NO x ), volatile organic compounds (VOCs) and carbon monoxide (CO), in line with previous analyses (AMAP, 2015). Methane concentrations are not directly calculated in WRF-Chem 3.5.1 and are set to a constant level throughout the troposphere [1.893 ppm in this study; [START_REF] Blasing | Recent greenhouse gas concentrations[END_REF] in the 2012 and 2050 simulations. Longer simulations would be needed to quantify the contribution from methane oxidation and effects related to methane feedbacks on tropospheric ozone.

©2018 American Geophysical Union. All Rights Reserved. of global shipping through the Arctic Ocean. We assume that Arctic diversion shipping occurs in July and August, following [START_REF] Corbett | Arctic shipping emissions inventories and future scenarios[END_REF], and that diversion shipping emissions are equally divided between the Northern Sea Route and the Northwest Passage. All future shipping emissions are based on "High Growth" projections, described in [START_REF] Corbett | Arctic shipping emissions inventories and future scenarios[END_REF] and [START_REF] Winther | Emission inventories for ships in the arctic based on satellite sampled AIS data[END_REF]. Emission totals of NO x , SO 2 and BC from Arctic shipping and Arctic flaring in 2012 and 2050 are given in Table 1. Arctic shipping mostly emits NO x and SO 2 , and flaring is an important local source of BC [as already noted by [START_REF] Stohl | Black carbon in the arctic: the underestimated role of gas flaring and residential combustion emissions[END_REF]. Table 1 shows that flaring emissions from ECLIPSEv5 are relatively stable between 2012 and 2050 [in agreement with earlier findings by [START_REF] Peters | Future emissions from shipping and petroleum activities in the arctic[END_REF], but that shipping emissions of BC, NO x and SO 2 from [START_REF] Corbett | Arctic shipping emissions inventories and future scenarios[END_REF] and [START_REF] Winther | Emission inventories for ships in the arctic based on satellite sampled AIS data[END_REF] increase very strongly in summer 2050 due to diversion shipping through the Arctic Ocean. Arctic shipping SO 2 emissions (north of 60°N) decrease by -31% in spring 2050 due to shipping emission controls reducing sulphur fuel contents [START_REF] Imo | Report of the marine environment protection committee on the sixty-first session[END_REF], but, in summer 2050, this decrease in emission factors is more than compensated for by the projected increase in traffic, and total Arctic shipping SO 2 emissions increase.

Simulations

The simulations performed in this study are presented in Table 2. The CONTROL simulation includes all sources of emissions. We also perform sensitivity simulations without emissions from Arctic ships (NOSHIPS), Arctic flares (NOFLARES), mid-latitude anthropogenic emissions south of 60°N (NOANTHRO MIDLAT, including ships) and biomass burning emissions (NOFIRES, including wildfires and anthropogenic agricultural fires).

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These control and sensitivity simulations are performed with 2012 and 2050 emissions (but NOFIRES uses the same biomass burning emissions for 2012 and 2050).

A simulation for the year 2008 using the same setup as CONTROL (on domain d1) was presented and validated against surface and airborne measurements in Marelle et al.

[2017], showing good agreement with observed Arctic BC and sulfate. The root mean square error (RMSE) at the Arctic surface in spring and summer 2008 was found to be 14.2 ng m -3 for BC, and 0.261 µg m -3 for sulfate, and the corresponding correlation coefficients 0.87 for BC and 0.73 for sulfate. WRF-Chem was however shown to be biased high for surface ozone (RMSE 7.56 ppbv), despite a good correlation coefficient of 0.73. This high bias was due to ozone overestimations over sea ice during spring, due to the lack of halogen chemistry in the model. This has only limited impacts on the attribution results presented in this study, since we show in Section 4 that only one source (remote anthropogenic emissions) has significant impacts on ozone in the Arctic lower troposphere during spring. In addition, the radiative effect of Arctic ozone is not very sensitive to concentrations in the boundary layer [START_REF] Rap | Satellite constraint on the tropospheric ozone radiative effect[END_REF].

Sensitivity simulations NOANTHRO MIDLAT, NOFIRES, NOSHIPS and NOFLARES are used to evaluate the impacts of a given emission source on modeled quantities, e.g. CONTROL(2012) -NOSHIPS(2012), to assess the effect of Arctic shipping emissions in 2012. In Sections 3, 4 and 5, results are shown in terms of relative contributions, e.g., [CONTROL(2012) -NOSHIPS( 2012)] / CONTROL(2012). Since we do not study every potential source and since non-linearities might be present in the model, the sum of contributions may not add up to 100 %. This is especially the case for ozone, since large natural sources (stratospheric, lightning) are excluded from our analysis. In addition, the ©2018 American Geophysical Union. All Rights Reserved.

ozone response from switching off sources completely may not be additive, because ozone chemistry is highly non-linear [START_REF] Wu | Chemical nonlinearities in relating intercontinental ozone pollution to anthropogenic emissions[END_REF].

Since emissions from Arctic shipping and flaring activities are only 0.2 % and 0.7 % of total NO x and BC anthropogenic and fire emissions in the d1 domain, they may only have relatively small effects on pollutant concentrations, BC deposition, or on the radiative budget in the Arctic. As a result, such effects may be locally small compared to the effects of internal meteorological variability in the model [e.g., [START_REF] Stofferahn | Investigation of aerosol effects on the arctic surface temperature during the diurnal cycle: part 1 total aerosol effect[END_REF], making them difficult to quantify. Therefore, in order to separate the signal from these local Arctic emissions from internal variability, we use a filtering approach based on ensemble runs and the Student t-test [START_REF] Stofferahn | Investigation of aerosol effects on the arctic surface temperature during the diurnal cycle: part 1 total aerosol effect[END_REF]. A first CON-TROL simulation is performed on domain d1, and used to set the boundary conditions for the sensitivity simulations NOSHIPS, NOFLARES and a second CONTROL performed on domain d2. Four ensemble members are then generated for each of these runs by initializing the model at 00 UTC, 06 UTC, 12 UTC and 18 UTC on 15 February. A two-tailed Student t-test is later used to assess whether the average effect of, for example, Arctic shipping (ensemble average for CONTROL -NOSHIPS) is different at the 95 % significance level from the variability between the 4 different (in this case CONTROL -NOSHIPS) realizations. This filtering is not used for the NOANTHRO MIDLAT and NOFIRE simulations because of the high computational cost of performing ensembles on the large d1 domain, and because emission changes associated with these sources are large and thus less sensitive to internal variability (see discussions in Sections 3, 4 and 5).

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deposition in the Arctic

This Section examines the impacts of the different emission sources (anthropogenic activity in the mid-latitudes, biomass burning, Arctic shipping, and Arctic oil and gas flaring) on ozone mixing ratios and aerosol concentrations (excluding dust and sea salt) at the surface (0 to 50 m altitude), and their impacts on BC deposition fluxes in the Arctic. Since local emissions are directly emitted at the Arctic surface, their impacts are expected to be higher at low altitudes. The vertical distributions of these contributions are discussed in the next section.

Figure 2 shows the simulated ozone and aerosol (BC; sum of sulfate + ammonium + nitrate + organic aerosol (OA)) surface concentrations in spring and summer 2012, and the contributions from each source. Similarly, Figure 3 presents the contributions from each source to BC deposition in the Arctic. For reasons noted earlier, individual contributions do not necessarily add up to 100 %. In addition, we do not quantify the contributions from local Arctic emissions other than ships and flares or from sources outside of the d1 domain.

In Figure 2, results from the CONTROL simulation (left-hand column) are in agreement with the well-known seasonal cycle of pollution in the remote Arctic [START_REF] Quinn | Arctic haze: current trends and knowledge gaps[END_REF], increasing in winter and spring and lower during summer. These lower concentrations in summer are clear at higher latitudes (north of 70°N), but at lower latitudes, aerosol concentrations increase over land during summer due to boreal biomass burning emissions. In spring 2012, mid-latitude anthropogenic emissions are an important source of Arctic surface ozone (15 to 30 % of total ozone) and aerosols (∼ 70 % of total sul-fate+nitrate+ammonium+OA), and of BC deposition at the surface (∼ 60 %, Figure 3).

Biomass burning is a weak source of surface BC in spring 2012, but has a large effect on BC deposition (up to 50 % in Russia). Earlier studies have shown that agricultural fires in Eurasia could be effectively transported to the Arctic at low altitudes in spring during specific events [START_REF] Stohl | Arctic smoke -record high air pollution levels in the european arctic due to agricultural fires in eastern europe in spring 2006[END_REF][START_REF] Warneke | Biomass burning in siberia and kazakhstan as an important source for haze over the alaskan arctic in april 2008[END_REF][START_REF] Marelle | Transport of anthropogenic and biomass burning aerosols from Europe to the Arctic during spring 2008[END_REF], but this does not seem to occur significantly in spring 2012. During summer 2012, when boreal wildfires occur, biomass burning emissions are a major source of short-lived pollutants at the surface (15 to 50 % for ozone, 50 to 100 % for BC and sulfate+nitrate+ammonium+OA) and of deposited BC in the Arctic (up to 80 %). Anthropogenic emissions are also an important source of aerosols in the central Arctic and in the European and Atlantic sectors (50 %) during summer, and a significant but small source of summertime surface ozone (5 to 20 %).

The results for BC are in general agreement with the attribution studies by Stohl et al.

[2013] and [START_REF] Xu | Source attribution of arctic black carbon constrained by aircraft and surface measurements[END_REF], who found that the main sources of BC measured at the Arctic surface during spring are anthropogenic emissions in Europe and Asia, while biomass burning is a major summertime BC source. Compared to these studies, and to the springtime analysis of [START_REF] Qi | Sources of springtime surface black carbon in the arctic: an adjoint analysis for april 2008[END_REF], our results suggests an even smaller role of biomass burning during spring (< 10 %, excluding Siberia), and a higher contribution in summer from anthropogenic BC emissions, especially over the sea ice pack and Greenland, although there could be significant interannual variability due to variations in wildfire emissions.

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Earlier studies [e.g., Walker et al., 2012;Wespes et al., 2012;[START_REF] Arnold | Biomass burning influence on high-latitude tropospheric ozone and reactive nitrogen in summer 2008: a multi-model analysis based on polmip simulations[END_REF]] also found that Arctic ozone is sensitive to biomass burning emissions during summer. However, Wespes et al. [2012] estimated that surface ozone in the Arctic is far more sensitive to anthropogenic emissions, while our results indicate that biomass burning is a larger source in many parts of the Arctic. These differences could be due to the focus of Wespes et al. [2012] on the limited period of the ARCTAS campaign flights in June and July 2008.

Differences in emission inventories and methodologies may also play a role. Arnold et al.

[2015] showed that there is significant inter-model variability in simulating Arctic ozone from boreal burning emissions, in particular due to treatments of VOC oxidation.

As illustrated in Figure 2, Arctic flaring emissions are a major source of present-day surface BC, but do not contribute much to surface concentrations of ozone or to other aerosol types. 10 to 20 % of total BC concentrations over the central Arctic in spring are due to Arctic flares, with up to 50 to 100 % over snow-and ice-covered regions in northern Russia and over the Kara Sea. In these regions, flaring emissions are also responsible for 50 to 100 % of the total BC deposition (Figure 3). Flares have a more localized influence on Arctic BC during summer compared to spring, due to less efficient transport and more active loss by wet deposition at this time of year [START_REF] Raut | Cross-polar transport and scavenging of siberian aerosols containing black carbon during the 2012 access summer campaign[END_REF] (The increase in total BC deposition in summer in our simulations is also seen in Figure 3, bottom left panel).

Our results show that flaring has a large influence (> 10 %) on surface BC in the European Arctic, the central Arctic and the Western and central Russian Arctic, but little influence outside of these regions. This is partly at odds with Stohl et al. ©2018 American Geophysical Union. All Rights Reserved.

[2017], who also found little influence from flaring BC on surface concentrations in eastern Siberia. [START_REF] Winiger | Siberian arctic black carbon sources constrained by model and observation[END_REF] concluded that this limited influence in eastern Siberia was incompatible with high Russian flaring BC emissions such as those present in the ECLIPSEv5 inventory. However, our simulations reproduce this limited flaring influence in Siberia using the same ECLIPSEv5 BC emissions used in the studies by Stohl et al.

[2013] and [START_REF] Winiger | Siberian arctic black carbon sources constrained by model and observation[END_REF], indicating that ECLIPSEv5 emissions are not necessarily inconsistent with the findings of [START_REF] Winiger | Siberian arctic black carbon sources constrained by model and observation[END_REF]. [START_REF] Winiger | Siberian arctic black carbon sources constrained by model and observation[END_REF] supposed that these inconsistencies could not be due to limitations in their model (FLEXPART), i.e. to an overestimated BC lifetime in FLEXPART, because "a too-long BC lifetime in the model would also lead to a general overestimation of observed BC, which is not the case". However, a recent case study using WRF-Chem found that Arctic flaring BC was more efficiently removed than BC from other emission sources during transport [transport efficiency lower than 30 % [START_REF] Raut | Cross-polar transport and scavenging of siberian aerosols containing black carbon during the 2012 access summer campaign[END_REF], even though WRF-Chem slightly overestimates BC in the Arctic [START_REF] Marelle | Improvements to the wrf-chem 3.5.1 model for quasihemispheric simulations of aerosols and ozone in the arctic[END_REF]. The reason for this discrepancy could thus be that FLEXPART overestimates the flaring BC lifetime specifically, or that WRF-Chem underestimates it, by e.g., overestimating precipitation (drizzle) in the region of the flaring emissions, or overestimating the hydroscopicity of aerosols containing flaring BC.

Additional research is needed to resolve this issue, including more dedicated measurements of flaring BC in the Arctic in order to validate emission inventories, BC removal and BC transport.

In 2012, shipping emissions have no significant effect on BC concentrations, or on BC deposition fluxes during spring, but are an important source of surface aerosols and ozone during summer, when Arctic shipping emissions and photochemistry are the highest.

©2018 American Geophysical Union. All Rights Reserved.

Summertime Arctic shipping contributes significantly to ozone (15 to 25 % of total ozone in the Norwegian, Barents and Kara Seas), BC (10 to 30 %), and to nitrate, sulfate, and ammonium aerosols (10 to 30 %). This can be due to a combination of increased precursor emissions (NO x , SO 2 ), to changes in aerosol chemistry [i.e. increased condensation of ammonia with increased nitrate and sulfate; [START_REF] Seinfeld | Atmospheric Chemistry and Physics -from Air Pollution to Climate Change[END_REF] and to enhanced oxidant levels due to NO x emissions. In regions influenced by Arctic shipping, shipping emissions are responsible for up to 50 % of total surface hydroxyl radical (OH) levels during summer (not shown). Increases in OH are known to lower the lifetime of greenhouse gases such as CH 4 [START_REF] Levy | Normal atmosphere: Large radical and formaldehyde concentrations predicted[END_REF], and have also been shown to significantly reduce the lifetime of SO 2 in the Arctic [START_REF] Fuglestvedt | Climate penalty for shifting shipping to the arctic[END_REF].

The results for present-day Arctic shipping can be compared to the high-resolution However, an important caveat is that this does not include the effect of changing mete-©2018 American Geophysical Union. All Rights Reserved. [2013], we find that diversion shipping, even in this "High

Growth" future scenario, does not strongly increase BC deposition over snow and ice (at most 4 to 8 % locally during summer; the snow and ice limit is shown in Figure 1). This is due to the short residence time of Arctic shipping BC (1.4 days during summer 2050), limiting transport away from the shipping lanes and onto ice or snow. Deposition of shipping BC over sea ice could be even lower using realistic weather conditions and seaice for 2050, due to the lower sea-ice available for BC to deposit on [START_REF] Wang | A sea ice free summer arctic within 30 years: An update from cmip5 models[END_REF] and to the reduced aerosol lifetime from increased precipitation [START_REF] Jiao | Changing black carbon transport to the arctic from present day to the end of 21st century[END_REF]. In our simulations, BC lifetime in the Arctic is mostly controlled by wet removal (96 % of the total BC deposition in the Arctic). Although there is to our knowledge no direct observational constraint on the short BC lifetime found in the Arctic summertime marine ©2018 American Geophysical Union. All Rights Reserved.

boundary layer, we note that, using a similar setup, WRF-Chem was able to reproduce the observed summertime BC concentrations near the Arctic Ocean [START_REF] Marelle | Improvements to the wrf-chem 3.5.1 model for quasihemispheric simulations of aerosols and ozone in the arctic[END_REF] at Zeppelin (Svalbard), Nord (Greenland) Alert (Canada), and Barrow/Utqia ġvik (Alaska), giving some confindence in these results.

Future (2050) local and remote emission contributions to vertical distributions Arctic aerosols and ozone

The radiative effects of aerosols and ozone do not scale with surface concentrations, and are more strongly related to total column burdens. Radiative effects are also known to be very sensitive to vertical pollutant distributions [START_REF] Lacis | Radiative forcing of climate by changes in the vertical distribution of ozone[END_REF][START_REF] Rap | Satellite constraint on the tropospheric ozone radiative effect[END_REF][START_REF] Flanner | Arctic climate sensitivity to local black carbon[END_REF]. In addition, wet deposition of BC is sensitive to aerosol concentrations at higher altitudes, in or below clouds. For these reasons, and to better understand the origin of Arctic aerosols and ozone, Figures 6 and7 show the seasonally averaged zonal mean BC and ozone contributions from different emission sources in 2050 (results in 2012 are qualitatively similar, except that shipping has much smaller impacts). These figures also show the average cloud cover in the CONTROL simulation, indicating that clouds are often present (mean cloud cover up to 25 %) at the surface and at altitudes from 4 to 8 km in the Arctic.

Figure 6 indicates that mid-latitude anthropogenic and biomass burning emissions are responsible for the majority of the Arctic-wide burden of aerosol pollution. Remote anthropogenic sources have a strong influence on BC throughout the troposphere during spring, and are also a strong source of BC during summer, especially between 3 to 9 km and at the surface north of 75°N. Biomass burning has a relatively small influence on BC concentrations in spring, but are estimated to be a major source of BC at low altitudes ©2018 American Geophysical Union. All Rights Reserved. [2015] showed that even emissions in the southern hemisphere could be responsible for approximately 10 % of Arctic BC burdens. At the surface, the sum of individual BC contributions is also less than 100 % during spring.

This is because we focus in this study on remote emissions and on the ermerging Arctic gas flaring and shipping emissions, but do not quantify the contribution from other local Arctic BC sources, such as domestic combustion, which has been shown to be a relatively ©2018 American Geophysical Union. All Rights Reserved.

strong source of surface Arctic BC in winter and spring [START_REF] Stohl | Black carbon in the arctic: the underestimated role of gas flaring and residential combustion emissions[END_REF]. However, these points do not affect the conclusions of this study, since the contribution of one source of interest (e.g. Arctic Flares) to Arctic BC can be estimated without estimating the contribution from all the other potential sources.

With regard to ozone, Figure 7 shows that non-methane remote anthropogenic emissions make an important contribution to ozone in the lower Arctic troposphere during Arctic flaring emissions have an important effect on the BC zonal mean, 5 to 20 %, comparable to results from [START_REF] Stohl | Black carbon in the arctic: the underestimated role of gas flaring and residential combustion emissions[END_REF]. This effect is mostly confined below 2 km.

As discussed in Section 3, flaring has a more localized influence during summer, due to stronger total deposition and to the northern shift of the Arctic Front. Arctic flares have a negligible effect on ozone levels.

©2018 American Geophysical Union. All Rights Reserved.

In 2050, Arctic shipping makes a relatively small contribution to zonal mean BC concentrations, even at the surface during summer (at most, 10 % below a few hundred meters at latitudes 70°N to 80°N). However, future Arctic shipping has a larger effect on summertime ozone in the lower troposphere (5 to 20 % below 2 km), especially at higher latitudes.

5. Present-day and future Arctic radiative effects due to local and remote emissions.

In order to compare the contribution of each source to the local radiative budget, we calculate with the WRF-Chem model the Arctic-wide average (60°N to 90°N) of the radiative effects at the top-of-atmosphere (TOA) of aerosols and ozone from local and remote sources of Arctic pollution.

Calculating the radiative effects of short-lived pollutants in WRF-Chem

The RRTMG module in the WRF-Chem model calculates radiative fluxes at TOA, taking into account predicted aerosols. In this work, RRTMG was also modified to use model-predicted ozone for radiative calculations, instead of climatological values. In these simulations, the RRTMG module is called 3 times at every radiative time step, first with all species taken into account, then without aerosols, and without ozone, in order to calculate the total direct radiative effects at TOA of aerosols and ozone. The direct effect from a specific source (e.g. Arctic shipping) is calculated as the difference in total direct effect between the simulations with (CONTROL) and without (e.g. NOSHIPS) for this source. The direct effect of aerosols includes the effect of changes in BC, sulfate, nitrate, ammonium and OA concentrations due to a particular source, but the effect of these different components is not estimated separately here. The calculation of direct effects ©2018 American Geophysical Union. All Rights Reserved.

does not take into account the effect of rapid adjustments in stratospheric temperatures, tropospheric temperatures and water vapor; i.e., calculated direct effects presented here are the so-called "instantaneous radiative forcings" [e.g., Myhre et al., 2013a].

WRF-Chem also calculates aerosol activation in liquid clouds and the resulting ef- [START_REF] Twomey | Accounting for non-linear chemistry of ship plumes in the geos-chem global chemistry transport model[END_REF][START_REF] Albrecht | Aerosols, cloud microphysics, and fractional cloudiness[END_REF]. Semi-direct aerosol effects [START_REF] Hansen | Radiative forcing and climate response[END_REF] are also taken into account, since predicted aerosol optical properties (and ozone concentrations) are coupled to radiation calculations within WRF-Chem, and have an influence on heating rates, temperature and relative humidity profiles, causing additional changes in cloud formation, cloud properties, and cloud lifetime. As a result, we calculate the direct+indirect+semi-direct radiative effects at TOA from a specific source as the difference of total upwelling radiative flux at TOA between the simulations with and without a particular source. Since the direct effect of aerosols and ozone is also known separately from the calculations presented above, we can extract from these runs the indirect+semi-direct effects for each source. The radiative effect of snow albedo changes due to BC deposition is not included in our calculations, nor the effect of ozone produced from methane and related long-term feedbacks, although they can be substantial and account for approximately half of the direct effects of BC and ozone respectively [AMAP , 2015].

©2018 American Geophysical Union. All Rights Reserved.

For Arctic shipping and flaring, the calculations are performed for each ensemble member, and a Student t-test is then performed to assess whether each radiative effect (e.g. direct aerosol effect of shipping) is significant at the 95 % level, following the approach presented in Section 2.3. Only statistically significant values are given and discussed in the following. 2017]. In our simulations, the direct aerosol radiative effect is net positive in the Arctic in spring (0.9 W m -2 ), as for half of the AEROCOM phase II models [START_REF] Sand | Aerosols at the poles: an aerocom phase ii multi-model evaluation[END_REF], due to the high albedo of snow and ice. It is well known that even aerosols with low singlescattering albedos (i.e. containing little black carbon) can have a positive direct effect over highly reflective surfaces [START_REF] Pueschel | Physical and radiative properties of arctic atmospheric aerosols[END_REF]. In summer, we estimate a strong negative direct aerosol effect (-3.1 W m -2 , see Figure 8), lower by -2.7 W m -2 than [START_REF] Sand | Aerosols at the poles: an aerocom phase ii multi-model evaluation[END_REF]. This difference could be due, in part, to the fact that several AEROCOM phase II models do not represent SOA and ammonium aerosols, which could enhance the global direct aerosol effect by 30 % [Myhre et al., 2013b]. In addition, AEROCOM phase II models calculate the direct aerosol effect as a difference between present-day (2000) and preindustrial (1850) emissions, when our simulations use the difference between presentday ( 2012) emissions and zero emissions. This last point could be especially important for the DRE of biomass burning aerosols, since preindustrial biomass burning emissions are far from zero due to the contribution of natural wild fires in preindustrial times. In order to compare our results to [START_REF] Sand | Aerosols at the poles: an aerocom phase ii multi-model evaluation[END_REF], we also calculated the average JJA 2012 aerosol optical depth at 550 nm (AOD) in WRF-Chem, interpolated at 9 Arctic observing stations (Alert, Canada; Ny lesund, Svalbard; Barrow, Alaska; Kangerlussuaq, Greenland; Resolute Bay, Canada; Bonanza Creek, Canada; Yakutsk, Russia; Andenes, Norway; Tiksi, Russia). We found that the mean AOD in WRF-Chem at these sites is 0.17, substantially higher than in [START_REF] Sand | Aerosols at the poles: an aerocom phase ii multi-model evaluation[END_REF] where the multimodel mean is 0.078 (individual models from 0.028 to 0.15). This difference in AOD could be due to stronger aerosol transport to the Arctic in summer 2012, or to intermodal differences (e.g. a longer aerosol lifetime in the Arctic free troposphere in WRF-Chem). However, this difference ©2018 American Geophysical Union. All Rights Reserved.

Direct and indirect radiative effects of aerosols and ozone in the Arctic

in AOD is unlikely to be due to differences in emissions since summertime anthropogenic and biomass burning emissions from AEROCOM phase II [START_REF] Lamarque | Historical (1850-2000) gridded anthropogenic and biomass burning emissions of reactive gases and aerosols: methodology and application[END_REF] for BC, SO 2 , and Organic Carbon in domain d1 are 20 %, 17 %, and 19 % higher than in the present study, while only NO x emissions are lower, by 8 %.

The sum of aerosol indirect and semi-direct effects due to present-day anthropogenic and biomass burning emissions in spring (-2.1 W m -2 ) and summer (-3.7 W m -2 ) are of the same magnitude but stronger than estimates for the indirect effect of aerosols in the high Arctic (68°N to 90°N) for 2003 by [START_REF] Shindell | Local and remote contributions to arctic warming[END_REF], -0.25 W m -2 in spring;

-1.75 W m -2 during summer.

The total (SW+LW) radiative effect of ozone from midlatitude anthropogenic emissions is ∼ 0.1 W m -2 in spring and summer. The effect of ozone from biomass burning emissions during summer is also ∼ 0.1 W m -2 . These values are similar to the annual average radiative effect of ozone in the Arctic of 0.15 W m -2 estimated by [START_REF] Collins | Global and regional temperature-change potentials for near-term climate forcers[END_REF].

Note that these values do not include the effect of methane emissions, which are estimated to be responsible for nearly half of the historical radiative forcing of ozone [START_REF] Stevenson | Tropospheric ozone changes, radiative forcing and attribution to emissions in the atmospheric chemistry and climate model intercomparison project (accmip)[END_REF]. Present-day Arctic shipping aerosols have a small positive direct effect during summer (∼ 6 mW m -2 in 2012, ∼ 55 mW m -2 in 2050), whereas earlier studies found a negative direct effect [e.g., Ødemark et al., 2012]. This positive net direct effect from shipping aerosols seems to be driven by the effect of BC over sea ice, even though shipping aerosols ©2018 American Geophysical Union. All Rights Reserved.

are mostly found over the open ocean and only occasionally over snow and ice. [START_REF] Sand | Aerosols at the poles: an aerocom phase ii multi-model evaluation[END_REF] showed that, during summer, anthropogenic and biomass burning aerosols in the Arctic have a strong positive direct effect over sea ice and a very weak negative effect over open water. In 2050, the positive direct effect of shipping aerosols increases due to higher emissions, but remains low compared to the effects of remote sources. This is a consequence of the relatively small contribution of Arctic shipping emissions to the total aerosol burden in the Arctic. In addition, aerosols from ships are mostly located below and within clouds (Figure 6), which can reduce their direct effect but increase their indirect and semi-direct effects.

Present-day indirect effects from Arctic shipping emissions (spring and summer) are not found statistically significant by our method (p value = 0.15 during summer). Previous studies [START_REF] Ødemark | Short-lived climate forcers from current shipping and petroleum activities in the arctic[END_REF][START_REF] Marelle | Air quality and radiative impacts of arctic shipping emissions in the summertime in northern norway: from the local to the regional scale[END_REF] have reported large negative indirect effects from Arctic shipping emissions during summer, but we find here that internal variability in a coupled model such as WRF-Chem is large enough to mask this relatively strong signal. These results suggest that larger ensembles or longer integration periods are needed to quantify moderate indirect effects in models. In contrast, we estimate a large negative indirect and semi-direct aerosol radiative effect from Arctic shipping of -0.8 W m -2 in summer 2050, This is due to increased diversion shipping in the scenario employed here. This value is similar in magnitude to the indirect effects of aerosols from remote anthropogenic and biomass burning emissions. This large future effect is likely due to the indirect effects of increased concentrations of hydrophillic sulfate, ammonium and nitrate aerosols, which are significantly higher in 2050 due to summertime diversion shipping. Note that we do not include the effect of CO 2 emissions, which would be reduced ©2018 American Geophysical Union. All Rights Reserved.

globally by the use of shorter diversion shipping routes through the Arctic, reducing global warming in the long-term [START_REF] Fuglestvedt | Climate penalty for shifting shipping to the arctic[END_REF]. In addition, Dalsøren et al.

[2013] showed that moderate diversion shipping in 2030 combined with strong emission reduction measures could lead to a decrease in the Arctic sulfate burden, causing warming compared to the present day. This is not the case in our study due to the extreme growth in traffic in the "High Growth" diversion scenario for 2050. These results indicate that predictions of future shipping impacts on climate are very sensitive to assumptions used to estimate future emission scenarios.

Surprisingly, we find that ozone from Arctic shipping has a small negative effect at TOA (∼ -15 mW m -2 in summer 2012, ∼ -4 mW m -2 in summer 2050), even though, as a greenhouse gas, the radiative effect of ozone is usually positive at TOA. To our knowledge, this is in contrast with all previous estimates of the radiative effect of ozone from Arctic ships, e.g. Ødemark et al. [2012], [START_REF] Dalsøren | Environmental impacts of shipping in 2030 with a particular ©2018[END_REF], [START_REF] Fuglestvedt | Climate penalty for shifting shipping to the arctic[END_REF]. This negative effect is due to the LW effect of ozone and to temperature inversions in the Arctic. Figure 7 shows that ozone pollution from Arctic shipping is confined close to the surface. In the model, air temperatures in these lower layers are often warmer than surface temperatures, a situation known as a temperature inversion, which is commonly observed in the Arctic [e.g., [START_REF] Devasthale | Quantifying the clearsky temperature inversion frequency and strength over the arctic ocean during summer and winter seasons from airs profiles[END_REF]. As a result, enhanced ozone in these warmer atmospheric layers increases LW emission and heat loss to space. In the CONTROL simulation, temperature inversion frequencies in the Arctic are 70 % during spring, 58 % during summer. Using clear-sky satellite measurements, Devasthale et al.

[2010] estimated similarly high inversion frequencies of 88 to 92 % during winter, and 69 to 86 % during summer. [START_REF] Rap | Satellite constraint on the tropospheric ozone radiative effect[END_REF] also showed that increasing ozone at the surface ©2018 American Geophysical Union. All Rights Reserved.

in the Arctic and Antarctic could cause a negative LW effect [Figure 1b in [START_REF] Rap | Satellite constraint on the tropospheric ozone radiative effect[END_REF], although they estimated that this effect would be compensated by an associated positive SW effect. This compensation does not occur in our simulations, either because the SW effect is lower due to ozone enhancements being often located below clouds, or because the LW effect is larger than in [START_REF] Rap | Satellite constraint on the tropospheric ozone radiative effect[END_REF] due to stronger or more frequent temperature inversions. Overall, this negative net effect from ozone can be expected to depend strongly on the vertical ozone profile from a given source, and on the strength, pattern, and frequency of temperature inversions in the model. Since Arctic shipping O 3 increases LW emission from the warm near-surface layers, it will also increase LW backradiation to the surface. As a result, the LW effect associated with shipping O 3 could warm the surface but cool the lower troposphere, and the combination of these two competing effects may have only limited effects on Arctic surface temperature, although it could decrease atmospheric stability. [START_REF] Flanner | Climate response to negative greenhouse gas radiative forcing in polar winter[END_REF] have recently studied the effect of greenhouse gas enhancements in inversion layers on surface temperature, and found that the likely resulting effect would still be a warming of the Arctic surface.

In 2050, the negative LW effect from shipping O 3 is larger than in 2012 (Figure S1a and S1b in the electronic supplement), because of the higher O 3 shipping enhancements from larger emissions. However, in 2050 the positive SW effect is increasing more than the LW effect, especially over sea ice (Figure S1c and S1d in the electronic supplement), and the resulting future total effect (SW+LW, shown in Figure 8) is thus reduced. We attribute this large increase in the SW effect to the high diversion shipping emissions in the central Arctic ocean, which increase O 3 over the ice pack, where the SW O 3 effect is further enhanced by the high albedo that effectively doubles SW radiation interacting ©2018 American Geophysical Union. All Rights Reserved.

with O 3 . Compared to these simulations using constant meteorology and sea-ice, it is likely that there would be less O 3 over sea ice in 2050 along with reduced sea ice, since diversion routes would only be operating far away from the ice edge, although ozone has a lifetime of several days and can also be transported far from where ship emission occurs.

At the same time, there would also be less inversions in 2050 since sea ice cover is a good proxy for temperature inversions in the Arctic [START_REF] Pavelsky | Atmospheric inversion strength over polar oceans in winter regulated by sea ice[END_REF].

Conclusions

Local Arctic emissions from shipping and resource extraction are currently growing and could become an important source of pollution in the Arctic, relative to anthropogenic and biomass burning sources emitted largely outside the Arctic. This study investigates the impacts of remote emissions (midlatitude anthropogenic, biomass burning) and emerging local emissions (Arctic shipping, Arctic gas flaring) on the distributions and radiative effects of aerosols and ozone in the Arctic. In this study, we have considered the effect of changing future emissions, but not the effect of the changing Arctic climate, including changing sea ice, which could also influence future Arctic aerosols and ozone and their impacts. [START_REF] Voulgarakis | How different would tropospheric oxidation be over an ice-free arctic?[END_REF] showed that, in an ice-free Arctic, ozone would increase significantly during spring due to reduced bromine chemistry, and decrease significantly during summer due to reduced photolysis. Concerning aerosols, [START_REF] Jiao | Changing black carbon transport to the arctic from present day to the end of 21st century[END_REF] have showed that warming-induced transport changes and wet removal increases could decrease Arctic BC burdens by 14 %, further reducing the contribution of long-range transport to Arctic aerosols and ozone.

In this study, sensitivity experiments using the WRF-Chem model, run with and without particular emissions, are used to quantify source contributions to aerosol and ozone ©2018 American Geophysical Union. All Rights Reserved. Shipping also contributes to 10 to 30 % of surface aerosol concentrations in this region.

©2018 American Geophysical Union. All Rights Reserved.

Arctic gas flaring is estimated to be a major source of current surface BC in spring and summer, and of BC deposited over snow and ice in spring over northern Russia and the Kara Sea. We show that these significant contributions could continue in the future, and that the relative influence of local flaring BC in the Arctic could increase due to projected anthropogenic BC emission reductions in the mid-latitudes. In contrast to some previous studies, we do not find that gas flaring is a significant source of Arctic surface Here, we use ensemble simulations to diagnose statistically significant direct and indirect radiative effects. Our results show that present-day biomass burning and remote anthropogenic aerosols give a combined indirect plus semi-direct effects of -2.1 W m -2 at TOA ©2018 American Geophysical Union. All Rights Reserved.

in spring and -3.7 W m -2 in summer. In comparison, we find a combined direct effect of +0.9 W m -2 during spring, and -3.1 W m -2 during summer. Compared to these remote sources, we estimate small present-day radiative effects from shipping and gas flaring with 2012 emissions (at most ∼ 25 mW m -2 for the direct aerosol effect of Arctic flaring, both in spring and summer). The indirect aerosol effect due to present-day local Arctic emissions could not be assessed using our method, since the values were found to be too small to be statistically significant. However, a major finding of this study is that the increased aerosol load due to future shipping could cause a very strong negative indirect and semi-direct effect of -0.8 W m -2 , which is comparable in magnitude to the direct and indirect effects due to anthropogenic and biomass burning emissions. In our simulations, this large effect is due to the location of shipping aerosols, which are often present within clouds, particularly in the summer when Arctic diversion shipping occurs.

The radiative impacts of BC deposition could not be investigated here due to the lack of a detailed snow-albedo model within our version of WRF-Chem. However, the results presented here could be used in future work to assess the radiative effects of remote and local sources due to these processes.

Third, in contrast to previous studies, we diagnose a small significant negative radiative effect at TOA due to ozone from Arctic shipping (∼ -15 mW m -2 in summer 2012, ∼ -4 mW m -2 in summer 2050). This is due to the frequent temperature inversions in the Arctic, and to the fact that ozone produced from Arctic shipping is often located in atmospheric layers that are warmer than the surface.

Our results indicate that, even though emissions from local sources are relatively low compared to global totals, they already have significant impacts in the Arctic. Arctic ©2018 American Geophysical Union. All Rights Reserved.

shipping emissions are one of the major sources of surface OH and ozone, and Arctic flares are a major source of surface BC concentrations and BC deposition. This influence can also be expected to increase in the future, especially if diversion shipping develops rapidly. In this case, Arctic shipping could become a major source of surface aerosol and ozone pollution in the summer, and could potentially have very large negative local radiative effects at TOA due to their indirect effects. However, even for worst-case future local emission scenarios, mid-latitude anthropogenic emissions and biomass burning emissions can be expected to remain the main sources of ozone and aerosol burdens in the Arctic troposphere, as well as the main sources contributing to aerosol and ozone radiative effects in the Arctic. However, in order to improve our confidence in these results, improved quantification of aerosol-cloud feedbacks as well as processes influencing the vertical distribution and loss of pollutants are needed, together with refined methodologies to assess pollutant radiative effects under a range of possible emission scenarios. ©2018 American Geophysical Union. All Rights Reserved. ©2018 American Geophysical Union. All Rights Reserved. ©2018 American Geophysical Union. All Rights Reserved. ©2018 American Geophysical Union. All Rights Reserved.

  clouds. Wet removal of BC in WRF-Chem is performed by explicitly calculating activation in clouds of the BC-containing internally mixed aerosols, based on their size distribution and chemical composition and subsequent removal by precipitation. In addition, wet removal by impaction is also calculated. Radiative calculations for the longwave (LW) and shortwave (SW) fluxes are performed by RRTMG [Rapid Radiative Transfer Model for Global applications; Iacono et al., 2008], and take into account the effect on radiative transfer of model predicted aerosols and ozone. Initial and boundary meteorological conditions for meteorology, as well as sea surface temperatures and sea ice, are from NCEP FNL (National Center for Environmental Prediction, final analysis). WRF winds and temperatures are nudged (grid nudging) to FNL above the boundary layer. Boundary and initial conditions for aerosols and trace gases (domain d1) are from the global model MOZART-4 [Model for Ozone and Related chemical Tracers, version 4; Emmons et al., 2010]. MOZART-4 and FNL fields are updated every 6 hours.

  Anthropogenic emissions are taken from the ECLIPSEv5 inventory [Evaluating the Climate and Air Quality Impacts of Short-Lived Pollutants; Klimont et al., 2016], except sub-Arctic shipping emissions from RCP8.5 [Representative Concentration Pathway 8.5; Riahi et al., 2011] and Arctic shipping emissions from Winther et al. [2014]. Biomass burning emissions for 2012, including wildfires and agricultural burning, are from FINNv1.5 [Fire INventory from NCAR, version 1.5; Wiedinmyer et al., 2014]. In order to avoid double-counting, agricultural fire emissions from ECLIPSEv5 are not used. Emissions of terrestrial biogenic compounds, sea salt, dimethylsulfide (DMS), mineral dust, and lightning NO x are calculated online by the model, as described in Marelle et al. [2017]. Future anthropogenic emissions in 2050 follow the "Current legislation" (CLE) scenario for ECLIPSEv5, assuming full enforcement of current and planned environmental regulations. As a result, between 2012 and 2050, ECLIPSEv5 March to August emissions of NO x , SO 2 and BC within the d1 domain change by +24 %, +27 % and -27 %. In this study the focus is on the changes in anthropogenic emissions. For this reason, wildfire emissions and agricultural burning, collectively denoted as biomass burning, as well as natural emissions, are the same in the 2012 and 2050 simulations. In addition, initial and boundary conditions (as well as emissions within MOZART-4 driving these) do not change between the 2012 and 2050 scenarios Local emissions from flaring associated with oil and gas extraction, primarily in northern Russia, are included as part of the ECLIPSEv5 dataset, and local Arctic shipping emissions are from the inventory of Winther et al. [2014]. In 2050, additional "diversion shipping" emissions from Corbett et al. [2010] are used, corresponding to a diversion of 5% ©2018 American Geophysical Union. All Rights Reserved.

3. 1 .

 1 Present-day (2012) surface concentrations and BC deposition from remote and local emissions ©2018 American Geophysical Union. All Rights Reserved.

  [2013] and Xu et al. [2017], who found significant flaring influence in spring in northern Alaska (Barrow) and northern Canada (Alert), but in agreement with Qi et al. [2017b] and Winiger et al.

(

  15 km×15 km) WRF-Chem simulations for northern Norway in July 2012 presented in[START_REF] Marelle | Air quality and radiative impacts of arctic shipping emissions in the summertime in northern norway: from the local to the regional scale[END_REF]. Both studies find that Arctic ships are responsible for up to 30 % of BC and sulfate concentrations along the Norwegian coast during summer. However, this study predicts larger ozone enhancements from shipping over the Norwegian and Barents Seas (3 to 6 ppbv) compared to[START_REF] Marelle | Air quality and radiative impacts of arctic shipping emissions in the summertime in northern norway: from the local to the regional scale[END_REF] (1 to 1.5 ppbv).Ødemark et al. [2012] also found relatively low ozone enhancements of 2 to 3 ppbv in a global model study, but used 65 % lower Arctic shipping NO x emissions. Ozone production from shipping emissions could be overestimated in the present study, since this is a known artifact of models run at lower resolutions[START_REF] Huszar | Modeling the regional impact of ship emissions on no x and ozone levels over the eastern atlantic and western europe using ship plume parameterization[END_REF], but Vinken et al.[2011] estimated that this effect was at most 1 to 2 ppbv in the Arctic. Alternatively, simulations presented in[START_REF] Marelle | Air quality and radiative impacts of arctic shipping emissions in the summertime in northern norway: from the local to the regional scale[END_REF] could have underestimated shipping ozone production in the Arctic, due to 25 % lower NO x emissions and due to the use of a limited simulation ©2018 American Geophysical Union. All Rights Reserved. domain covering Norwegian coastal areas, thereby not taking into account additional ozone production downwind of emissions. 3.2. Future (2050) surface concentrations and BC deposition from remote and local emissions Results for spring 2050 are shown in Figures 4 and 5. Even though Arctic shipping emissions of BC increase by 55 % between spring 2012 and 2050, they remain too low to have a strong impact on surface BC concentrations or BC deposition. Between 2012 and 2050, the contribution from remote anthropogenic emissions to surface BC and BC deposition decreases from 60 % to 50 % approximately, because of the decrease in global anthropogenic BC emissions in the ECLIPSEv5 CLE scenario (-27 % in domain d1). The contribution from biomass burning emissions, assuming constant 2012 emissions, remains low during spring. Flares also remain an important springtime source of BC and BC deposition, especially in northwestern Russia. The relative contribution of flaring to Arctic BC increases slightly compared to 2012 (from 10 % to 15 % in the central Arctic) because of the small projected increase in flaring BC emissions and the strong decline in global BC emissions. In summer 2050, Figure 4 shows that local shipping emissions become the main source of Arctic surface ozone (up to 35 %, 10 ppbv) and surface aerosol pollution (up to 70 %, 250 ng m -3 for BC; 50 %, 1 µg m -3 for other aerosols) along diversion shipping lanes in use in July and August. Increases in sulfate, nitrate, and ammonium aerosols from Arctic shipping are again likely due to a combination of higher emissions (NO x , SO 2 ), increased oxidant levels (OH), and changes to aerosol chemistry (as discussed in Section 3.1.

  orology and sea ice in 2050, since they are kept constant between 2012 and 2050 in our simulations. These climate-related changes could have an important effect on the ozone chemistry during sumer in the Arctic, by e.g. increasing dry deposition (due to decreased atmospheric stability and larger open ocean surfaces) and decreasing photolysis [due to the decreased albedo from reduced snow and sea ice cover; Voulgarakis et al., 2009].Shipping emissions are responsible for most of the total surface BC along Arctic shipping lanes in summer 2050, but do not cause a strong increase in BC deposition fluxes at the surface (Figure5). In our simulations, BC deposition is mainly due to wet removal, which also depends on BC concentrations aloft that are insensitive to surface Arctic shipping emissions (Section 4). The contribution of shipping emissions to total BC deposition is higher along the Northwest Passage than over the Northern Sea Route, because of lower background deposition along the Northwest Passage. However, in agreement with[START_REF] Browse | Impact of future arctic shipping on high-latitude black carbon deposition[END_REF] andMénégoz et al.

  during summer, especially south of 75°N, close to boreal fire sources. These results are in agreement with those from[START_REF] Xu | Source attribution of arctic black carbon constrained by aircraft and surface measurements[END_REF] for spring. During summer,[START_REF] Ikeda | Tagged tracer simulations of black carbon in the arctic: transport, source contributions, and budget[END_REF] and[START_REF] Stohl | Black carbon in the arctic: the underestimated role of gas flaring and residential combustion emissions[END_REF] found even higher biomass burning contributions to BC.In Figure6, at higher altitudes, the sum of the individual quantified BC contributions (ANTHRO MIDLAT+FIRES+FLARES+SHIPS) adds up to less than 100 % of CON-TROL BC. In this study, we only quantify the contributions of the following sources of Arctic aerosols and ozone: anthropogenic emissions south of 60°N, biomass burning emissions, shipping emissions north of 60°N and flaring emissions north of 60°N. All of the other sources (e.g., the stratospheric source, sources outside of the domain boundary, Arctic emissions from the residential or road transport sectors) are included in CONTROL and in all of the sensitivity simulations, but their individual contributions to Arctic BC are not quantified, since no simulations are performed with these sources turned off. It is clear in Figure6that some of these unquantified contributions have an important influence on high-altitude BC in the Arctic in spring. Since BC transport to the Arctic is quasi-isentropic, these unquantified contributions to high-altitude Arctic BC are likely to originate from the lower latitudes, e.g. from the tropics or even the southern hemisphere(Figure 4.3 of AMAP [2015]), south of the d1 domain boundary conditions specified by the MOZART4 model. Indeed, AMAP

  fects on cloud properties, including cloud albedo and cloud lifetime, for both grid-scale clouds [Morrison microphysics scheme; Morrison et al., 2009] and sub-grid clouds [KF-CuP scheme; Berg et al., 2015]. As a result, changes in aerosol concentrations, composition and size between two simulations, e.g., CONTROL and NOSHIPS, may also change cloud properties [aerosol first and second indirect effects;

Figure 8

 8 Figure8shows the calculated radiative effects at TOA in the Arctic (latitude > 60°N)

  Between 2012 and 2050, global anthropogenic emissions of BC decrease in the ECLIP-SEv5 CLE scenario, while SO 2 and NO x emissions increase. As a result, the direct effect of aerosols from remote anthropogenic sources is more negative in 2050 compared to 2012, changing by ∼ -0.6 W m -2 in both spring and summer. Such a change would reverse estimated recent trends in the Arctic direct aerosol effect, since Breider et al. [2017] calculated changes of ∼ +0.48 W m -2 between 1980 and 2010 due to declining Arctic aerosol loads and reductions (increases) in SO 2 (BC) emissions. It is important to note that, unlike in ©2018 American Geophysical Union. All Rights Reserved. the ECLIPSEv5 CLE scenario, some scenarios project strong reductions in future sulfur dioxide emissions, reducing the sulfate load globally and in the Arctic [e.g. Representative Concentration Pathways scenarios, Lamarque et al., 2011]. Such reductions would cause a positive change in direct radiative effect relative to present-day [Gillett and Von Salzen, 2013].Biomass burning emissions are kept constant in our experiments, and as a result the associated direct radiative effects also remain approximately constant, with some small differences in 2050 likely due to model non linearities. However, since these non linearities are higher for the cloud response, indirect effects from biomass burning are noticeably different in summer 2050 and in summer 2012 despite constant emissions.5.2.2. Current and future radiative effects from local Arctic flaring and shippingArctic flaring emissions have small but significant positive direct radiative effects (20 to 30 mW m -2 ) in spring and summer 2012 and 2050 (Figure8), which are comparable to earlier annual estimates fromAMAP [2015]. This is due to their large influence on Arctic BC. However, we find no significant indirect and semi-direct aerosol effects from Arctic flares, and very weak radiative effects for ozone. Arctic flaring emissions are relatively stable between 2012 and 2050 in the CLE scenario, and as a consequence radiative effects from Arctic flares do not change much between present and future in our simulations.

  concentrations (surface, troposphere), BC deposition fluxes, and radiative effects, in spring and summer for the present-day (2012) and the future (2050). Future projections use a scenario taking into account current planned emission reduction legislation (ECLIPSEv5 CLE), projecting lower 2050 BC and higher SO 2 and NO x emissions relative to 2012 in the Northern Hemisphere, with slowly growing Arctic flaring emissions. We also use high-growth projections of future shipping emissions in the Arctic, including future diversion shipping through the Arctic Ocean. Our results assume constant biomass burning emissions compared to 2012. The key novel points in this study are the following:First, this study quantifies the relative impacts of emerging local sources of Arctic pollution (Arctic shipping and Arctic gas flaring emissions), compared to remote sources of pollution. The importance of this issue has been highlighted in recent papers,[START_REF] Arnold | Arctic air pollution: Challenges and opportunities for the next decade[END_REF][START_REF] Law | Local arctic air pollution: Sources and impacts[END_REF], but this comparison has not to our knowledge been investigated in previous studies. In line with previous work on Arctic pollution, we find that long-range transport of pollution from mid-latitude anthropogenic emissions and biomass burning emissions is the main source contributing to pollution burdens in the Arctic (for 2012 and 2050 emissions), with anthropogenic pollution dominating in spring and biomass burning during summer. However, we find that local sources already have an important influence on surface and lower tropospheric concentrations of ozone and aerosols (BC, NO - 3 , NH + 4 , SO 2 - 4 ). With 2012 emissions, Arctic shipping makes an important contribution to surface ozone (15 to 25 % of total surface ozone) in the Norwegian and Barents Sea region during summer. This is caused by a strong increase in summertime surface OH concentrations (∼ by a factor of 2 in the Norwegian Sea) due to shipping NO x emissions.

  BC outside of northern Europe, northern Russia and the Kara Sea. Due to the projected growth in Arctic shipping emissions associated with diversion shipping in 2050, Arctic shipping could become a major source of surface ozone (up to +10 ppbv) and aerosols (locally, the main source of surface BC) during the Arctic summer. Diversion shipping does not contribute significantly to BC deposition over snow and ice due to efficient wet BC removal over the open Arctic Ocean at this time of year. The relative contribution of remote anthropogenic emissions could be further reduced by emission control in the midlatitudes: the ECLIPSEv5 CLE scenario used for future projections contains lower BC emissions in 2050 than in 2012. However, such predictions are very sensitive to the uncertain evolution of future global emissions. Second, this study gives an estimation of the indirect aerosol effects from remote and local Arctic sources using a model resolving aerosol activation in clouds explicitly. Quantifying radiative effects due local Arctic emissions is complicated by model internal variability due to small perturbations in concentration distributions and aerosol-cloud feedbacks.

Figure 1 .

 1 Figure 1. Simulation domains (d1, d2) and areas of > 50 % average snow and sea ice cover in

Figure 2 .

 2 Figure 2.

Figure 3 .

 3 Figure 3. Left column: surface BC deposition in (top) spring 2012 and (bottom) summer 2012

Figure 4 .

 4 Figure 4.

Figure 5 .

 5 Figure 5. Same as Figure 3, for spring 2050 (MAM, top) and summer 2050 (JJA, bottom).

Figure 6 .

 6 Figure 6. Spring (top 5 panels) and summer (bottom 5 panels) zonal mean BC concentrations

Figure 7 .

 7 Figure 7.

Figure 8 .

 8 Figure 8. Average Arctic instantaneous radiative effects (RE) at TOA due to each source, in

  spring, while biomass burning emissions have a very limited role, in agreement with Wespes et al.[2012]. Anthropogenic emissions make a small relative contribution in the

upper troposphere during spring, due to the large stratospheric source [not quantified here, see e.g.

Wespes et al., 2012]

. However, absolute enhancements due to remote anthropogenic emissions are significant, 5 to 13 ppbv at altitudes 5 to 8 km. During summer, biomass burning emissions contribute more than remote anthropogenic emissions to lower tropospheric ozone. This contrasts to results from Wespes et al. [2012], who found that Northern American and European anthropogenic emissions were the dominant sources of summertime Arctic ozone at low altitudes. As already noted, Wespes et al. [2012] focused on a limited time period during summer 2008, using a different model, and significant disagreement remains between models regarding boreal fire impacts on Arctic ozone Arnold et al. [2015].

Table 1 .

 1 Emission totals for local Arctic sources (latitude > 60°N) in spring (MAM) and summer (JJA) 2012 and 2050. Relative emission increases from 2012 to 2050 are also given. Emission source BC emissions (kton) NO x emissions (kton) SO 2 emissions (kton)

	2012 emissions, spring
	Arctic ships	0.210	28.4	10.5
	Arctic flares	12.7	8.75	6.49
	2012 emissions, summer
	Arctic ships	0.297	39.0	16.9
	Arctic flares	9.58	6.58	4.87
	2050 emissions, spring
	Arctic ships	0.325 (+55 %)	31.1 (+9.5 %)	7.22 (-31 %)
	Arctic flares	13.4 (+5.5 %)	9.20 (+5.1 %)	6.55 (+0.92 %)
	2050 emissions, summer
	Arctic ships	4.35 (+1400 %)	445 (+1500 %)	135 (+1200 %)
	Arctic flares	10.1 (+5.4 %)	6.91 (+5.0 %)	4.92 (+1.0 %)

©2018 American Geophysical Union. All Rights Reserved.

Table 2 .

 2 List of simulations. The effect of changing biomass burning emissions is not investigated: all 2050 simulations use biomass burning emissions for 2012, and 2012 NOFIRES include all 2050 emissions but no biomass burning emissions.

	Simulation name	Description	Ensemble	Years
	CONTROL	All emissions	Yes (N = 4) 2012 and 2050
	NOSHIPS	No shipping emissions north of 60°N	Yes (N = 4) 2012 and 2050
	NOFLARES	No flaring emissions north of 60°N	Yes (N = 4) 2012 and 2050
	NOANTHRO MIDLAT No anthropogenic emissions south of 60°N	No	2012 and 2050
	NOFIRES	No biomass burning emissions	No	2012 and 2050

©2018 American Geophysical Union. All Rights Reserved.

Acknowledgments. We acknowledge funding from the European Union (Framework 7 programme) ICE-ARC (Ice, Climate, Economics -Arctic Research on Change) project, under Grant Agreement 603887 and the French Chantier Arctique Pollution in the Arctic System (PARCS) project. Computer resources were provided by IDRIS HPC under GENCI, and by IPSL CICLAD/CLIMSERV. Louis Marelle acknowledges funding from TOTAL SA through an ANRT CIFRE PhD grant. We thank colleagues at PNNL (Jerome D. Fast, Manish Shrivastava, Larry K. Berg, Richard C. Easter) and CICERO (Maria Sand) for helpful discussions. The WRF-Chem model is available at

http://www2.mmm.ucar.edu/wrf/users. Meteorological and chemical data from NCEP and MOZART4, used for initial and boundary conditions, can be found at https://rda. ©2018 American Geophysical Union. All Rights Reserved. ucar.edu/ and https://www.acom.ucar.edu/wrf-chem/mozart.shtml. DMS oceanic data is available at https://www.bodc.ac.uk/solas_integration/implementation_ products/group1/dms/documents/dmsclimatology.zip. RCP8.5 shipping emissions, ECLIPSEv5 anthropogenic emissions, Arctic shipping emissions for local and diversion shipping, and FINNv1.5 fire emissions are available at https://tntcat.

iiasa.ac.at/RcpDb/dsd?Action=htmlpage&page=download, http://www.iiasa.ac. at/web/home/research/researchPrograms/air/ECLIPSEv5.html, http://envs.au. dk/en/knowledge/air/emissions/projects/arctic-ship-emissions/, http://coast. cms.udel.edu/ArcticShipping/, http://bai.acom.ucar.edu/Data/fire/.


Dan Lu

Sha Zhu

‡ Lukasz

F Sobala

Ganeko Bernardo-Seisdedos

Oscar Millet

Yongmin Zhang

Jesus Jiménez- Barbero

Gideon J Davies

Matthieu Sollogoub

From 1,4-Disaccharide to 1,3-Glycosyl-Carbasugar: Synthesis of a Bespoke Inhibitor of Family GH99 Endo--Mannosidase

Understanding of enzyme reaction mechanism can lead to the design of enzyme inhibitors. A Claisen rearrangement was used to allow the conversion of an -1,4 disaccharide into an -1,3-linked glycosylcarbasugar to target the endo--mannosidase from glycosidase family GH99 which, unusually, is believed to act through a 1,2 anhydrosugar "epoxide" intermediate. Using NMR and X-ray crystallography, it is shown that glucosylcarbasugar--aziridines can act as reasonably potent endo--mannosidase inhibitors, likely by virtue of their shape-mimicry and the interactions of the aziridine nitrogen with the conserved catalytic acid/base of the enzyme active-site. O O O O O O Claisen rearrangement a-1,4-disaccharide a-1,3-glycosyl-carbasugar a-Glc Bn a-Glc Bn a-aziridine in BxGH99

Understanding enzyme reaction mechanisms is essential for the rational design of inhibitors. Glycoside hydrolases (GH), also called glycosidases, are a vast group of enzymes that are classified, based upon their amino-acid sequence similarity, into over 150 distinct families in the Carbohydrate Active Enzymes database (CAZy). 1,2 The vast majority of these enzymes can be divided into two classes according to their reaction mechanism and the relative configuration of substrate and product: those that act with retention or inversion of anomeric configuration. The retaining glycosidases utilize a double nucleophilic displacement which results in net retention of configuration at the anomeric center. The retaining reaction classically involves nucleophilic amino-acid side-chain and proceeds via a covalent glycosyl-enzyme intermediate. Some enzymes also use neighboring group participation reactions (notably those active on beta linked GlcNAc). 3 Recently, a new neighbouring-group retaining mechanism involving an intermediate epoxide and hence no covalent bond to the enzyme was proposed for endo--mannosidases and endomannanases (-Glc-1,3--Man-OR and -Man-1,3--Man-OR substrates) of the GH99 family. 4,5 In this putative mechanism, nucleophilic attack by the manno-O2 atom, allows catalysis via a 1,2 anhydro sugar "epoxide" intermediate. (Scheme 1). In terms of shape and charge changes, a positive charge is very likely to be developed and the conformation of the pyranose changes from 4 C 1 of the substrate to the 4 E/ 4 H 5 of the epoxide. Based on these considerations, inhibitors that have been designed for this important class of enzymes 6 are -1,3-disaccharide analogues and these fall into two categories: those which mimic the charge and those which mimic its shape. In the first class, we find the usual deoxymannojirimycin (1), isofagomine (2) or noeuromycin derivatives. 4,7 A clear influence of the amine position has been demonstrated. It has also been shown that the glucal derivative 3 is an inhibitor of the enzyme due to its shape, which corresponds to a 4 H 5 conformation. 7 Here, we decided to combine both properties in a new inhibitor and create an amine with a likely 4 E/ 4 H 5 conformation. To achieve both the half-chair conformation and introduce an amino group, we decided to synthesize an aziridine on a carbasugar for obvious stability reason. Furthermore, according to the mechanism, the catalytic acid E336 of Bacteroides xylanisolvens GH99 (BxGH99) should be in its acidic form and therefore prone to release a proton to the aziridine, hence we decided to investigate the synthesis and inhibitory ability of glucosyl-carbasugar--aziridine

(Scheme 1)

There are many ways to synthesize carbasugars, 8 including those containing a disaccharide motif. 9 The most direct way would be to start directly from the corresponding disaccharide, 10 but the necessary 1,3-disaccharides (like Glc--1,3-Glc) are rare and expensive. The synthesis of the two monosaccharides and their subsequent assembly seemed tedious. We therefore decided to use a trick based on the use of Claisen rearrangement to make carbasugars nicely developed by Nagarajan on monosaccharides. 11 In this reaction, a glucal functionalized with an alkene in position 6 affords a carba-glucal with an aldehyde in position 6. This is due to the symmetry of the molecule, and in the operation, the protecting group in position 4 becomes the one in position 3 in the new numbering of the carbasugar. (Scheme 2)

This approach was used to synthesize a 1,3-pseudodisaccharide from a 1,4-disaccharide. The synthesis would start from maltose, that would be converted into the doubly unsaturated maltal 5, which in turn could undergo Claisen rearrangement to give keto-alkene 6. This operation is key to the total synthesis as we not only obtain the desired 1,3pseudo-disaccharide but also the alkene which will be converted into the desired epoxide Maltal was prepared according to Haworth,12 with a slight modification in the deprotection step (see SI). Next, the differentiation of the two primary hydroxyl groups was required. For this, we decided to protect the glucoside with a benzylidene group. This reaction had to be performed with great care (Scheme 4) by controlling the amount of camphor sulfonic acid used (0.1 equiv). Then, the other primary alcohol could be selectively silylated, subsequent benzylation and de-silylation afforded the alcohol 8. Oxidation using Dess-Martin periodinane (DMP) followed by olefination produced dialkene 9. Claisen rearrangement on 9 was performed under micro-wave irradiation at 240 °C and was immediately followed by the aldehyde reduction to afford alcohol 10 in 76% yield. (Scheme 4) Scheme 4. Key Claisen rearrangement and preparation of the substrate 10.

Scheme 5. Synthesis of aziridine carbasugars 4.

With the alkene 10 in hand, we investigated its aziridination. We opted for a three-step route: epoxidation, azide-ring opening, and Staudinger azidereduction-aziridination; 13 this strategy also operates with overall inversion of configuration of the three-membered ring. We first benzylated the remaining hydroxyl in 10 to give 11, which was epoxidized using methyl(trifluoromethyl)dioxirane generated in situ 14 to give the α-epoxide 12 in 46% yield and the β-epoxide 13 in 26% yield, which were separated by silica gel flash chromatography. The stereochemistry of these epoxides was determined thanks to a NOESY experiment. α-Epoxide 12 displayed cross correlations between H-5 and H-3, while in β-epoxide 13 H-5 showed cross-correlations with H-3 but also with H-1 and H-2 (Scheme 5 and SI). This reaction therefore produced the undesired α-epoxide as the major product, but all our attempts to improve the situation with various epoxidation reactions gave lower yields. For instance, mCPBA which was previously often used to synthesize epoxides from cyclohexenes polyols 15 did not improve the selectivity nor the yield. Further treatment of the isolated epoxide 13 with sodium azide furnished a mixture of 1,2-trans azido alcohols 14/15, which under the action of PPh 3 afforded α-aziridine 16 in 52% yield after these two steps. Final deprotection, using Birch conditions, 16 afforded the unprotected aziridinated pseudo-sugar 4 (Scheme 5).

The properties of aziridine 4 were investigated with GH99 family members Bt (Bacteorides thetaiotaomicron) and BxGH99. A crystal structure of the BxGH99 enzyme in complex with the aziridine 4 at a resolution of 1.27 Å (PDB accession: 6HMG) was obtained by soaking of BxGH99 crystals with an aqueous solution of the compound. The nitrogen atom of the aziridine makes a 2.6 Å hydrogen bond with one of the side chain oxygen atoms of BxGH99 residue E336, which acts as an acid/base in the proposed GH99 mechanism (Scheme 1). In a ternary complex with the aziridine and α-1,2-mannobiose (obtained at a resolution of 1.03 Å, PDB 6HMH), this distance changes to 2.7 Å. The interaction is shown in Figure 1A. In both cases, the pseudopyranose ring is in a 4 H 5 conformation, as predicted for the azirdine and likely resembles the intermediate/transition-state of the reaction. Indeed, it binds similarly to ManGlucal and GlcChex 17 (Figure 1B), which was also observed in a 4 H 5 conformation in complex with BxGH99 (PDB 5M5D and 5MC8). 7 Using NMR we have determined the dissociation constant (K D ) of the aziridine with 15 N-labelled BtGH99 to be 11.5 ± 0.9 μM (Figure 1C). The compound, therefore, binds to BtGH99 with an affinity 2-fold higher than GlcDMJ (24 μM) and slightly higher than ManGlucal (15 μM). However, it is not a potent inhibitor in comparison with GlcIFG, ManIFG or ManNOE, all of which bind to BtGH99 with a K D in the nanomolar range. In conclusion, we have devised a strategy to synthesize 1,3-glycosyl-carbasugars designed to interact with the family GH99 endo--mannosidases. The obtained inhibitor has improved activity in comparison with the shape mimics, but does not reach the affinity of isofagomine or noeuromycin analogues. This probably indicates that the aziridine is not charged owing to its lower pKa. In future work, we will be able to use this synthetic strategy to synthesize other analogues to study the mechanism of this family of glycosidases which is not yet firmly established.

  Scheme 2. Claisen rearrangement leading to interchange of position 3 and 4 from the glucal to the carba-glucal.

7

 7

Figure 1 .

 1 Figure 1. (A) Divergent stereo ("wall-eyed") view of 4 in complex with BxGH99. Catalytic residues surrounding the -1 subsite and the interaction between the aziridine N and E336 Oε are shown. The 2mF o -DF c synthesis contoured at 1 e -/Å 3 . (B) Comparison of the ligands observed in the 4 H 5 conformation in complex with BxGH99. 7 Green: the aziridine 4, gold: ManGlucal (from PDB 5M5D), grey: GlcChex (from PDB 5MEL). The sugar moiety in the -2 subsite is not shown. (C) The result of NMR titration of BtGH99 with 4. The protein concentration in the solution was 58 μM.

We thank Chinese Scholarship Council (CSC).

ASSOCIATED CONTENT Supporting Information

Experimental details, spectra and X-ray crystallography. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author

* E-mail: matthieu.sollogoub@sorbonne-universite.fr

Author Contributions

All authors have given approval to the final version of the manuscript. / ‡These authors contributed equally.