Tao Wang

Carmen Kalalian

Daniel Fillon

Sébastien Perrier

Jianmin Chen

Florent Domine

Liwu Zhang

Christian George

Sunlight Induces the Production of Atmospheric Volatile Organic Compounds (VOCs) from Thermokarst Ponds

Keywords: permafrost, thermokarst, volatile organic compounds (VOCs), Arctic, photochemistry

SYNOPSIS:

Photoproduction of VOCs from thermokarst ponds water is not directly correlated to the amount of dissolved organic matter but rather to the availability of organic matters at the top layers of the liquid phase.

ABSTRACT: Ground subsidence caused by permafrost thaw causes the formation of thermokarst ponds, where organic compounds from eroding permafrost accumulate.

We photolyzed water samples from two such ponds in Northern Quebec and discovered the emission of volatile organic compounds (VOCs) using mass spectrometry. One pond near peat-covered permafrost mounds was organic-rich, while the other near sandy mounds was organic-poor. Compounds up to C10 were detected, comprising O, N, and S atoms. The main compounds were methanol, acetaldehyde, and acetone.

Hourly VOC fluxes under actinic fluxes similar to local solar fluxes might reach up to 1.7 nmol C m -2 s -1 . Unexpectedly, the fluxes of VOCs from the organic-poor pond were greater than those from the organic-rich pond. We suggest that different segregation of organics at the air/water interface may partly explain this observation. This study indicates that sunlit thermokarst ponds are a significant source of atmospheric VOCs, which may affect the environment and climate, via ozone and aerosol formation.

Further work is required for understanding the relationship between pond organic composition and VOC emission fluxes.

INTRODUCTION

The top 3 m of terrestrial permafrost contains ~ 1000 Pg of carbon in the form of frozen vegetal debris that has accumulated over the past millennia [START_REF] Mishra | Spatial heterogeneity and environmental predictors of permafrost region soil organic carbon stocks[END_REF][START_REF] Palmtag | A high spatial resolution soil carbon and nitrogen dataset for the northern permafrost region based on circumpolar land cover upscaling[END_REF] . This is significantly more than the atmospheric reservoir, 750 Pg 3 , so that the mineralization of permafrost carbon induced by global warming has the potential to dramatically accelerate climate change. Projections of CO2 and CH4 emissions by thawing permafrost show a very wide range of values, most of which are between 7 and 100 Pg C by the year 2100 under the emission scenario RCP4.5 [START_REF] Gasser | Pathdependent reductions in CO 2 emission budgets caused by permafrost carbon release[END_REF][START_REF] Macdougall | Sensitivity of carbon budgets to permafrost carbon feedbacks and non-CO 2 forcings[END_REF][START_REF] Schaefer | The impact of the permafrost carbon feedback on global climate[END_REF][START_REF] Schuur | Climate change and the permafrost carbon feedback[END_REF] . However, these estimates are based on gradual permafrost thaw and ignore numerous processes, including rapid erosion and abrupt permafrost thaw [START_REF] Turetsky | Carbon release through abrupt permafrost thaw[END_REF] , increased lightning and fires, and the rapid northward migration of trees [START_REF] Chen | Future increases in Arctic lightning and fire risk for permafrost carbon[END_REF] . Much greater emissions are therefore likely [START_REF] Turetsky | Carbon release through abrupt permafrost thaw[END_REF][START_REF] Natali | Permafrost carbon feedbacks threaten global climate goals[END_REF] and additional process studies are required for more reliable projections.

Many model parameterizations of the permafrost contribution to the atmospheric CO2 budget rely on field or laboratory warming experiments that observe static processes [START_REF] Natali | Permafrost degradation stimulates carbon loss from experimentally warmed tundra[END_REF][START_REF] Schaedel | Divergent patterns of experimental and model-derived permafrost ecosystem carbon dynamics in response to Arctic warming[END_REF] . Recent experiments however stress that lateral hydrological export could explain unexpectedly high losses of permafrost carbon [START_REF] Plaza | Direct observation of permafrost degradation and rapid soil carbon loss in tundra[END_REF] . Indeed, ice melting in permafrost leads to ground subsidence and geomorphological changes with the formation of erosion gullies and thermokarst ponds [START_REF] Olefeldt | Circumpolar distribution and carbon storage of thermokarst landscapes[END_REF] . Erosion processes lead to the accumulation of dissolved and particulate organic carbon (DOC and POC) in thermokarst ponds [START_REF] Miner | Permafrost carbon emissions in a changing Arctic[END_REF][START_REF] Wauthy | Increasing dominance of terrigenous organic matter in circumpolar freshwaters due to permafrost thaw[END_REF] , where it can be mineralized by microbial respiration and photochemistry [START_REF] Bowen | Arctic Amplification of Global Warming Strengthened by Sunlight Oxidation of Permafrost Carbon to CO 2[END_REF][START_REF] Laurion | Dissolved organic matter photolysis in Canadian arctic thaw ponds[END_REF][START_REF] Matveev | Methane and carbon dioxide emissions from thermokarst lakes on mineral soils[END_REF] . These latter processes are seldom included in models of permafrost carbon and in particular photolytic processes are ignored.

Photolysis of DOC and POC leads to the formation of CO2 and its release to the atmosphere. However, products of incomplete oxidation are also formed and have been observed in the liquid phase [START_REF] Ward | Complete and Partial Photo-oxidation of Dissolved Organic Matter Draining Permafrost Soils[END_REF][START_REF] Ward | Photochemical alteration of organic carbon draining permafrost soils shifts microbial metabolic pathways and stimulates respiration[END_REF][START_REF] Ward | Assessing the prevalence, products, and pathways of dissolved organic matter partial photo-oxidation in arctic surface waters[END_REF] . The more volatile of these products can be released into the atmosphere and affect climate through several processes. The first one is their subsequent oxidation to CO2. Since gaseous emission measurements from lakes focus on CO2 and CH4 [START_REF] Matveev | Methane and carbon dioxide emissions from thermokarst lakes on mineral soils[END_REF][23][START_REF] Holgerson | Large contribution to inland water CO 2 and CH 4 emissions from very small ponds[END_REF][START_REF] Virkkala | The ABCflux database: Arcticboreal CO2 flux observations and ancillary information aggregated to monthly time steps across terrestrial ecosystems[END_REF] , neglecting this delayed CO2 source may underestimate the greenhouse gas (GHG) emissions by lakes. Both field and laboratory studies indicate that incomplete photo-oxidation may represent up to half the photolysis products in lakes [START_REF] Ward | Assessing the prevalence, products, and pathways of dissolved organic matter partial photo-oxidation in arctic surface waters[END_REF][START_REF] Mazoyer | The dominant role of sunlight in degrading winter dissolved organic matter from a thermokarst lake in a subarctic peatland[END_REF] . The second one is through the formation of ozone (O3), a potent GHG [START_REF] Liu | Stratospheric ozone depletion and tropospheric ozone increases drive Southern Ocean interior warming[END_REF] , as many of the reactive organic compounds emitted are likely to have a strong O3 formation potential [START_REF] Li | Persistent growth of anthropogenic non-methane volatile organic compound (NMVOC) emissions in China during 1990 -2017: drivers, speciation and ozone formation potential[END_REF] . Lastly, the further atmospheric oxidation of emitted compounds may lead to the formation of less volatile compounds that contribute to aerosol formation [START_REF] Hallquist | The formation, properties and impact of secondary organic aerosol: current and emerging issues[END_REF] . These aerosols affect climate through light scattering and cloud droplet formation [START_REF] Farmer | Atmospheric Processes and Their Controlling Influence on Cloud Condensation Nuclei Activity[END_REF][START_REF] Moise | Optical Properties of Secondary Organic Aerosols and Their Changes by Chemical Processes[END_REF] .

Here we performed laboratory experiments to explore the release of volatile organic compounds (VOCs) from the photolysis of waters from thermokarst ponds in palsa and lithalsa environments. Palsas are peat-covered permafrost mounds whose collapse feeds large amounts of organic matter into thermokarst ponds. Lithalsas are also permafrost mounds, but without peat, and the amount and nature of organic matter released to thermokarst ponds are therefore expected to be different. Investigating the contrasted environments informs us about the variability of VOC emissions from thermokarst ponds. Below we will demonstrate that both sampled ponds are photochemically active.

We will focus our attention on the different behaviors, and we discuss some aspects of the mechanisms involved and their underlying atmospheric implications at the sampling period and sites. The aim of this study is to encourage follow-up investigations to cover a wider range of conditions and spatiotemporal coverage.

MATERIALS AND METHODS

Sample Collection.

The water samples used in this study were sampled in September 2019 from two contrasted thermokarst ponds located within the Canadian subarctic region at Kuujjuarapik (55°13'11'' N, 77°42'52" W) and Umiujaq (56°33'31" N, 76°28'57" W), as shown in Figure 1.

The Kuujjuarapik pond was located at the base of a collapsing palsa, and its water was visibly colored. The Umiujaq pond was surrounded by collapsing lithalsas with sandy soil, with water less colored than that from the Kuujjuarapik pond. The vegetation near Umiujaq was mostly lichen (Cladonia sp., mostly C. stellaris) and dwarf birch (Betula glandulosa) as detailed by Gagnon et al. [START_REF] Gagnon | The carbon sink due to shrub growth on Arctic tundra: a case study in a carbon-poor soil in eastern Canada[END_REF] . The soil organic content is low in this area, around 4 kg m -232 . In the sporadic permafrost zone, i.e., where we collected the water samples, numerous palsas and lithalsas have formed [START_REF] Coulombe | Coupling of sedimentological and limnological dynamics in subarctic thermokarst ponds in Northern Québec (Canada) on an interannual basis[END_REF] , as carbon reservoirs [START_REF] Fewster | Imminent loss of climate space for permafrost peatlands in Europe and Western Siberia[END_REF] .

To collect the liquid samples, amber glass containers, initially rinsed with concentrated HNO3 and then several times with ultrapure water, were then prefilled with ultrapure water (18.2 MΩ.cm) and emptied at each site, and then filled with pond water. The triplicate samples were taken from the top 10 cm of the ponds. Blanks were also collected by opening and closing a bottle filled with ultrapure water at each site.

All samples were frozen after collection and kept frozen until the actual photochemical experiments. The water samples were used for experiments as received i.e., unfiltered.

All results given below are blank-corrected, where the signals arising from blanks were subtracted from those of actual experiments. The VOC production from blanks was always significantly lower than those from collected samples (see Figure S1).

Experimental Setup. The approach used here was similar to that in our previous studies [START_REF] Wang | Superoxide and Nitrous Acid Production from Nitrate Photolysis Is Enhanced by Dissolved Aliphatic Organic Matter[END_REF][START_REF] Wang | Snowmelt Leads to Seasonal Nitrous Acid Formation Across Northwestern China[END_REF] . Briefly, a water-and Pyrex-filtered Xenon arc lamp delivered light irradiation in the actinic region (i.e., at wavelengths longer than ~ 280 nm) to a cylindrical quartz cell (2 cm diameter and 10 cm length) half-filled with 6 mL of the unfrozen pond water sample (Figure S2). The headspace of this reactor was then continuously purged with purified air at a flow rate of 200 mL min -1 , and its content of VOCs can be monitored by mass spectrometry (see below). After an initial stabilization period, the lamp was switched on, simulating solar irradiation for 180 min. The temperature fluctuations of the reactor were small, i.e., within ± 2°C (Figure S3). The experimental temperature (~ 25℃) is comparable to the maximum air temperature during the warm seasons in northern Quebec [START_REF] Lackner | snow and soil data from a low-Arctic valley in the forest-tundra ecotone in Northern Quebec[END_REF] . All experiments were performed in triplicates.

Figure S4 gives the spectral profiles of experimental and natural irradiation. Here, irradiance is calculated for the wavelength range of 280 -420 nm because only radiation therein initiates photochemistry, since shorter wavelength may cause photolysis, while the energy of longer wavelength is not sufficient to induce photochemical reactions.

Laboratory works commonly used this wavelength range to infer the photochemical production and fluxes of VOCs [START_REF] Bernard | Photosensitized Formation of Secondary Organic Aerosols above the Air/Water Interface[END_REF][START_REF] Rossignol | Atmospheric photochemistry at a fatty acid-coated air-water interface[END_REF][START_REF] Abis | Measurement report: Biogenic volatile organic compound emission profiles of rapeseed leaf litter and its secondary organic aerosol formation potential[END_REF] . The experimental irradiance was 8.0 mW cm -2 as previously set [START_REF] Ciuraru | Unravelling New Processes at Interfaces: Photochemical Isoprene Production at the Sea Surface[END_REF][START_REF] Ciuraru | Photosensitized production of functionalized and unsaturated organic compounds at the air-sea interface[END_REF] , with an actinic flux of 1.20 × 10 16 photon cm -2 s -1 . The surface of irradiated water sample in this laboratory study was several orders of magnitude lower than in nature, which reduces the flux of products. In order to meet our detection limits, the experimental irradiance was set to be higher than the natural levels at the sampling sites, but it still compares to the midday conditions elsewhere and should thus be regarded as realistic on Earth [START_REF] Brüggemann | Interfacial photochemistry at the ocean surface is a global source of organic vapors and aerosols[END_REF][START_REF] Seinfeld | Atmospheric Chemistry and Physics, From Air Pollution to Climate Change[END_REF] .

Measurement of VOCs.

The VOCs produced upon irradiation were monitored using a high-resolution proton transfer reaction time-of-flight mass spectrometer (HR-PTR-ToF-MS, 8000, Ionicon Analytic GmbH, Austria). This technique was previously described by Müller et al. [START_REF] Müller | A compact PTR-ToF-MS instrument for airborne measurements of volatile organic compounds at high spatiotemporal resolution[END_REF] and uses H3O + as a soft ionization agent. This MS continuously sampled 200 mL min -1 gas flow at the reactor's outlet through a heated silicon steel tube. Spectra were acquired with a time resolution of 10 s at a mass resolution of 4500 m/Δm.

A calibration standard (TO-14A Aromatic Mix, Restek Corporation, USA) containing 14 VOCs (100 ppb for each diluted in N2) was utilized to calibrate and test the instrument performance. The sensitivity of standard compounds ranged between 15 and 70 cps/ppb, depending on the actual mass. As it was not possible to calibrate the instruments for each compound detected, the protonation rate constant was assumed to be 2 × 10 -9 cm 3 s -1 , and therefore the mean sensitivity was set to be 30 cps/ppb. The mass calibration was performed by internal standards: hydronium ion isotope (H3 [START_REF] Laurion | Dissolved organic matter photolysis in Canadian arctic thaw ponds[END_REF] O + , m/z = 21.022) and protonated acetone (C3H7O + , m/z = 59.049) as commonly selected.

The PTR-MS viewer software (version 3.2.8) was used for the raw spectral analysis, mass calibration, peak fitting, and area extraction. The mixing ratios of VOCs were finally determined based on the existing protocols [START_REF] Cappellin | On data analysis in PTR-TOF-MS: From raw spectra to data mining[END_REF] . More details are provided in Supporting Information (Text S1).

Aqueous Phase Analysis. For the liquid analysis, the solution before and after the VOC photochemical measurements were filtered, in contrast to the actual experiments, by a 0.22 μm polytetrafluoroethylene (PTFE) membrane filter (Pall Corporation, USA).

Each treated sample was analyzed using a Q-Exactive Hybrid Quadrupole-Orbitrap mass spectrometer (Thermo Scientific, USA) to characterize the liquid molecules included. In addition, each sample's pH, optical properties, and concentration of nonpurgeable organic carbon (NPOC) were measured. More details are provided in Supporting Information (Text S2).

Data Processing. The detected molecules in both phases can be characterized by a variety of indices, including oxygen-to-carbon ratio (O/C), hydrogen-to-carbon ratio (H/C), ring and double-bound equivalence (RDBE), aromaticity equivalent (Xc), carbon oxidation state (OSc), and modified aromaticity index (AImod). Kendrick mass defect (KMD) analysis was performed to identify an array of compounds containing similar repeated functional groups [START_REF] Xu | Using ESI FT-ICR MS to Characterize Dissolved Organic Matter in Salt Lakes with Different Salinity[END_REF] . Compounds whose elemental compositions only differ by the number of subunits would possess the same KMD level and line up horizontally in the KMD plot [START_REF] Bahureksa | Soil Organic Matter Characterization by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR MS): A Critical Review of Sample Preparation, Analysis, and Data Interpretation[END_REF] . In this study, two subunits (CH2, COO) were considered as key to describe the observed photochemistry. The characterization approaches are detailed in Text S3.

Correlation analysis and principal component analysis (PCA) were applied to the temporal VOC concentration profiles during the second half of light exposure to gain more insights into the relevant chemistry. Independent samples t-test was used to compare the experimental results of two different samples, whereas paired samples ttest compared the scenarios of pre and post irradiation for a specific sample. The outcome can be characterized by the p value, which indicates significant discrepancy or variation, when lower than 0.05. The measurement results from mass spectrometers are reserved for at least two decimals.

Carbon Flux Calculation. Following our previous studies on the photochemistry

and VOC production at the air/sea interface [START_REF] Bernard | Photosensitized Formation of Secondary Organic Aerosols above the Air/Water Interface[END_REF][START_REF] Rossignol | Atmospheric photochemistry at a fatty acid-coated air-water interface[END_REF][START_REF] Ciuraru | Unravelling New Processes at Interfaces: Photochemical Isoprene Production at the Sea Surface[END_REF][START_REF] Fu | Photosensitized Production of Atmospherically Reactive Organic Compounds at the Air/Aqueous Interface[END_REF][START_REF] Tinel | Mechanistic Insights on the Photosensitized Chemistry of a Fatty Acid at the Air/Water Interface[END_REF] , we calculated the carbon flux associated with the emission of a given VOC normalized to the surface of water available under the simulated sunlight, denoted as SFVOCs (nmol C m -2 s -1 ), according to:

S𝐹𝐹 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 = � 𝐿𝐿 × 𝐶𝐶 𝑖𝑖 × 𝑁𝑁 𝑖𝑖 × 10 -3 𝑉𝑉 𝑇𝑇 × 𝐴𝐴 𝑚𝑚 𝑖𝑖=1 ( 1
)
where L is the flow rate of air (~ 3.33 mL s -1 ), Ci is the concentration of VOC i (ppb),

Ni is the number of carbon atoms in VOC i (dimensionless), VT is the molar volume of gas at temperature T (24.4 L mol -1 at 24.3℃), A is the area of surface water exposed to air (17.85 × 10 -4 m 2 ), m is the number of organic species produced over the interface (38 for Kuujjuarapik and 55 for Umiujaq).

Given that the irradiance (I) of the natural study site differs from the experimental setting, we estimated the natural VOC fluxes, denoted as NFVOCs (nmol C m -2 s -1 ), by using a linear relationship between the photoinduced VOC production and irradiance as previously reported in other cases [START_REF] Ciuraru | Unravelling New Processes at Interfaces: Photochemical Isoprene Production at the Sea Surface[END_REF][START_REF] Ciuraru | Photosensitized production of functionalized and unsaturated organic compounds at the air-sea interface[END_REF][START_REF] Brüggemann | Interfacial photochemistry at the ocean surface is a global source of organic vapors and aerosols[END_REF] , as shown by equation ( 2). This correction method facilitates the comparison between the VOC emission fluxes derived from the different laboratory experiments and/or in-field measurements.

N𝐹𝐹 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 = S𝐹𝐹 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉-𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 + (S𝐹𝐹 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉-𝑙𝑙𝑖𝑖𝑙𝑙ℎ𝑡𝑡 -S𝐹𝐹 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉-𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 ) × 𝐼𝐼 𝑛𝑛𝑑𝑑𝑡𝑡𝑛𝑛𝑑𝑑𝑑𝑑𝑙𝑙 𝐼𝐼 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑑𝑑𝑖𝑖𝑚𝑚𝑒𝑒𝑛𝑛𝑡𝑡𝑑𝑑𝑙𝑙 (2)
where SFVOCs-dark and SFVOCs-light are respectively the carbon fluxes attributed to the VOCs emitted before and during irradiation, Iexperimental is the experimental irradiance (8.0 mW cm -2 ), Inatural is the natural irradiance. Hourly Inatural during the warm season of 2019 (1 st June -30 th September) was measured at a meteorological site near the Umiujaq sampling site [START_REF] Domine | Automatic monitoring of the effective thermal conductivity of snow in a low-Arctic shrub tundra[END_REF][START_REF] Lackner | Radiation data from a tundra vegetation zone in the low-Arctic Tasiapik valley in the forest-tundra ecotone in Northern Quebec[END_REF] . Temporal variations of Inatural are given in Figure S5. Solar irradiance at Kuujjuarapik is very similar to that at Umiujaq (see Figure S4), therefore we used the Umiujaq data for both sites. Because the experimental irradiance was 22.7

folds of the natural daily mean level, the 180 min of irradiation in the laboratory simulates ~ 3 days' photochemistry in the field. Similar photoinduced VOC production was previously observed across other water samples under strong and weak irradiation in a smog chamber system [START_REF] Bernard | Photosensitized Formation of Secondary Organic Aerosols above the Air/Water Interface[END_REF] , and therefore no irradiance threshold exists in such type of investigations. Applying equation 2 (as described above) aims only to underline some differences in the pattern of emitted VOCs at both sampling sites, without capturing the full environmental variability of this Arctic region.

Implication Evaluation. To estimate whether the photochemically produced VOCs could locally (i.e., at the sampling sites) affect the O3 budget under a given NOx concentration, O3 formation potential, OFP (μg m -3 ), was calculated according to [START_REF] Li | Persistent growth of anthropogenic non-methane volatile organic compound (NMVOC) emissions in China during 1990 -2017: drivers, speciation and ozone formation potential[END_REF] :

𝑂𝑂𝐹𝐹𝑂𝑂 = �[𝑋𝑋] 𝑗𝑗 × 𝑀𝑀𝐼𝐼𝑀𝑀 𝑗𝑗 𝑛𝑛 𝑗𝑗=1 (3)
where [X]j is the concentration of VOC j (μg m -3 ), MIRj is the maximum incremental reactivity (MIR) coefficient of VOC j, n is the number of VOC with documented MIR (22 for Kuujjuarapik and 33 for Umiujaq). MIR can be taken from the updated research results (https://intra.engr.ucr.edu/~carter/SAPRC/, last access: 10 th , November 2022).

The calculated OFP can be normalized by the available air/water interfacial area to the simulated efflux of O3, denoted as SFO3 (nmol m -2 s -1 ), which is further corrected by the outdoor irradiance as that done for carbon flux to the natural FO3, denoted as NFO3 (nmol m -2 s -1 ), according to:

S𝐹𝐹 𝑉𝑉 3 = 𝐿𝐿 × 𝑂𝑂𝐹𝐹𝑂𝑂 × 10 -3 𝑀𝑀 𝑉𝑉 3 × 𝐴𝐴 (4) N𝐹𝐹 𝑉𝑉 3 = S𝐹𝐹 𝑉𝑉 3 × 𝐼𝐼 𝑛𝑛𝑑𝑑𝑡𝑡𝑛𝑛𝑑𝑑𝑑𝑑𝑙𝑙 𝐼𝐼 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑑𝑑𝑖𝑖𝑚𝑚𝑒𝑒𝑛𝑛𝑡𝑡𝑑𝑑𝑙𝑙 ( 5
)
where MO3 is the molar mass of O3 (48.0 g mol -1 ).

RESULTS

As soon as the pond water was illuminated in the actinic region, a strong net photochemical production of a variety of VOCs can be observed (Figure 2a). The production is sustained and increasing, demonstrating that products are building up as long as the light was presented. Such production stopped when the light was switched off, with the concentration decreasing over time. While Figure 2a shows the integrated signal over all VOCs, more product details are presented in (2.0%), C2H2O (2.0%), C2H6O (1.8%), C4H8 (1.7%), followed by the other 45 products (accounting up to 21.8%). Also, for a given VOC, its relative production is negatively correlated with its molecular weight (Figure S7).

Overall, three dominant species, CH4O, C2H4O, and C3H6O that were normally identified as methanol, acetaldehyde, and acetone (or possibly propanal or methyl vinyl ether), are responsible for > 60% of the total VOC concentrations for both samples. A statistical treatment (PCA in Figure S8) further reveals that these three compounds are not correlated and do originate from distinct processes, depending on the type of thermokarst pond surface water. Furthermore, positive correlations are common for the Umiujaq VOCs, whereas more negative correlations are observed among the Kuujjuarapik VOCs (Figure S9). In general, the Kuujjuarapik VOCs are more oxygenated, unsaturated, and aromatic than the Umiujaq ones, as reflected by the higher O/C and OSc, lower H/C and higher RDBE, and higher Xc and AImod, respectively (Table S2). Such molecular differences are certainly associated with the compositions of permafrost organic carbon in the two contrasted ponds studied here.

The photochemistry of both samples also differs significantly in the filtered liquid phase. The NPOC levels were 1320 ± 16 and 1070 ± 28 μM for the waters from Kuujjuarapik and Umiujaq, respectively, before irradiation. After irradiation, as shown in Figure S10, the NPOC level for Kuujjuarapik significantly increased to 1823 ± 170 μM (p < 0.05), whereas that of Umiujaq generally showed no significant change, with a value of 1003 ± 123 μM (p > 0.05). Moreover, as shown in Figure S11, the pH increased from 6.9 ± 0.0 to 7.4 ± 0.1 for the Umiujaq water (p < 0.05), while it decreased from 7.1 ± 0.0 to 6.9 ± 0.0 for the Kuujjuarapik water (p < 0.05). Lastly, the Umiujaq sample was observed to photobleach with irradiation, while for Kuujjuarapik the absorbance increased over time for wavelengths below 300 nm (Figure S12).

Our observations indicate that the organics contained in the thermokarst ponds are and methanol, which account for > 60% of the concentration of photoproduced VOCs (see Figure 2d), also explain nearly half of the photoinduced VOC-associated carbon flux over sunlit thermokarst ponds.

In order to further reveal the potential importance of the discovered photochemistry, we speculate that the photoinduced VOC production for a longer time scale can be estimated by extrapolating the measurement results to the natural irradiance of the warm season (Figure S13). The highest daily NFVOCs over the thermokarst ponds would occur around the summer solstice. As irradiance decreased since then, the NFVOCs over both ponds generally decreased. Since NFVOCs is influenced by temperature, irradiance, and the properties of water, the atmospheric relevance of this photochemistry should be accurately evaluated according to the actual situation.

DISCUSSION

To the best of our knowledge, this is the first detection and quantification of photochemically produced non-methane VOCs from thermokarst ponds. The weak VOC emissions before irradiation are probably explained by the diffusion to the gas phase of volatile species photoproduced under natural conditions before sampling. This is supported by the positive correlation between the individual VOC concentrations before and during irradiation (Figure S14). Thermokarst ponds are therefore a photochemical reactor that is able to transform permafrost carbon into atmospheric VOCs. VOCs have been reported to be also emitted directly by thawing permafrost, without the intervention of liquid phase photochemistry [START_REF] Kramshøj | Biogenic volatile release from permafrost thaw is determined by the soil microbial sink[END_REF][START_REF] Kramshøj | Volatile emissions from thawing permafrost soils are influenced by meltwater drainage conditions[END_REF][START_REF] Li | Overlooked organic vapor emissions from thawing Arctic permafrost[END_REF] . Current understanding is that these VOCs were produced by microbial metabolism, mostly anaerobic, and accumulated over the years before being released by thawing [START_REF] Kramshøj | Biogenic volatile release from permafrost thaw is determined by the soil microbial sink[END_REF][START_REF] Kramshøj | Volatile emissions from thawing permafrost soils are influenced by meltwater drainage conditions[END_REF][START_REF] Li | Overlooked organic vapor emissions from thawing Arctic permafrost[END_REF] . Here, however, we observed rapid VOC emission which is well correlated to irradiation conditions, so that production can be ascribed to photochemistry with reasonable certainty, rather than microbial production. This is in line with the observation of photoproducts over the liquid phase by others [START_REF] Brüggemann | Interfacial photochemistry at the ocean surface is a global source of organic vapors and aerosols[END_REF] . Given that many photoinduced reactions at the air/water interface are significantly enhanced in comparison to the corresponding gas-or liquidphase bulk processes [START_REF] Anglada | Photoinduced Oxidation Reactions at the Air-Water Interface[END_REF] , and that our experiments were performed in the absence of any gaseous oxidants (and over very short residence time of the gas phase), the observed photochemical production of VOCs is mainly related to processes occurring at the air/water interface, with a possible contribution of aqueous bulk photochemistry. Depending on the actual chemistry taking place, partial oxidation or decarboxylation leads to the formation of DOCr, which is more volatile and therefore more easily diffuses to the gas phase [START_REF] Ward | Complete and Partial Photo-oxidation of Dissolved Organic Matter Draining Permafrost Soils[END_REF][START_REF] Ward | Assessing the prevalence, products, and pathways of dissolved organic matter partial photo-oxidation in arctic surface waters[END_REF][START_REF] Ward | Sunlight controls water column processing of carbon in arctic fresh waters[END_REF] . Given that the DOC loading of the Kuujjuarapik pond water significantly increased after irradiation, photolysis of POC to DOC may be responsible for the photochemistry in this palsa-affected surface water. In contrast, the decrease in absorbance of the Umiujaq water after irradiation indicates a decrease in photoreactive compounds, implying a lower transformation of POC to DOC.

Interestingly, the Kuujjuarapik pond water, which is organic-rich because it is affected by peat, is less efficient in the photochemical production of VOCs than the Umiujaq one which is organic-poor. Based on our previous research on the photochemistry of humic acids in multiphase systems similar to the one explored here [START_REF] Bernard | Photosensitized Formation of Secondary Organic Aerosols above the Air/Water Interface[END_REF][START_REF] Ciuraru | Unravelling New Processes at Interfaces: Photochemical Isoprene Production at the Sea Surface[END_REF] , we are considering that the photoproduction observed for the thermokarst pond surface water is strongly impacted by the processes occurring at the air/water interface, and in particular by the availability of organic matters at the top layers of the liquid phase. In other words, if more appropriate precursors and oxidants are located at the sunlit air/water interface, more VOCs should be produced. Such availability is highly pH-dependent, as for example more long-chain organic acids are increasingly soluble with increasing pH. Herein, the photo-decreased pH of the Kuujjuarapik water indicates the generation of organic acids derived from the photolysis of POC, during which the availability of surface acids decreases, whereas the photo-increased pH of the Umiujaq water implies the higher VOC production efficiency relying on photodecarboxylation that can be described as the breakdown of carboxylic acids (R-COOH)

to hydrocarbons (R-H) and CO2 by sunlight [START_REF] Ward | Complete and Partial Photo-oxidation of Dissolved Organic Matter Draining Permafrost Soils[END_REF][START_REF] Perez | Structures and Chemical Rearrangements of Benzoate Derivatives Following Gas Phase Decarboxylation[END_REF] :

𝑀𝑀 -𝐶𝐶𝑂𝑂𝑂𝑂𝑂𝑂 ℎ𝑣𝑣 � � 𝑀𝑀 -𝑂𝑂 + 𝐶𝐶𝑂𝑂 2 (6)
Another indication can be found in the evolution of the optical property of surface water. The photobleaching of the Umiujaq water implies that large compounds may photodegrade into smaller fragments with increased volatility. By contrast, the photoenhanced absorbance of the Kuujjuarapik water indicates that photochemistry induces the increase of chemical functionalities, as evidenced by the photo-increased liquid aromaticity (Table S3). This variation may be attributed to the significant photolysis of POC, followed by the conversion of non-condensed aromatics to monoaromatics with higher AImod [START_REF] Koch | From mass to structure: an aromaticity index for high-resolution mass data of natural organic matter[END_REF] . Overall, the photo-productivity of thermokarst pond water may thus be better predicted by considering the segregation and processes at the air/water interface than bulk organic concentration. This is in line with previous research in other aqueous systems, where the prevalence of interface processes was demonstrated [START_REF] Bernard | Photosensitized Formation of Secondary Organic Aerosols above the Air/Water Interface[END_REF][START_REF] Ciuraru | Unravelling New Processes at Interfaces: Photochemical Isoprene Production at the Sea Surface[END_REF] .

As additionally suggested by the KMD plots for CH2 (Figure S15) and COO (Figure S16), most photo-enhanced VOCs would "theoretically" form after the removal of one or more target subunits (e.g., CH2 or COO) from the photodegraded liquid compounds.

As an example, for both samples, C4H4O2 may generate via the photo-dealkylation of C5H6O2 and its homologous series, and the photo-decarboxylation of C4H6O3 or C5H6O5 would produce C3H6O. More suggested "precursor-product" pairs are listed alongside the KMD plots in the Supporting Information. This points to the fact that, as expected, we do observe multigeneration products from permafrost carbon conversion, in agreement with the time profiles seen in Figure 2a. In comparison with the Kuujjuarapik DOC, the Umiujaq DOC favors the photochemical production of VOCs via decarboxylation channels. The efficient carboxyl loss follows the photo-increased pH of the Umiujaq water and may be associated with more significant release of CO2.

The relevant mechanisms may be much more complex than expected [START_REF] Chen | Source and formation process impact the chemodiversity of rainwater dissolved organic matter along the Yangtze River Basin in summer[END_REF] , depending on the actual composition of the liquid phase.

Uncertainties may exist when reproducing the natural conditions. Firstly, the production and release of volatile species are obviously changing with temperature [START_REF] Heath | Effects of Temperature, Oxygen Level, Ionic Strength, and pH on the Reaction of Benzene with Hydroxyl Radicals at the Air-Water Interface in Comparison to the Bulk Aqueous Phase[END_REF][START_REF] Leng | Temperature-Dependent Henry's Law Constants of Atmospheric Amines[END_REF] .

Secondly, the carbons in thermokarst ponds are refreshed because (i) the permafrost is always thawing by warming [START_REF] Olefeldt | Circumpolar distribution and carbon storage of thermokarst landscapes[END_REF][START_REF] Jones | Modern thermokarst lake dynamics in the continuous permafrost zone, northern Seward Peninsula, Alaska[END_REF] , rainfall [START_REF] Magnússon | Extremely wet summer events enhance permafrost thaw for multiple years in Siberian tundra[END_REF] , and wildfire 9 ; (ii) wind above the shallow pond will mix its liquid phase, including the vertical transport from deeper layers to interface [START_REF] Vonk | Reviews and syntheses: Effects of permafrost thaw on Arctic aquatic ecosystems[END_REF] ; and (iii) the headwater streams feed the thermokarst ponds by bringing in soil materials, especially during rain episodes [START_REF] Ward | Sunlight controls water column processing of carbon in arctic fresh waters[END_REF][START_REF] Vonk | Reviews and syntheses: Effects of permafrost thaw on Arctic aquatic ecosystems[END_REF] . The refresh of thermokarst pond water will further change its physical properties (e.g., surface tension, liquid viscosity) that are associated with the performance of air/water interface. Third and perhaps most important, the "chemical history" (e.g., number of diurnal cycles) of permafrost carbons will also alter the liquid properties [START_REF] Ward | Complete and Partial Photo-oxidation of Dissolved Organic Matter Draining Permafrost Soils[END_REF][START_REF] Ward | Assessing the prevalence, products, and pathways of dissolved organic matter partial photo-oxidation in arctic surface waters[END_REF][START_REF] Mazoyer | The dominant role of sunlight in degrading winter dissolved organic matter from a thermokarst lake in a subarctic peatland[END_REF] , followed by the variation of sunlight intensity at various depths and at the aqueous interface [START_REF] Ward | Sunlight controls water column processing of carbon in arctic fresh waters[END_REF] . Those effects are certainly to be taken into account for the following studies.

IMPLICATIONS

It is worthwhile to mention that, the measurements performed by this study are too limited in space and time to provide a robust assessment of the VOC emission over the numerous thermokarst landscapes (3.6 × 10 6 km 2 , ~ 20% of the northern permafrost region [START_REF] Olefeldt | Circumpolar distribution and carbon storage of thermokarst landscapes[END_REF] ), but given the vast area of this continuously formed natural air/water interface and the considerable emission fluxes of VOCs under sunlight, there is an urgent need to take throughout research of this photochemistry over the thermokarst ponds/lakes on Earth. Besides, temporal difference caused by the variations of carbon compositions, temperature, and irradiance are warranted in future works, as an attempt to reveal more characteristics of the photochemistry over thermokarst landscapes.

Nevertheless, the photochemical production of atmospheric VOCs may strongly impact the local air composition by generating significant airborne O3 in the presence of NOx. Simply based on equations ( 3) and ( 4), the SFO3 is calculated to be 3.83 nmol m -2 s -1 for the Kuujjuarapik sample and 7.34 nmol m -2 s -1 for the Umiujaq sample, under the experimental irradiance. Such results are much higher than the SFO3 over an authentic air/sea surface (0.51 nmol m -2 s -1 ) investigated under the same experimental condition [START_REF] Ciuraru | Photosensitized production of functionalized and unsaturated organic compounds at the air-sea interface[END_REF] , suggesting that the photochemistry we discovered might present considerable impacts on the local ozone budget. Corrected by equation ( 5), the daily average NFO3 is 0.17 nmol m -2 s -1 for Kuujjuarapik and 0. from that of VOC concentrations, implying that the atmospheric impact of this photochemistry may not be directly determined by the magnitude of VOC emission. In addition to altering locally the O3 budget, the emissions of VOCs could also lead to an increase in secondary organic aerosol formation, similar to what has been observed for processes involving chromophoric materials at the air/sea interface [START_REF] Bernard | Photosensitized Formation of Secondary Organic Aerosols above the Air/Water Interface[END_REF] . This however needs to be verified by further measurements.

Even though the VOC flux data were not reported for typical thermokarst landscapes, a shallow post-glacial subarctic lake was recently explored for its in-field emission of six non-methane VOCs 67 , and the comparison results are listed in Table S4. At the end of subarctic warm season (i.e., September), the lake presented net deposition fluxes of methanol, acetaldehyde, acetone, and dimethyl sulfide, in contrast to the net emission fluxes of isoprene and monoterpenes, eventually causing a net deposition flux of total VOCs (-0.287 nmol C m -2 s -1 ). The negative flux measured for the lake demonstrates that processes other than photochemistry were involved. In this study, both thermokarst pond waters owned more significant fluxes of methanol, acetaldehyde, and acetone than other three VOCs, corresponding to the photoinduced VOC emission fluxes of 0.080 and 0.172 nmol C m -2 s -1 for Kuujjuarapik and Umiujaq, respectively. While in nature it is difficult to isolate the contribution of photochemical processes to VOC emissions, the comparable absolute VOC fluxes obtained in the field and laboratory imply that the photochemistry reported here may have considerable implications for the budget and composition of atmospheric VOCs over subarctic water bodies.

Another obvious extension of this work is the comparison of the carbon fluxes of VOCs, which are currently not in-field measured over thermokarst ponds, with those of CH4 and CO2, which have been frequently measured over water bodies in the Arctic.

The CO2 and CH4 fluxes have been measured above ponds close to those we sampled [START_REF] Matveev | Methane and carbon dioxide emissions from thermokarst lakes on mineral soils[END_REF][START_REF] Matveev | High methane emissions from thermokarst lakes in subarctic peatlands[END_REF] . The average fluxes of CO2 and CH4 were respectively 143.84 and 5.80 nmol C m -2 s -1 over a lithalsa-affected thermokarst lake, and 676.28 and 40.60 nmol C m -2 s -1 over a palsa-affected thermokarst lake. In comparison, the VOC fluxes we measured are lower by 1-3 orders of magnitude. However, measurement conditions are not comparable. Our experiments were performed over samples with a maximal depth of ~ 0.55 cm, in contrast to the water bodies with depths of typically 2.0 m over which the gaseous CO2 and CH4 were produced. Furthermore, one study on a thermokarst lake near Kuujjuarapik 26 concluded that "only half of the light-associated DOC losses were converted into CO2", so that the fraction of VOCs produced by pond water photolysis may actually be significant. Meaningful comparisons would require operating under similar conditions for CO2, CH4 and VOC measurements. This is an analytical challenge, left for future research. The photochemical production of VOCs can be briefly described as the evolution of soil carbon lost from thawing permafrost to dissolved carbon in thermokarst pond along with the increased temperature, followed by the photogeneration of volatile atmospheric carbon in surface water (solid line arrows). Overall, this photochemistry connects the Earth's cryospheric carbon reservoir to the atmosphere, via the hydrosphere. In detail, it comprises the release of permafrost carbon into thermokarst ponds (dash-dot line arrow), the photo-reaction of particulate organic carbon (POC) and dissolved organic carbon (DOC) in the liquid phase (short-dash line arrow), the further conversion of secondary DOC (DOCs) acting as VOC precursors (short-dash line arrow), the diffusion of volatile reacted DOC (DOCr) into the atmosphere as VOCs (dot line arrow), as well as the retention of DOCr in the liquid phase (dash line arrow). DOCs can also be completely oxidized to CO2.

  photochemically active and produce a variety of VOCs. Based on equation (1), under

Figure 4

 4 Figure 4 illustrates the generic mechanism summarizing the photochemical

Figure 1 .

 1 Figure 1. Overview of the studied thermokarst ponds.

Figure 2 .

 2 Figure 2. Characterization of the photoproduction of VOCs.

Figure 3 .

 3 Figure 3. Evaluation of the photoproduction of VOCs.

Figure 4 .

 4 Figure 4. Schematic diagram for the photoinduced VOC production.

Table S1 .

 S1 The nonmonotonic increases suggest that 2 nd generation products are formed in both the liquid and gas phases. The net concentrations of total VOCs produced by photochemistry, Half of the VOCs emitted from the sunlit Kuujjuarapik sample contain ≤ 3 carbons, and these compounds are responsible for ~ 87% of the concentration of the Kuujjuarapik VOCs (Figure 2c, top half). By contrast, for the Umiujaq VOCs, while the total concentration is mainly contributed by C1-C3 species, most compounds contain ≥ 4 carbons. The concentration-weighted mean molecular mass of the Umiujaq VOCs (53.26) is slightly greater than that of the Kuujjuarapik VOCs (50.98).Furthermore, the VOCs are classified into CHO, CH, CHON, CHN, and CHS subgroups according to their elemental compositions (Figure2c, bottom half). The CHO subgroup generally dominates the structure of VOCs observed, and the water sample from Umiujaq is more efficient than that from Kuujjuarapik in producing all these subgroups, except for the concentration of CHON and the number of CHS. The average molecular formulas can be given as C2.370H4.839O1.026N0.064S0.011 for the Kuujjuarapik VOCs and C2.651H5.189O0.948N0.065S0.003 for the Umiujaq VOCs.

	C3H4O (4.6%), C2H2O (3.2%), C2H4O2 (2.2%), CH3NO (2.2%), C4H8O (2.1%), C4H6O
	(1.4%), C2H5NO (1.3%) and other 28 compounds (accounting up to 17.8%). For the
	pond water from Umiujaq, the ten most abundant compounds are C2H4O (30.9%),
	CH4O (15.5%), C3H6O (14.8%), C3H4O (5.0%), C2H4O2 (2.4%), C4H8O (2.2%), C3H5N

determined by subtracting the background signal of each VOC (i.e., the averaged dark level before irradiation) by the corresponding maximum signal observed during the 180 min of irradiation, are 13.83 ± 2.01 ppb (mean ± standard error [SE]) and 20.14 ± 1.93 ppb for the samples from Kuujjuarapik and Umiujaq, contributed by 38 and 55 species, respectively. That is, the Umiujaq water exceeds the Kuujjuarapik water in the VOC photoproduction (p < 0.05, Figure

S6

). Thirty volatile products are common to both sites (Figure

2b

). Speciation of the observed photoproduced VOCs is given in Figure

2d

. As the most abundant product from the sunlit Kuujjuarapik pond surface water, CH4O accounts for 28.3% of the total VOC concentration, followed by C2H4O (20.8%), C3H6O (16.1%),

ACKNOWLEDGMENTS

The authors thank Letizia Abis, Xinke Wang, Sophie Tomaz, and Dandan Li (CNRS, IRCELYON) for their help in data analysis. F.D. and D.F. were funded by the French Polar Institute (IPEV, grant 1042) and NSERC discovery grant program. T.W. acknowledges the

ASSOCIATED CONTENT

Supporting Information. Description of experimental procedures, methods of data analysis, characteristics (irradiance and actinic flux) of the experimental and natural irradiation; detailed information and molecular characterization of the detected VOCs, photoinduced evolutions of liquid phase, calculation of ozone formation potentials.

AUTHOR INFORMATION

Corresponding Author

Liwu Zhang (zhanglw@fudan.edu.cn) Florent Domine (florent.domine@gmail.com) Christian George (christian.george@ircelyon.univ-lyon1.fr)

Author Contributions

T.W., F.D., S.P., and C.G. designed research; T.W., C.K., and S.P. performed research; T.W., C.K., S.P., and C.G. analyzed the data; T.W., D.F., F.D., L.Z., J.C., S.P., and C.G. wrote the paper.

Notes

The authors declare no competing interest.

National Natural Science Foundation of China (42205099) and the China Postdoctoral Science Foundation (2021M700792). L.W.Z. acknowledges the National Natural Science Foundation of China (22176036, 21976030, 22006020).