M Prokopiou

P Martinerie

C J Sapart

E Witrant

G A Monteil

K Ishijima

S Bernard

J Kaiser

I Levin

T Sowers

T Blunier

D Etheridge

E Dlugokencky

R S W Van De Wal

T Röckmann

Constraining N 2 O emissions since 1940 using firn air isotope measurements in both hemispheres

N 2 O is currently the 3 rd most important anthropogenic greenhouse gas in terms of radiative forcing and its atmospheric mole fraction is rising steadily. To quantify the growth rate and its causes, we performed a multi-site reconstruction of the atmospheric N 2 O mole fraction and isotopic composition using firn air data collected from Greenland and Antarctica in combination with a firn diffusion and densification model. The multi-site reconstruction showed that while the global mean N 2 O mole fraction increased from (290±1) nmol mol -1 in 1940 to (322±1) nmol mol -1 in 2008 the isotopic delta [values] of atmospheric N 2 O decreased by (-2.2±0.2) ‰ for δ 15 N av , (-1.0±0.3) ‰ for δ 18 O, (-1.3±0.6) ‰ for δ 15 N α , and (-2.8±0.6) ‰ for δ 15 N β over the same period. The detailed temporal evolution of the mole fraction and

Introduction

The rise of nitrous oxide (N 2 O) since pre-industrial times contributes significantly to radiative forcing (Forster et al., 2007). Over the past four decades, the N 2 O mole fraction has increased by 0.25 % per year, reaching 324 nmol mol -1 in 2011 (IPCC, ch.6, 2013). Therefore, the understanding of the biogeochemical cycle of N 2 O is important for a reliable assessment of future climate change. In addition, the destruction of N 2 O in the stratosphere provides an important source of nitrogen oxides (NO x ), which contribute to stratospheric ozone depletion [START_REF] Ravishankara | Nitrous oxide (N 2 O): the dominant ozone-depleting substance emitted in the 21 st century[END_REF].

Natural sources of N 2 O are microbial processes in soils and oceans, which produce N 2 O during nitrification and denitrification (Bouwman et al., 2013;Loescher et al., 2012;[START_REF] Santoro | Isotopic signature of N 2 O produced by marine ammonia-oxidizing archaea[END_REF]Galloway et al., 2004;[START_REF] Pérez | Identifying the agricultural imprint on the global N 2 O budget using stable isotopes[END_REF]Yung and Miller, 1997;[START_REF] Kim | Nitrogen-15 and Oxygen-18 Characteristics of Nitrous Oxide: A global perspective[END_REF]. The increase of N 2 O since pre-industrial times (hereafter referred to as "anthropogenic" increase) has been attributed largely to increased microbial production, resulting from the increased use of nitrogen fertilizers in agriculture. Industry (especially nylon production) and fossil fuel combustion present a smaller contribution to the anthropogenic source [START_REF] Davidson | The contribution of manure and fertilizer nitrogen to atmospheric nitrous oxide since 1860[END_REF][START_REF] Kroeze | Closing the global N 2 O budget: a retrospective analysis 1500-1994[END_REF]Mosier et al., 1998). N 2 O is primarily destroyed in the stratosphere via UV photolysis (90%) and reactions with excited oxygen atoms (10 %) [START_REF] Minschwaner | Absorption of Solar Radiation by O 2 : Implications for O 3 and lifetimes of N 2 O, CFCl 3 , and CF 2 Cl 2[END_REF], with a minor N 2 O fraction removed by surface sinks [START_REF] Syakila | Neglecting sinks for N 2 O at the earth's surface: does it matter?[END_REF].

Estimates of the total N 2 O source strength from various bottom-up and top-down studies suggest a mean value of roughly 17 Tg a -1 N equivalents at present. However, the range in both approaches is large, especially for bottom-up estimates, which range between 8.5 and 27.7 Tg a -1 N, whereas top-down estimates range between 15.8 and 18.4 Tg a -1 N (Potter et al., 2011 and references therein). Besides the total source strength, the contributions of individual source processes are also poorly constrained. i.e. δ 15 N sp = δ 15 N α -δ 15 N β . For consistency with many recent publications in the field, we here adopt the nomenclature from Yoshida and Toyoda (1999), α and β, for the two positions.

The different sources and sinks of N 2 O are associated with characteristic fractionation processes leading to different isotope ratios. For example, microbial sources emit N 2 O that is depleted in 15 N and 18 O relative to the tropospheric background. N 2 O that returns from the stratosphere after partial photochemical removal is enriched in both heavy isotopes [START_REF] Yoshida | Constraining the atmospheric N 2 O budget from intramolecular site preference in N 2 O isotopomers[END_REF]Yung and Miller, 1997;[START_REF] Kim | Nitrogen-15 and Oxygen-18 Characteristics of Nitrous Oxide: A global perspective[END_REF]. Stratospheric N 2 O also has a high 15 N site-preference compared to tropospheric N 2 O. The observed enrichment is caused by kinetic isotope fractionation in the stratospheric sink reactions [START_REF] Kaiser | Probing stratospheric transport and chemistry with new balloon and aircraft observations of the meridional and vertical N 2 O isotope distribution[END_REF]2002;[START_REF] Park | Measurements of N 2 O isotopologues in the stratosphere: Influence of transport on the apparent enrichment factors and the isotopologue fluxed to the troposphere[END_REF][START_REF] Röckmann | Isotopic enrichment of nitrous oxide ( 15 N 14 NO, 14 N 15 NO, 14 N 14 N 18 O) in the stratosphere and in the laboratory[END_REF][START_REF] Yoshida | Constraining the atmospheric N 2 O budget from intramolecular site preference in N 2 O isotopomers[END_REF].

The multi-isotope signature of N 2 O adds useful constraints on its budget. In particular, when the isotopic composition of tropospheric N 2 O is combined with the fractionation during its removal in the stratosphere, the isotopic composition of the global average source can be determined [START_REF] Ishijima | Temporal variations of the atmospheric nitrous oxide concentration and its δ 15 N and δ 18 O for the latter half of the 20th century reconstructed from firn air analyses[END_REF][START_REF] Bernard | Constraints on N 2 O budget changes since pre-industrial time from new firn air and ice core isotope measurements[END_REF]Röckmann et al., 2003;[START_REF] Kim | Nitrogen-15 and Oxygen-18 Characteristics of Nitrous Oxide: A global perspective[END_REF].

The temporal variations of the N 2 O isotopic composition are difficult to quantify on a short timescale because of its long residence time in the atmosphere. Longer time scales can be Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License.

reconstructed by using air trapped in Arctic and Antarctic firn and ice which provides a natural archive of past atmospheric composition. The firn phase is the intermediate stage between snow and glacial ice, which constitutes the upper 40-120 m of the accumulation zone of ice sheets. Within the firn, air exchanges relatively freely in the upper layers and with the overlying atmosphere (convective zone). With increasing depth the air pores shrink in size due to firn compaction, and air mixes primarily via slow diffusion in the diffusive zone. At densities larger than ≈815 kg m -3 , air is permanently trapped in closed bubbles in the ice and totally isolated from the atmosphere. The precise age range of air that can be retrieved from polar firn between the surface and bubble close-off depends on site specific characteristics like temperature, accumulation rate and porosity and typically ranges from several decades to 120 years.

For N 2 O, a number of studies have reported isotope measurements from different Arctic and Antarctic firn drilling sites showing a steady decrease of the heavy isotope content of N 2 O over the past decades [START_REF] Park | Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940[END_REF][START_REF] Ishijima | Temporal variations of the atmospheric nitrous oxide concentration and its δ 15 N and δ 18 O for the latter half of the 20th century reconstructed from firn air analyses[END_REF][START_REF] Bernard | Constraints on N 2 O budget changes since pre-industrial time from new firn air and ice core isotope measurements[END_REF]Röckmann et al., 2003;[START_REF] Sowers | Extending records of the isotopic composition of atmospheric N 2 O back to 1800 A.D. from air trapped in snow at the South Pole and the Greenland Ice Sheet Project II ice core[END_REF]. A more recent study by [START_REF] Park | Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940[END_REF] attempted to reconstruct the long-term trends in N 2 O isotopic compositions and its seasonal cycles to further distinguish between the influence of the stratospheric sink and the oceanic source at Cape Grim, Tasmania, demonstrating that isotope measurements can help in the attribution and quantification of surface sources in general.

Taking into account the long atmospheric lifetime of N 2 O and the fact that both hemispheres are well mixed on annual timescales, it is reasonable to assume that the results from these studies are representative for the global scale. However care needs to be taken because small differences in the diffusivity profiles of the firn column lead to large effect on the isotope signature (Buizert et al. 2012). Interestingly, for atmospheric methane (CH 4 ), another important greenhouse gas, a recent multi-site analysis on the carbon isotopic composition of showed large differences among reconstructions from different sites [START_REF] Sapart | Can the carbon isotopic composition of methane be reconstructed from multisite firn air measurements?[END_REF]. In particular, firn fractionation effects related to diffusion and gravitational separation and their implementation in models (Buizert et al., 2012) have large effects on the reconstructed signals. Small differences in the diffusivity profiles of the firn column lead to large effects on the isotope signatures. Therefore, more robust results may be obtained by combining isotope information from a number of different sites in a multi-site reconstruction, including a critical evaluation of diffusivity profiles. individual sites [START_REF] Allin | Chlorine isotope composition in chlorofluorocarbons CFC-11, CFC-12 and CFC-113 in firn, stratospheric and tropospheric air[END_REF][START_REF] Witrant | A new multi-gas constrained model of trace gas non-homogeneous transport in firn: evaluation and behavior at eleven polar sites[END_REF]Wang et al., 2012;Rommelaere et al., 1997). We then use an isotope mass balance model to infer the changes in the isotopic signature of the N 2 O source over time to investigate possible changes in the source mix.

Materials and Methods

Firn air Sampling

New firn air samples added in this study to the total dataset were collected in 2008 and 2009 during the firn campaign [START_REF] Sapart | Simultaneous stable isotope analysis of methane and nitrous oxide of ice core samples[END_REF] as part of the North Eemian Ice Drilling programme (NEEM) in Greenland (77.45º N 51.06º W). These data are combined with existing firn air data from four other sites. Information on the locations is provided in Table 1.

The firn air collection procedure is described in detail by Schwander et al. (1993). Here a brief description is presented. Essentially a borehole is drilled in the firn to a certain depth and then the firn air sampling device is inserted into the borehole. The device consists of a bladder, a purge line and a sample line. When the sampling device reaches the desired depth the bladder is inflated to seal the firn hole and isolate the air below the bladder from the overlying atmosphere, and air is pumped out from the pore space below the bladder.

Continuous online CO 2 concentration measurements are performed to verify that no contamination with contemporary air occurs during the extraction procedure. After the contaminating air has been pumped away, firn air is collected in stainless steel, glass or aluminium containers.

N 2 O isotope analysis

The firn air samples from NEEM are analyzed for N 2 O isotopocules at the Institute for Marine and Atmospheric research Utrecht (IMAU). The N 2 O mole fraction and isotopic composition are measured using continuous flow isotope ratio mass spectrometry (IRMS).

The method is described in detail by Röckmann et al. (2003b). Here only a brief summary is

Modelling trace gas transport in firn

In firn air, the interstitial gas is not yet isolated in closed-off bubbles, so diffusion processes and gravitational separation alter mole fractions and isotope ratios over time. Thus, firn air measurements cannot be used directly to derive the atmospheric history of trace gas signatures. Over time, atmospheric compositional changes are propagated downwards into the firn based on the diffusivity of the atmospheric constituent in question. Firn air diffusion models take these effects into account and thereby allow reconstruction of changes in the atmospheric composition from the firn profile.

In this study we use the LGGE-GIPSA firn air transport model to reconstruct the temporal evolution of N 2 O mole fraction and isotopic composition from the measured firn profiles (Allis et al., 2015;[START_REF] Witrant | A new multi-gas constrained model of trace gas non-homogeneous transport in firn: evaluation and behavior at eleven polar sites[END_REF]Wang et al., 2012;Rommelaere et al., 1997). depth, together with profiles of firn density and effective diffusivity. The latter parameter is determined as a function of depth for each firn-drilling site by modelling the mole fractions in firn for trace gases with well known atmospheric histories (Buizert et al., 2012;[START_REF] Witrant | A new multi-gas constrained model of trace gas non-homogeneous transport in firn: evaluation and behavior at eleven polar sites[END_REF]Rommelaere et al., 1997;Trudinger et al., 1997). A multi-gas constrained inverse method [START_REF] Witrant | A new multi-gas constrained model of trace gas non-homogeneous transport in firn: evaluation and behavior at eleven polar sites[END_REF] is used to calculate the effective diffusivity of each site for each specific gas. It is noteworthy that diffusivity is not constrained equally well at all sites because different sets of constraints (e.g. number of available reference gases) are used at different sites and because of different depth resolutions.

A Green-function approach as presented by Rommelaere et al. (1997), with an extension for isotopic ratios and revised to take into account the sparsity of the measurements [START_REF] Witrant | Input estimation from sparse measurements in LPV systems and isotopic ratios in polar firn[END_REF][START_REF] Witrant | A new multi-gas constrained model of trace gas non-homogeneous transport in firn: evaluation and behavior at eleven polar sites[END_REF]Wang et al., 2011) is used to assign a mean age and age distribution to a certain depth.

Due to the long N 2 O residence time in the atmosphere, the global variability of the isotopic composition of N 2 O is very small and no significant variations between individual background locations have been detected so far [START_REF] Kaiser | Complete and accurate mass spectrometric isotope analysis of tropospheric nitrous oxide[END_REF]. In particular, the isotope ratio difference between northern and southern hemisphere tropospheric air is expected to be only -0.06 ‰ (based on an interhemispheric mole fraction gradient of 1.2 nmol mol -1 [START_REF] Hirsch | Inverse modelling estimates of the global nitrous oxide surface flux from 1998-2001[END_REF]] and isotope ratio difference of -15 ‰ between average source and average tropospheric isotope delta). These differences are within the uncertainties of the firn air measurements used here and therefore the data from the northern and southern hemisphere are combined into a single dataset without including an interhemispheric gradient.

With the multi-site reconstruction method, we used the measurements from six firn air The deepest firn data from each site provide constraints furthest back in time and the oldest air samples that are included in the inversion are from the DML-98 and DC-99, which extend the reconstruction of atmospheric N 2 O back to the early 20 th century (Röckmann et al., 2003).

At the same, the correction for isotopic fractionation in firn is most uncertain for the deepest samples, where strong differences between individual firn air models have been reported (Buizert et al., 2012).

Scaling of different data sets

At present, no international reference materials for the isotopic composition of N 2 O exist. [START_REF] Kaiser | Complete and accurate mass spectrometric isotope analysis of tropospheric nitrous oxide[END_REF] and Toyoda et al. (1999) linked the isotopic composition of N 2 O in tropospheric air to the international isotopes scales for nitrogen isotopes (Air-N 2 ) and oxygen isotopes (either VSMOW or Air-O 2 ). Our measurements are linked to a standard gas cylinder of tropospheric air with known N 2 O mole fraction and isotopic composition based on the scale of [START_REF] Kaiser | Complete and accurate mass spectrometric isotope analysis of tropospheric nitrous oxide[END_REF] for δ 15 N av and δ 18 O values and Yoshida and Toyoda (1999) for position dependent 15 N values. However, the reference air cylinder used for the calibration was exhausted and had to be replaced three times over the years in which the different measurement that we combine in this study were performed. Although the cylinders were carefully compared, the long-time consistency of the isotope scale could not be guaranteed because long-time isotope standards are not available. In fact, analysis of the data from the convective zone for the different sites, show small but significant differences from the temporal trends that are well established from previously published data from the German Antarctic Georg von Neumayer station for 1990 to 2002 [START_REF] Röckmann | High-precision determination of the changing isotopic composition of atmospheric N 2 O from 1990 to 2002[END_REF]2005). The linear trends reported in that paper are (-0.040±0.003) ‰ a -1 for δ 15 N av , (0.014±0.016) ‰ a -1 for δ 15 N α , (-0.064±0.016) ‰ a -1 for δ 15 N β and (-0.021±0.003) ‰ a -1 for δ 18 O. Since they were derived from direct air samples (unaffected by firn fractionation), these trends can be used as a reference to re-scale the different firn air results from different dates. To do so, data from the diffusive zone (ρ < 815 kg m -3 ) for each individual site were scaled to one reference site, DC-99, taking into account the temporal differences in sampling and the model-assigned mean age of the firn air samples (see below). DC-99 was chosen as reference site because it has most measurements in the diffusive zone. Also, the precision of these measurements was high because high volume cylinders were available from which many measurements could be performed and averaged. To test the sensitivity to the choice of reference site, we repeated the re-scaling using NEEM-09 as reference, which generated almost identical results within uncertainty bars (Appendix C).

The average difference between the samples from the diffusive zone at a given site and the interpolated DC-99 results was compared to the expected temporal trend between the sampling date of each station and DC-99, using the temporal trends established by [START_REF] Röckmann | High-precision determination of the changing isotopic composition of atmospheric N 2 O from 1990 to 2002[END_REF], as shown in the equations below. The effect of this scaling is that the temporal trend in the past decade is effectively forced to follow the atmospheric measurements at Neumayer station [START_REF] Röckmann | High-precision determination of the changing isotopic composition of atmospheric N 2 O from 1990 to 2002[END_REF].

After re-scaling the firn isotopic data we detected some individual data points that clearly deviated from the general trends. These were considered outliers, because they exceeded the 2σ error, and were removed from the dataset. All of these values are site-specific 15 N values, specifically, the following, were excluded: NEEM-EU-08 hole depth -4.9 m, -34.72 m, - presented in the next section are based on the scaling procedure and removal of the outliers as described above (Appendix B).

Global N 2 O (isotope) budget calculations

The tropospheric budget is controlled by N 2 O emissions from natural and anthropogenic sources at the surface and by the exchange between troposphere and stratosphere. A simple two-box model is used to quantitatively understand the emissions and the budget changes of The change in the tropospheric N 2 O reservoir is given by the following mass balance equations [START_REF] Allin | Chlorine isotope composition in chlorofluorocarbons CFC-11, CFC-12 and CFC-113 in firn, stratospheric and tropospheric air[END_REF]:

n T dχ T dt =E nat +E anth -F exch (χ T -χ S ) (1) n S dχ S dt =F exch (χ T -χ S ) -L (2)
where n is the amount of air and χ S and χ T are the mole fractions of N 2 O in the stratosphere and troposphere respectively. Annual fluxes between the two reservoirs, F exch , are calculated based on previous estimates (Appenzeller et al., 1996;[START_REF] Holton | On the global exchange of mass between stratosphere and troposphere[END_REF]. The loss due to stratospheric sink is determined by:

L= n T χ T !n S χ S τ (3)
where τ is the atmospheric lifetime of 123 -19 +29 a.

n T dχ T δ T dt =E nat δ nat +E anth δ anth +F exch (χ S δ S -χ T δ T ) (4) n S dχ S δ S dt =F exch (χ T δ T -χ S δ S ) -Lδ L (5)
Solving equations 4 and 5 and substituting equations 1 and 2 we reach the final isotope equations:

n T dδ T dt = E nat χ T (δ nat -δ T )+ E anth χ T (δ anth -δ T ) + F exch χ S χ T (δ S -δ T ) (6) n S dδ S dt = F exch χ T χ S (δ T -δ S ) - L χ S ε L (7)
where δ T is either δ 15 N av , δ 18 O, δ 15 N α , δ 15 N β from the multi-site reconstruction as shown below. δ nat and δ anth is the isotopic composition of the natural and anthropogenic N 2 O source, respectively (our target quantity). ε L is the apparent fractionation factor associated with stratospheric destruction.

δ S is also not known in this case, but can be calculated using the analogue from Röckmann et al. (2003) by employing the observed apparent Rayleigh fractionation in the stratosphere (𝜀 app ). Based on this, the relative isotope ratio difference between the stratosphere and the troposphere can be calculated by:

δ S = δ T +1 χ S χ T ε app -1 (8)
where 𝜀 !"" represents the stratospheric fractionation constant associated with this removal process. Here, we used the average 𝜀 !"" of all lowermost stratospheric measurements from [START_REF] Kaiser | Probing stratospheric transport and chemistry with new balloon and aircraft observations of the meridional and vertical N 2 O isotope distribution[END_REF] (Table 3). Note that slightly different fractionations 𝜀 !"" have been used

in previous studies by [START_REF] Röckmann | Isotopic enrichment of nitrous oxide ( 15 N 14 NO, 14 N 15 NO, 14 N 14 N 18 O) in the stratosphere and in the laboratory[END_REF] and [START_REF] Park | Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940[END_REF][START_REF] Park | Measurements of N 2 O isotopologues in the stratosphere: Influence of transport on the apparent enrichment factors and the isotopologue fluxed to the troposphere[END_REF] and the sensitivity to these differences will be examined below.

Furthermore we assume that the N 2 O lifetime and 𝜀 !"" remained constant from pre-industrial time to 2008, thus the annual strength removal can be scaled down from its current value at χ T = 322 nmol mol -1 to the pre-industrial level of χ T,pi =270 nmol mol -1 and the relative Furthermore it is hypothesized that during the pre-industrial period only natural emissions occurred without any anthropogenic input. After the industrialization (≈1750) any increase in the emission budget is considered to be due to anthropogenic input while natural emissions remain constant, allowing separation of E nat and E anth .

Hence, the isotopic signature of the pre-industrial (natural) N 2 O source calculated this way also represents the isotopic signature of the natural source at present, and consequently the average isotopic composition of N 2 O originating from "anthropogenic" sources (δ anth ) can be estimated.

Uncertainty estimation using random scenarios

The The covariance between the uncertainty on the reconstruction in one year i and the uncertainty in another year j is defined as:

cov i,j =r i,j σ i σ j (9) r i,j =f ( i-j ) (10)
The correlation (r i,j ) is maximum between two consecutive years, and decreases as the time difference increases. We carried out the same analysis for the different N 2 O isotopocules: for each isotopocule (δ value), we generated a covariance matrix 𝐁 𝜹 , constrained by the uncertainty ranges provided by the atmospheric reconstructions and the correlation coefficients defined in Eq.6 and Eq.7

to generate a set of 50 random scenarios within the uncertainty envelopes. For each of these random scenarios, we calculated the corresponding source signature scenario and the range in the results provides an uncertainty estimate of the isotopic source signatures.

Results

Mean age

The mean age of N 2 O in air sampled from different depths in the firn for all datasets that are used in this study is shown in Fig. 1. The strong change in the mean age gradient that is clearly visible in each profile reflects the transition between the diffusive and bubble close-off zones, which occurs at a specific depth and mean age for each site (marked with x on Fig. 1).

Fig. 1 also shows that for each site the few samples that are collected within the bubble closeoff zone provide the constraints for most of the reconstructed record (for instance, at BKN-03, 50 m depth is the beginning of the bubble close-off zone). In addition to the mean age, the width of the age spectrum also increases with depth. Therefore, the temporal resolution of signals that can be reconstructed from the firn air measurements reduces with depth and approaches the one of ice core samples towards the bottom of the bubble close-off zone.

The Greenland sites (NH) have similar meteorological and glaciological conditions (Table 1), thus the differences between the mean age profiles in Fig. 1 The atmospheric history that has been reconstructed from these firn datasets using the multisite inversion (using the data from NEEM-09, NEEM-EU-08, NGRIP-01, BKN-03, DC-99, DML-98) as described in section 2.4 is shown in the left panels of Fig. 2. The solid line shows the scenario that leads to the best fit with all firn data in the middle panel, and the dashed lines show the upper and lower range of possible scenarios that would still produce an acceptable fit to the data within the uncertainty bars. Color-coded symbols show data plotted at their respective mean age (as derived from the firn air model). When the best-fit scenario is used as input for the forward firn air model for each individual site, the model produces the vertical profiles that are shown as coloured lines together with the data in the middle panels. For the sites that were included in the multi-site reconstruction, the firn profiles based on the best-fit scenarios generally match the experimental data points well, which is expected after a successful inversion procedure and with consistent data sets. The right panels in Fig. 2 show the differences between these model results and the data. For the data that were used in the multi-site inversion the model-data differences are generally very small, although individual firn drilling sites in some cases show small systematic deviations, in particular in the close-off zone. This means that when inversions would have been performed on individual sites, the optimal reconstructions would be slightly different. The advantage of the multi-site reconstruction is that the reconstructed scenario is constrained by all sites and all sampling depths. Despite the small differences between individual sites, the left panels show that all data fall within the uncertainty bars of the reconstructed scenario of the inversion.

From 1940 to 2008 the total changes of the δ values of atmospheric N 2 O are (-2.2±0.2) ‰ for δ 15 N av , (-1.0±0.3) ‰ for δ 18 O, (-1.3±0.6) ‰ for δ 15 N α and (-2.8±0.6) ‰ for δ 15 N β respectively (Fig. 2, left panels). The average linearized trends are (-0.032±0.004) ‰ a -1 for δ 15 N, (-0.014±0.008) ‰ a -1 for δ 18 O, (-0.019±0.015) ‰ a -1 for δ 15 N α and (-0.041±0.020) ‰ a -1 for δ 15 N β . These overall trends are slightly lower compared to previous studies that used only the data at individual sites [START_REF] Ishijima | Temporal variations of the atmospheric nitrous oxide concentration and its δ 15 N and δ 18 O for the latter half of the 20th century reconstructed from firn air analyses[END_REF][START_REF] Bernard | Constraints on N 2 O budget changes since pre-industrial time from new firn air and ice core isotope measurements[END_REF]Röckmann et al., 2003;[START_REF] Sowers | Extending records of the isotopic composition of atmospheric N 2 O back to 1800 A.D. from air trapped in snow at the South Pole and the Greenland Ice Sheet Project II ice core[END_REF] and other studies that used data from the same period, which were not used in the present study [START_REF] Park | Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940[END_REF]. However, the differences lie well within the combined uncertainties. We note that comparisons of average linear trends can be flawed when the firn air records have different length and the temporal profiles do not change linearly (see below). Trends for δ 15 N α are smaller in magnitude than for δ 15 N β , while results

from [START_REF] Bernard | Constraints on N 2 O budget changes since pre-industrial time from new firn air and ice core isotope measurements[END_REF] showed stronger changes for δ 15 N α than for δ 15 N β . However, in that study the trends were largely determined from measurements on young ice core samples with comparatively higher measurement errors and scatter.

Data from two sites were not included in the multi-site inversion and are used as independent in light blue and SP-95 in blue) however, agree only for the more recent atmospheric history (Fig. 2, left panels). For mean ages before 1990 most of the points are outside the uncertainty envelopes of the multi-site reconstruction. Inter-laboratory calibration differences might be a possible explanation for the discrepancy, but the differences are not a systematic shift, and they are larger than offsets among laboratories that were established in the past [START_REF] Sapart | Simultaneous stable isotope analysis of methane and nitrous oxide of ice core samples[END_REF][START_REF] Kaiser | Complete and accurate mass spectrometric isotope analysis of tropospheric nitrous oxide[END_REF]. In fact, the data reported by [START_REF] Sowers | Extending records of the isotopic composition of atmospheric N 2 O back to 1800 A.D. from air trapped in snow at the South Pole and the Greenland Ice Sheet Project II ice core[END_REF] were actually measured with good agreement in two different laboratories.

To evaluate our scaling approach we repeated the multi-site reconstruction using the original non re-scaled data and the data that were re-scaled to NEEM-09 instead of DC-99 (see Appendix C). The data rescaled to NEEM-09 gave very similar results (within uncertainties)

to the one re-scaled to DC-99 as was expected, thus the results do not depend on the choice of the site used for re-scaling. Without re-scaling, the overall change of N 2 O mole fraction and isotopic composition remained the same, but an additional decadal variability was introduced for δ 15 N av , δ 15 N α and δ 15 N β . In addition to that, the uncertainty envelopes doubled because of the scale inconsistencies. All scaling approaches produce results that are consistent with our preferred scaling to DC-99 within the uncertainty envelopes. We conclude that DC-99 rescaling removed the discrepancies that would cause larger errors if the original data were used instead, but the re-scaling does not introduce artificial signals (see Appendix C). The regularization of the inversion results using a rugosity factor introduces a free parameter, which is chosen to eliminate overfitting of experimental uncertainties and which controls the smoothness of the reconstruction. The value of this parameter is set based on a robust generalized cross validation criterion, ensuring that the resolution obtained from the inverse model is similar to the experimental data while taking into account the sparsity of the measurements [START_REF] Witrant | Input estimation from sparse measurements in LPV systems and isotopic ratios in polar firn[END_REF]. A sensitivity experiment where the weight of the regularization term is increased, which leads to comparable results as linear tropospheric histories presented in Fig. 2 was performed (Appendix C). This combined with the fact that straight lines can be drawn within the uncertainty envelopes of the reconstructed scenarios and the sensitivity tests (see Appendix C) indicates that the isotopic trends are not significantly different from straight lines within the current uncertainties. The increase in the N 2 O mole fraction of (32±1) nmol mol -1 over the reconstruction period can be explained in the mass balance model by an increase in the emissions from (11.9±1.7) Tg a -1 N in 1940 to (16.4±1.7) Tg a -1 N in 2008. The emissions increased with an increasing trend until 1975, then the annual increase continued, but at a slower rate up to 1990, and from then on the annual emissions have stayed approximately constant or even decreased slightly.

Reconstruction of the N 2 O emission history

The corresponding changes in the mole fraction (increasing growth rate before 1990, no increase and possibly slight decrease in the growth rate afterwards) are difficult to discern due to the long atmospheric lifetime of N 2 O. On average, the annual growth rate from 1995 to 2008 period is 0.7 nmol mol -1 a -1 , corresponding to average annual emissions of 3.5 Tg a -1 N.

The temporal evolution of the N 2 O isotope signatures

The results from the isotope budget calculations are presented in The magenta lines represent 50 scenarios generated randomly within the reconstructed uncertainty range, as described in section 2.6. The middle panels show the temporal changes in the isotope signatures of the total N 2 O source, with their accompanied uncertainties, as calculated from the atmospheric mass balance model (section 2.5). The total source is split into an assumed constant "natural" and an increasing "anthropogenic" component and the right panels show the isotopic evolution of the "anthropogenic" component.

Results show that the average δ 15 N av of the total N 2 O source, over the reconstruction period, is (-7.6±0.6) ‰ where the uncertainty is calculated using the 1σ uncertainty from the scenarios with respect to the mean value (magenta lines). There is no statistically significant long-term trend, but a temporal variability is observed on the decadal scale that might mask this trend.

δ 15 N av first decreased from (-6.5±0.6) ‰ in 1940 to (-8.5±0.6) ‰ in 1965, then slowly increased again to (-6.6±0.6) ‰ in 1985, followed by another decrease to (-8.5±0.6) ‰ in 2008. These oscillations originate from the slightly curved trends in the isotopic reconstructions for δ 15 N av in Fig. 5 (left panels).

When the source is split into a constant natural and a varying anthropogenic component, the variability is primarily projected on the anthropogenic part and the temporal variations increase accordingly. However, also the uncertainties increase substantially, because the differences between the individual scenarios are attributed to only a small fraction of the total source.

The δ 15 N av signature of the anthropogenic source has an average value of (-18.2±2.6) ‰. It initially increases (the small initial decrease is not significant) from (-21.5±2.6) ‰ in 1940 to (-8.6±2.6) ‰ in 1990 where it starts to slowly decrease reaching (-15.4±2.6) ‰ in 2008.

During the early part of the reconstruction period (<1970), when the "anthropogenic" contribution was only a small fraction of the total source, the uncertainty ranges of the source signatures are large. Therefore, the uncertainties for the early part (<1970) were excluded from the uncertainty 1σ uncertainties from the generated scenarios. This applies to all anthropogenic isotope signatures.

The budget calculations suggest an overall trend towards more enriched anthropogenic emissions but the uncertainties are very large. Mathematically, this trend arises from the fact that the isotope reconstructions yield relatively linear temporal isotope trends, whereas the source strength increases in a strongly non-linear fashion (Fig. 4). In the beginning of the record a small increase in the source strength needs to produce a similar absolute isotope shift as a larger increase in the source strength in later years. This can only be satisfied by a lower δ 15 N av value for the small "anthropogenic" emissions in the early part of the firn record. A constant δ 15 N av source signature would result in a small temporal change in δ 15 N av of atmospheric N 2 O in the beginning of the record and increasing isotope trends with increasing emissions, similar to the exponential curves that were fit to the firn air data in Röckmann et al. (2003).

The δ 18 O of the total source varies within (27.2±2.6) ‰ over the entire period. δ 18 O does not show significant decadal scale oscillations because the reconstructed scenario for δ 18 O is even more strictly linear than the δ 15 N av scenario. For this reason, as explained above, in the best fit scenario the δ 18 O of the anthropogenic source for the initial 30 years has a more depleted value starting with (15.2±2.6) ‰ in year 1940, reaching (31.1±2.6) ‰ in year 1975 and remaining around this value until 2008 (Fig. 5). However, the relatively larger uncertainty envelopes for the atmospheric history of δ 18 O actually allow scenarios with smaller δ 18 O changes in the beginning of the record and larger changes in the later period, which means that the reconstruction does not exclude a constant value for the anthropogenic δ 18 O source signature. The available dataset thus does not allow quantifying a long-term trend in δ 18 O.

For the position dependent 15 N signatures of the total source no significant long-term trends were detected. For δ 15 N α no decadal scale variability is observed, whereas for δ 15 N β a temporal variability is observed similar to the δ 15 N av . The uncertainty ranges for δ 15 N α and δ 15 N β are about a factor 2 greater than for δ 15 N av , which is due to the larger analytical error that leads to higher uncertainties in the scenario reconstructions. δ 15 N α varies in the range (-3.0±1.9) ‰, δ 15 N β in the range (-11.7±2.3) ‰.

The temporal evolution of δ 15 N α of the anthropogenic source looks similar to that of δ 18 O, but with even larger variations and uncertainties with a total average of (-8.1±1.7) ‰. δ 15 N α increased from (-18.2±1.7) ‰ in 1940 to an average of (-5.4±1.7) ‰ in 1975 and retained this value until 2008. In contrast, δ 15 N β is similar to that of δ 15 N av with a total anthropogenic source average of (-26.1±8.4). δ 15 N β initially decreases from (-19. 2012) and bottom-up approaches using emission databases (Syakila and Kroeze, 2013;[START_REF] Kroeze | Closing the global N 2 O budget: a retrospective analysis 1500-1994[END_REF]. A more recent study by Thompson et al. (2014b) performed inversions of atmospheric measurements for 2006 to 2008 with multiple models and reported emissions of 16.1-18.7 Tg a -1 N for 2008 in agreement with our findings.

To investigate the effect of lifetime choice on the N 2 O isotopic signatures [START_REF] Prather | Measuring and modeling the lifetime of nitrous oxide including its variability[END_REF] we performed a sensitivity study where we linearly changed the N 2 O lifetime from 123 years pre-industrially (≈1750) to 119 years in modern times (2008). The results are shown in Appendix D, where the effect on the emission strength and isotopic composition is discussed in detail. Results from this sensitivity study showed that the effect of a decreasing lifetime gives higher N 2 O emissions for year 2008 while keeping the same pre-industrial value, confirming a sensitivity in the choice of lifetime in line with [START_REF] Prather | Measuring and modeling the lifetime of nitrous oxide including its variability[END_REF].

Consequently this sensitivity only influences the anthropogenic isotopic signature by 10 %, meaning that the resulting values can increase by (2.0±1.0) ‰. The lifetime effect is most pronounced for the earliest part of the record (<1970) where the reconstruction uncertainties are larger than this systematic uncertainty.

The increase in N 2 O emissions over the past decades resulted in an overall decrease of all isotopic signatures of atmospheric N 2 O with time. The isotopic signature of the total source of N 2 O (Fig. 4, middle panels) is strongly depleted in all heavy isotopes compared to tropospheric N 2 O (Table 3), which is due to the strong enrichment associated with the removal in the stratosphere. In Table 3 the isotopic composition for the pre-industrial period (≈1750) (δ nat,pi ) is compared with the derived anthropogenic source signature derived from our multi-site reconstruction (δ anth , averaged from 1940 to 2008). The results show that the anthropogenic source is more depleted in heavy isotopes than the natural one for all signatures, confirming results from previous studies that used forward firn air modelling on measurements from individual sites [START_REF] Park | Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940[END_REF][START_REF] Ishijima | Temporal variations of the atmospheric nitrous oxide concentration and its δ 15 N and δ 18 O for the latter half of the 20th century reconstructed from firn air analyses[END_REF]Röckmann et al., 2003). is strongly simplified and several other sources and factors may contribute (Toyoda et al., 2015), we use the results from our box model calculations (Table 3) in a similar way to estimate the fraction of the two source categories according to the following simple mass balance calculation:

F high = δ 15 Ν meas sp ! δ 15 Ν low sp δ 15 Ν high sp ! δ 15 Ν low sp (11)
This returns a fractional contribution of the δ 15 N high sp component of (19±4) % to the total preindustrial emissions and (35±11) % to the total present source. The errors were derived by propagating the errors of the δ 15 N sp . endmembers and δ 15 N meas sp as given above. We note that the errors associated with the precise isotopic composition of the endmembers are correlated if δ 15 N sp .for the two endmembers remain relatively constant. Therefore, the change in the relative fraction of the two categories is likely better constrained than the absolute values.

Splitting the total present emission strength into a natural (pre-industrial, 11.0 Tg a -1 N) and anthropogenic (5.4 Tg a -1 N) component, we derive a fraction of the δ 15 N high sp component (which includes nitrification) of (54±26) % for the "anthropogenic" emissions. This is another piece of evidence for agricultural sources being the main contributor to the N 2 O increase, because nitrification-dominated agricultural emissions can be associated with the δ 15 N high sp component.

The temporal changes of the derived fraction of nitrification are in good qualitative agreement with the results from [START_REF] Park | Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940[END_REF], who reported a change of (13±5) % from 1750 to (23±13) % today. However, the absolute numbers derived from our study are higher than the results from [START_REF] Park | Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940[END_REF]. The difference is due to the fact that different apparent isotope fractionations during stratospheric removal (ε app ) are used in the mass balance model (Table 3; eq. 7,8). In this study we used the averaged lowermost stratospheric apparent isotope fractionations from [START_REF] Kaiser | Probing stratospheric transport and chemistry with new balloon and aircraft observations of the meridional and vertical N 2 O isotope distribution[END_REF], which we consider more representative than the numbers used by [START_REF] Park | Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940[END_REF]. Using different values for ε app causes a shift in the isotopic source signatures from the mass balance model. The choice of this value thus adds a systematic source of uncertainty to the absolute value of the δ 15 N high sp fractions reported above (F high ). The reconstruction of mole fraction and isotopic composition was used to investigate temporal changes in the isotopic signature of N 2 O emissions over the study period. The average total source for δ 15 N av and δ 15 N β shows no statistically significant long-term trend but likely significant decadal scale variability. For δ 18 O and δ 15 N α of the total N 2 O source, no significant temporal changes can be detected with the present dataset because the uncertainties are large, especially in the beginning of the reconstruction period.

When the total source is split into a constant natural and a varying anthropogenic component, the reconstruction of the δ values of the anthropogenic source indicates a significant increase of δ 15 N av from the early to the modern part of the record. This originates from the near-linear isotope histories of the best guess scenario, which would imply that small emissions in the early part had a similar absolute effect on the δ values as stronger emissions in the latter part.

A similar effect for δ 18 O is likely, but not significant given the larger uncertainties for this signature.

Nevertheless, the isotope signal in δ 15 Ν av may also be a signal for changing source contributions over time. Bottom-up models suggest that N 2 O emitted from agricultural soils Results from the mass balance model yield an increase in 15 N site preference between the preindustrial and modern total N 2 O source. The increase in δ 15 N sp of (16±11) % between the preindustrial and modern source is in qualitative agreement with increased emissions from nitrification processes associated with agriculture.

  Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License. Here we combine new N 2 O isotope measurements from the NEEM site in Greenland with previously published firn air N 2 O isotope records from 4 different sites from Greenland and Antarctica to reconstruct records of the N 2 O isotopic composition over the last 70 years. We use the multi-gas firn transport model established by the Laboratoire de Glaciologie et Géophysique de l'Environnement and Grenoble Image Parole Signal Automatique (LGGE-GIPSA) to obtain an atmospheric scenario that is constrained by and consistent with all

  Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License. In the "forward version" of LGGE-GIPSA, a physical transport model uses a historic evolution of atmospheric N 2 O mole fractions to calculate the vertical profiles of mole fractions in firn. For the isotopocules, further simulations are performed separately to calculate their respective vertical profiles. Important parameters needed to constrain the model are the site temperature, accumulation rate, depth of the convective layer and close-off

  drillings at five sites (NEEM-09, NEEM-EU-08, NGRIP-01, BKN-03, DC-99, DML-98) to constrain our model and determine a set of atmospheric reconstructions that fits all sites. Data from[START_REF] Ishijima | Temporal variations of the atmospheric nitrous oxide concentration and its δ 15 N and δ 18 O for the latter half of the 20th century reconstructed from firn air analyses[END_REF] and[START_REF] Sowers | Extending records of the isotopic composition of atmospheric N 2 O back to 1800 A.D. from air trapped in snow at the South Pole and the Greenland Ice Sheet Project II ice core[END_REF] [NGRIP-01 and SP-01, SP-95 respectively] were not included in our multi-site reconstruction because no data for δ 15 N α and Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License. δ 15 N β were published for those sites. These datasets were used for independent validation of δ 15 N av and δ 18 O. To quantify the isotope fractionation due to diffusion and gravitational settling within the firn, a forward firn transport model simulation was carried out with a realistic N 2 O mole fraction scenario (based on the Law Dome record, MacFarling Meure et al., 2001), but with a constant isotopic N 2 O history. This allows determining the role of transport isotope fractionation occurring in the firn, in the absence of isotopic changes in the atmosphere. The results are used to subtract the firn fractionation effects from the measured signals, which can then be used to assess the atmospheric history. Compared to the signal, the effect of firn fractionation is minor for δ 15 N, but important for δ 18 O especially at the lower accumulation rates in the Southern Hemisphere (see Appendix A).

  Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License.

  61.95 m and -74.5 m, and NEEM-09 hole depth 1.0 m, 0.2 m and -69.4 m. The mole fraction data that can be obtained from the NEEM air isotope measurements were substituted with more precise measurements of N 2 O mole fraction by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) the Institute of Environmental Physics, University of Heidelberg (IUP), the Centre of Ice and Climate, University of Copenhagen (CIC) and National Oceanic and Atmospheric Administration (NOAA). In this way we combine all available N 2 O mole fraction data. The mole fraction data were scaled to the most recent international scale, NOAA-2006A from the CSIRO scale or the NOAA-2000 scale. Conversion of the NOAA-2000 data to the Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License. NOAA-2006A scale is done using a conversion factor available by National Oceanic and Atmospheric Administration (NOAA) (http://www.esrl.noaa.gov/gmd/ccl/scales/N2O_scale.html). Converting from the CSIRO to the NOAA-2006A scale, though, requires the reference cylinder details, which were not available. Instead we used a trend scenario, based on the CSIRO atmospheric scale combined with Law Dome data and assuming a constant interhemispheric gradient. This trend scenario was then compared with the data provided on NOAA-2006A scale, and a polynomial fit was generated, which was then used to convert the data to the NOAA-2006A scale. All results

  N 2 O. The model consists of a tropospheric N 2 O reservoir (index T) into which N 2 O is emitted from natural (E nat ) and anthropogenic (E anth ) sources. N 2 O is then transported to the stratosphere (index S) where part of it is destroyed by photochemical reactions (F sink ), and the remainder returns from the stratosphere to the troposphere (TS exch ).

  Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License.The isotopic budgets are calculated by simply multiplying the reservoir sizes with the corresponding δ values of the different flux terms:

  Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License. enrichment of stratospheric N 2 O relative to tropospheric N 2 O described by Eq. 8 remains constant over time. The effect of changing the N 2 O lifetime is examined below.

  precision of the calculated N 2 O emissions (E nat , E anth ) depends primarily on the precision of the atmospheric reconstruction of the N 2 O mole fraction (χ T ). However, the uncertainty envelope provided by the firn air reconstruction is insufficient to quantify the uncertainty on the atmospheric N 2 O reconstruction: the year-to-year variability of N 2 O is constrained by the N 2 O lifetime in the troposphere, which is very small in comparison to the width of the reconstructions confidence interval. Possible realistic N 2 O scenarios are scenarios that are within the confidence intervals provided by the atmospheric reconstructions, and that have realistic year-to-year variability. Mathematically, this can be represented by an uncertainty variance covariance matrix B, where the diagonal elements (variances) are the yearly uncertainties on the atmospheric N 2 O mole fractions, and the off-diagonals are the covariances of the uncertainties of different years.

  Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License.We generated an ensemble of 50 random realistic N 2 O scenarios within the uncertainty envelope of the firn atmospheric N 2 O reconstruction constrained by the covariance matrix B.For each of these atmospheric N 2 O scenarios, we calculated the corresponding N 2 O emission time series. The range of emissions from these scenarios then provides a realistic estimate for the uncertainty in N 2 O emissions.

  are small. The Antarctic sites (SH) show clear differences because the meteorological and glaciological variables differ strongly from site to site. As a result the firn-ice transition is at a different depth for each location (e.g., the firn-ice transition zone for DML-98 is located at about 73.5 m compared to about 99.5 m at DC-99). Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License. 3.2 Experimental results and multi-site reconstruction Mole fraction and isotopic composition of N 2 O in firn air are presented versus depth of the firn air sampling in the middle panels of Fig. 2. The mole fraction decreases with depth in qualitative agreement with the known increase of N 2 O in the atmosphere over time. In contrast, all isotope deltas slowly increase with depth in the upper firn and show stronger heavy isotope enrichment in the close-off zone, both indicating heavy isotope depletion in atmospheric N 2 O with time.

  Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License.

  validation of the reconstructed scenarios. The data points from Ishijima et al. (2007) (NGRIP-01, yellow) are within the range of scenarios reconstructed by the inverse model and thus independently validate our results. The δ 15 N av and δ 18 O data from Sowers et al. (2002) (SP-01

  Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License.

Fig. 3

 3 Fig. 3 shows the temporal evolution of the global N 2 O mole fraction as inferred from the firn air reconstruction in the top panel, and in the bottom panel the emission strength in Tg a -1 N calculated with the mass balance model (Section 2.5). The solid black line denotes the best estimate scenario, which is used as input in the mass balance model. The magenta lines show the ensemble of random scenarios generated to quantify the uncertainty of the emissions (see Section 2.6).

Fig. 4 .

 4 The left panels show the atmospheric trends. The solid black lines represent the best-fit scenarios while the dashed black lines represent the upper and lower uncertainty envelope of the firn air reconstructions. Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License.

  Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License.

  1±8.4) ‰ to (-42.0±8.4) ‰ in 1955 only to increase again to (-10.6±8.4) ‰ in year 1990 and then decrease again to (-26.0±8.4) ‰ in 2008. Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License.4 DiscussionFrom the combination of the firn air reconstruction with a simple two-box model we conclude that N 2 O emissions increased from (11.9±1.7) Tg a -1 N in 1940 to (16.4±1.7) Tg a -1 N in 2008. This agrees, within uncertainties, with previous firn reconstruction studies from[START_REF] Ishijima | Temporal variations of the atmospheric nitrous oxide concentration and its δ 15 N and δ 18 O for the latter half of the 20th century reconstructed from firn air analyses[END_REF] andPark et al. (

  Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License. (0±5) ‰. Park et al., (2012) used these two endmembers to calculate a change in the relative fractions of these source classes over time based on their firn air data. Although this approach

  Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License. Nevertheless, this systematic uncertainty should not alter the overall change in F high from preindustrial to modern times and the results from our multi-site reconstruction of the isotopic composition of N 2 O thus confirm the suggestion by Park et al. (2012) that the relative importance of the high-SP component (presumably nitrification) has increased with increasing mole fraction since pre-industrial times.5 ConclusionsThe temporal evolution of the total N 2 O emission fluxes and the source isotopic composition have been estimated in a top-down approach using a multi-site reconstruction of N 2 O mole fraction and isotopic composition from 6 firn air samplings at 5 different Arctic and Antarctic locations in a two-box model. The results from a mass balance model constraints the source strength and suggest a total increase in N 2 O emissions of (4.5±1.7) Tg a -1 N between the 1940 and 2008 due to anthropogenic processes. This agrees with previous top-down estimates, but deviates from bottom-up model estimates, which suggest higher N 2 O emission increases. A significant source of the uncertainty in such top-down estimates is a possible change in the N 2 O lifetime over the reconstruction period, which we have quantified following the recent results from[START_REF] Prather | Measuring and modeling the lifetime of nitrous oxide including its variability[END_REF].

  Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License.was the dominant contributor to the anthropogenic N 2 O increase in the first decades. Smaller contributions due to emissions from more enriched sources, like industry, automobiles and coal combustion increased, which may have contributed to an isotope enrichment of the emissions, which is not detectable within the error bars for the other isotope signatures.

  Figure 1. N 2 O mean ages in firn versus depth. The dashed lines represent the sites from the 902 NH (North Greenland Ice-core Project [NGRIP-01], North Eemian Ice-core Project [NEEM-903 09, NEEM-EU-08]) and the solid lines the SH sites (South Pole [SP-01, SP-95], Dome C 904 [DC-99], Dolomite [DML-98] and Berkner Island [BKN-03]). The numbers accompanying 905 the sites are the corresponding drilling years. Marker X indicates the transition between the 906 firn diffusive zone and the bubble close-off zone for each site. Dashed orange line NGRIP-01, 907 dashed brown NEEM-EU-08, dashed red NEEM-09, purple line BKN-03, black DML-98, 908 green DC-99, blue SP-95 and light blue SP-01. 909

Figure 2 .

 2 Figure 2. Left: Reconstructed atmospheric scenarios (black solid line with dashed lines indicating the 2 σ uncertainty intervals) and results of the firn air samples (corrected for firn fractionation) plotted at their respective assigned mean age. Middle: corresponding depth profiles, symbols show the measurements and solid lines the results of the forward model using the best estimate scenario as input. Right: model data discrepancies. Orange: NGRIP-01 (Bernard et al, 2006), Yellow: NGRIP-01 (Ishijima et al., 2007), Brown: NEEM-EU-08, Red NEEM-09, Purple: BKN-03, Black: DML-98, Green: DC-99, Blue: SP-95 and Light Blue: SP-01. Data from NGRIP-01 (Ishijima et al., 2007), SP-95 and SP-01 were not used in the atmospheric reconstruction and are only plotted for comparison purposes here.

Figure 3 :

 3 Figure 3: Top panel. N 2 O mole fraction history from the multisite reconstruction (solid black line with uncertainty envelopes as dashed black lines) and the scenarios within the uncertainty envelopes that were used in the mass balance model (magenta lines) to evaluate the uncertainties of the atmospheric modelling results. Bottom panel. N 2 O production rate as calculated from the mass balance model. The solid black line represents the result for the best fit reconstruction while magenta lines represent the results for the individual scenarios from the top panel.

Figure 4 :

 4 Figure 4: Left panels: Historic evolution of δ 15 N, δ 18 Ο, δ 15 N α and δ 15 N β in N 2 O as derived from the firn air reconstruction. Middle panels: isotope signatures of the total emitted N 2 O. Middle panels: isotope signatures of the anthropogenic source, respectively. The solid black line represents the best-fit scenario while the dashed ones represent the respective uncertainties as determined by the reconstruction method. Magenta lines represent the emissions that are required to produce the magenta N 2 O histories in the left panels.

Figure C4 .

 C4 Figure C4. Comparison of the atmospheric reconstructions between different re-scaling methods. Solid and dashed green lines are the scenarios from data re-scaled to DC-99 used in this study. Solid red lines are the best-case scenario for the non re-scaled data and solid blue lines are the best-case scenarios from the data re-scaled to NEEM-09. The latter data series is shifted because of a calibration offset. When this is corrected for the data superimposes the green lines as expected.

  Due to the long steady-state lifetime of N 2 O in the atmosphere (123 -19 +29 a; SPARC Lifetimes Report 2013), temporal and spatial gradients are small, making it difficult to resolve localised sources. Measurements of the isotopic composition of N 2 O may help to constrain the atmospheric N 2 O

	Brenninkmeijer	and	Röckmann,	2000;	Yoshida	and	Toyoda,	1999),

budget. The N 2 O molecule is linear (NNO) and the two N atoms are chemically distinguishable; thus they tend to attain different isotopic compositions. Beyond oxygen (δ 18 O, δ 17 O) and average δ 15 N av ("bulk") signatures, N 2 O also displays site specific 15 N isotopic information. Site preference (δ 15 N sp ) is defined as the difference in δ 15 N between the central (2, µ or α) and terminal position (1, τ or β) of N atoms in N 2 O

[START_REF] Kaiser | Intramolecular 15 N and 18 O fractionation in the reaction of N 2 O with O( 1 D) and its implications for the stratospheric N 2 O isotopic signature[END_REF]

Table 1 .

 1 Site information on the drilling locations of the North Greenland Ice core Project 881 (NGRIP-01), Berkner Island (BKN-03), North Greenland Eemian Ice drilling Project 882 (NEEM-EU-08, NEEM-09), Dome Concordia (DC-99) and Dronning Maud Land (DML-98), 883 where firn air samples were collected, and two key meteorological variables of each site. 884 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: 21 June 2016 c Author(s) 2016. CC-BY 3.0 License.

		Site	Location	Mean annual	Surface accumulation	Sampling
				temperature	rate (water equivalent)	year
				( o C)	(cm a -1 )
		NGRIP-01 75 o N 42 o W	-31	20		2001
		BKN-03	79 o S 45 o W	-26	13		2003
		NEEM-	77.4 o N 51.1 o W	-29	22		2008
		EU-08
		NEEM-09	77.4 o N 51.1 o W	-29	22		2009
		DC-99	75 o S 123 o E	-53	3		1999
		DML-98	75 o S 65 o E	-38	6		1998
	885
	886	Table 2. Detailed information in the mole fraction and the isotopic composition of the
	887	laboratory reference gases used for correcting each set of firn air samples.
		Site	Sampling	Mole	δ 15 N av	δ 18 O	δ 15 Ν β	δ 15 Ν α
			year	fraction	(‰)	(‰)	(‰)	(‰)
			(nmol mol -1 )
		NGRIP-01	2001	318	6.64	44.61	-2.79	16.07
		BKN-03	2003	318	6.64	44.61	-2.79	16.07
		NEEM-	2008	324	6.22	44.40	-3.08	15.52
		EU-08
		NEEM-09	2009	318	6.38	44.92	-2.66	15.41
		DC-99	1999	318	6.64	44.61	-2.79	16.07
		DML-98	1998	318	6.64	44.61	-2.79	16.07

Table 3 .

 3 Stratospheric isotope fractionation (ε app ) used in the mass balance model, and isotopic composition of the natural and anthropogenic source, and the respective results from Park et al. (2012).

		ε app ( ‰)	ε app ( ‰)	δ atm,pi ( ‰)	δ nat,pi ( ‰)	δ nat,pi ( ‰)	δ anth ( ‰)	δ anth ( ‰)
		this study*	Park et al.,	Park et al.,	this study*	Park et al.,	this study*	Park et al.,
			2012*	2012		2012		2012**
	δ 15 Ν	-16.2	-14.9	9.3±0.2	-5.2±0.2	-5.3±0.2	-18.2±2.6	-15.6±1.2
	δ 18 Ο	-13.4	-13.3	45.5±0.2	33.1±0.2	32.0±0.2	27.2±2.6	32.0±1.3
	δ 15 Ν α	-23.0	-22.4	18.8±1.0	-1.9±1.0	-3.3±1.0	-8.1±1.7	-7.6±6.2
	δ 15 Ν β	-9.4	-7.1	-0.6±1.1	-8.3±1.1	-7.5±1.1	-26.1±8.4	-20.5±7.1
	δ 15 Ν sp	-	-	19.4±1.5	6.4±1.5	4.2±1.5	18.0±8.6	13.1±9.4

*ε app values used in this study are averaged values from the lower stratosphere from

[START_REF] Kaiser | Probing stratospheric transport and chemistry with new balloon and aircraft observations of the meridional and vertical N 2 O isotope distribution[END_REF]

and ε app

Acknowledgements

We thank the teams involved in the firn air sampling at NEEM site during the 2008 and 2009 field seasons. NEEM is directed and organized by the Centre of Ice and Climate at the Niels Bohr Institute, University of Copenhagen, Denmark and the US National Science Foundation, Office of Polar Programs. It is supported by funding agencies and institutions in Belgium (FNRS-CFB and FWO), Canada (NRCan/GSC), China (CAS), Denmark (FIST), France (IPEV, CNRS/INSU, CEA and ANR), Germany (AWI), Iceland (RannIs), Japan (NIPR), Korera (KOPRI), The Netherlands (NWO/ALW and NWO/NPP), the United Kingdom (NERC NE/F021194/1) and the USA (US NSF, Office of Polar Programs). This project was financially supported by the Dutch Science Foundation (NWO), projects 851.30.020 & 865.07.001. Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: June 2016 c Author(s) 2016. CC-BY 3.0 License.

Anthropogenic N 2 O emissions are dominated by agricultural soil (70 %) with smaller contributions from automobiles, coal combustion, biomass burning and industry. Oceanic emissions were previously assumed to be only natural. However, the latest IPCC Assessment Report (IPCC, ch.6, 2013) for the first time breaks down oceanic emissions into a natural and an anthropogenic component, e.g. due to atmospheric N deposition to rivers [START_REF] Syakila | The global nitrous oxide budget revisited[END_REF][START_REF] License | Impacts of atmospheric anthropogenic nitrogen on the open ocean[END_REF][START_REF] Kroeze | New estimates of global emissions of N 2 O from rivers and estuaries[END_REF], thus estimating the anthropogenic component of oceanic N 2 O emissions to amount to 1 Tg a -1 N.

N 2 O emitted from agricultural soils and biomass burning is more depleted in δ 15 N and δ 18 O than the tropospheric background (Park et al., 2011;Goldberg et al., 2010;Ostrom et al., 2010;Tilsner et al., 2003;Perez et al., 2001;2000) while N 2 O emitted from other minor sources, such as automobiles, coal combustion and industry, has values closer to tropospheric N 2 O values [START_REF] Syakila | The global nitrous oxide budget revisited[END_REF][START_REF] Toyoda | Isotopomeric characterization of N 2 O produced, consumed and, emitted by automobiles[END_REF]Ogawa and Yoshida, 2005a;2005b).

Qualitatively, an increase of strongly depleted agricultural emissions in the first part of our reconstruction, followed by a decreasing relative contribution from agriculture and increasing contributions from more enriched sources like industry, automobiles and coal combustion, can explain the reconstructed changes of isotope signatures of both the total source and the anthropogenic component. The global N 2 O budget study from [START_REF] Syakila | The global nitrous oxide budget revisited[END_REF] indicates that agricultural emissions were 78 % of the total during the 1940-1980 period with little input from industry, vehicle exhaust and coal combustion. After 1980 the relative share of agricultural emissions dropped to 64 %, while the other sources increased, supporting our suggestion.

Additional evidence for potential changes in the N 2 O source composition between the preindustrial and present atmosphere may be derived from the position-dependent 15 N signatures, quantified by the 15 N site preference. Table 3 shows that the difference in the δ 15 N av signature between the pre-industrial and the anthropogenic source derived from our reconstruction is primarily due to a change at position δ 15 N β , whereas δ 15 N α remains relatively constant. This is reflected by a larger difference in δ 15 N sp between natural and anthropogenic emissions, which could indicate a temporal change in production processes. [START_REF] Sutka | Distinguishing Nitrous Oxide Production from Nitrification and Denitrification on the Basis Atmos[END_REF] suggested that there may be two distinct classes of N 2 O sources with different δ 15 N sp . N 2 O produced during nitrification and fungal denitrification had a high δ 15 N sp of (33±5) ‰ and N 2 O from denitrification and nitrifier denitrification had a low δ 15 N sp of Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-487, 2016 Manuscript under review for journal Atmos.

Appendix B: Data processing

In this study isotope deltas (δ) are used to denote the relative 15 N/ 14 N and 18 O/ 16 O ratio difference of N 2 O in firn air with respect to a standard reference,

where R represents the 15 N/ [START_REF] Kaiser | Correction of mass spectrometric isotope ratio measurements for isobaric isotopologues of O 2 ,CO, CO 2 , N 2 O and SO 2[END_REF], assuming a constant 17 O excess of 0.9 ‰.

There is a disagreement between reported trends of the position dependent δ 15 N av values reported in the literature from firn air on the one hand and archived air samples on the other hand [START_REF] Park | Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940[END_REF][START_REF] Ishijima | Temporal variations of the atmospheric nitrous oxide concentration and its δ 15 N and δ 18 O for the latter half of the 20th century reconstructed from firn air analyses[END_REF][START_REF] Bernard | Constraints on N 2 O budget changes since pre-industrial time from new firn air and ice core isotope measurements[END_REF][START_REF] Röckmann | High-precision determination of the changing isotopic composition of atmospheric N 2 O from 1990 to 2002[END_REF]Röckmann et al., 2003;[START_REF] Sowers | Extending records of the isotopic composition of atmospheric N 2 O back to 1800 A.D. from air trapped in snow at the South Pole and the Greenland Ice Sheet Project II ice core[END_REF]. In principle the temporal trend measured directly on archived air samples should be fully consistent with top firn air samples of the various data sets, which were collected over a decade or more, since the air in the diffusive zone is not very old. However, this is not the case. Using the high-precision determination of the temporal trend of the N 2 O isotope signatures on archived air samples from [START_REF] Röckmann | High-precision determination of the changing isotopic composition of atmospheric N 2 O from 1990 to 2002[END_REF] as reported in section 2.4 we rescale the different firn profiles to match this trend in the diffusive zone by interpolating the measurements from the diffusive zone of all sites to DC-99 (δ ΙΝΤ ). By using the firn model -assigned mean age of each sample, The maximum age difference from diffusive zone to surface corresponds to Δage = Δ DC t-t 0 = 10 a.

Below you can find the equations used:

Where 𝑚 is the slope connecting the two points we want to interpolate. The applied scaling N and (17.0±1.7) Tg a -1 N in year 2008 with a total increase of (4.7±1.7) Tg a -1 N. When keeping the lifetime constant, the results for the same years are (11.9±1.7) Tg a -1 N and (16.4±1.7) Tg a -1 N with a total increase of (4.5±1.7) Tg a -1 N. This shows that there is a sensitivity on the choice of lifetime for our mass balance model on the N 2 O atmospheric budget as was indicated by [START_REF] Prather | Measuring and modeling the lifetime of nitrous oxide including its variability[END_REF].

The N 2 O source isotopic signature shows no significant change with the choice of lifetime giving similar average source values for all source signatures as for when using a constant