Isotopic Constraints on the Atmospheric Sources and Formation of Nitrogenous Species in Biomass-Burning-Influenced Clouds

The interpretation of tropospheric cloud formation rests on understanding the sources and processes affecting aerosol constituents of the atmosphere that are preserved in cloudwater. 25 However, this challenge is difficult to be quantitatively addressed based on the sole use of bulk chemical properties. Nitrogenous aerosols, mainly ammonium (NH4 ) and nitrate (NO3 ), play an important role in tropospheric cloud formation. Here we collected cloudwater samples at the summit of Mt. Tai (1545 m above sea level) in Eastern China during a long-lasting biomass burning (BB) event, and measured for the first time the isotopic compositions (mean ± 1σ) of 30 cloudwater nitrogen species (δN-NH4 + = -6.53 ± 4.96‰, δN-NO3 = -2.35 ± 2.00‰, δO-NO3 = 57.80 ± 4.23‰), allowing insights into their sources and potential transformation mechanism within the clouds. Large contributions of BB to the cloudwater NH4 + (32.9 ± 4.6%) and NO3 (28.2 ± 2.7%) inventories were confirmed through a Bayesian isotopic mixing model, coupled with our newly-developed computational quantum chemistry module. Despite an overall reduction in total 35 anthropogenic NOx emission due to effective emission control actions and stricter emission standards for vehicles, the observed cloud δN-NO3 values suggest that NOx emissions from transportation may have exceeded emissions from coal combustion. δO-NO3 values imply that the reaction of OH with NO2 is the dominant pathway of NO3 formation (57 ± 11%), yet the contribution of heterogeneous hydrolysis of dinitrogen pentoxide was almost as important (43 ± 40


Introduction
Nitrogenous aerosols, mainly nitrate (NO3 -) and ammonium (NH4 + ), formed from the emissions of nitrogen oxides (NOx = NO + NO2) and ammonia (NH3), are major chemical components of aerosols, which serve as cloud condensation nuclei (CCN) and thus play an Biomass burning (BB) is an important source of N in the atmosphere (Lobert et al., 1990;Souri et al., 2017).During the harvest/hot season of eastern China, agricultural BB frequently occurs and modifies the concentration and composition of aerosols in the atmosphere (Chen et al.,70 2017; Zhang and Cao, 2015).For example, about 50% of the N derived from biomass combustion can be released as NH3 and NOx to form particulate NH4 + and NO3 -, which then account for over 80% of total nitrogenous species in BB smoke particles (Crutzen and Andreae, 1990).BB-induced aerosols have not only been associated with poor air quality and the detrimental effects on human health, they have also shown to exert manifold effects on the tropospheric clouds, altering regional 75 or even global radiation budgets (Chen et al., 2014;Norris et al., 2016;Voigt and Shaw, 2015).
The optical and chemical properties of clouds (and thus their radiative forcing) are directly related to the aerosol and precipitation chemistry (Seinfeld et al., 2016).Moreover, clouds represent reactors of multiphase chemistry, contributing to many chemical transformations that would otherwise not take place, or would proceed at much slower rates (Herrmann et al., 2015; 80 Lance et al., 2017;Ravishankara, 1997;Schurman et al., 2018;Slade et al., 2017).Understanding the sources and fate of nitrogenous species in BB-influenced clouds is particularly important to comprehensively assess the environmental impacts of BB.But this challenge is difficult to address based on the sole use of bulk chemical properties (as most often done in previous studies).
Given that the 15 N can be preserved between the sources and sinks of NOx and NH3, the N 85 isotopic composition of NO3 -(δ 15 N-NO3 -) and NH4 + (δ 15 N-NH4 + ) can be related to different sources of NOx and NH3, and thus delivers useful information regarding the partitioning of the origins of atmospheric/cloudwater NOx and NH3, respectively (Hastings et al., 2013;Michalski et al., 2005;Morin et al., 2008;Chang et al., 2018).Moreover, the oxygen isotope composition of NO3 -(δ 18 O-NO3 -) is largely determined by chemical reactions rather than the source, as it is primarily modulated by the atmospheric exchange reactions of the oxygen isotope.Therefore, δ 18 O-NO3 -has the potential to indicate the relative importance of various NO3 -formation pathways (Alexander et al., 2009;Elliott et al., 2009).To our knowledge, there are no reports on the N and O isotopic composition of nitrogenous species in cloudwater.
End of July 2015, a large-scale BB event occurred over eastern and northern China.We 95 took advantage of this special event to collect cloudwater samples at a high-altitude mountaintop site in the North China Plain, and to calibrate the isotopic signatures BB events leave in the N pool of clouds.Integrating cloudwater nitrogenous species isotope data (δ 15 N-NH4 + , δ 15 N-NO3 -and δ 18 O-NO3 -) in a Bayesian isotopic mixing model coupled with a newly developed computational quantum chemistry module (Chang et al., 2018), and using an isotopic mass balance approach, the 100 sources and production pathways of inorganic nitrogen in cloudwater were quantified.

Cloudwater Sample Collection
Mt. Tai (117 o 13ʹ E, 36 o 18ʹ N; 1545 m above sea level) is a world-recognized geopark of key natural, historical and cultural significance, located in the eastern North China Plain (Fig. 1).

105
It belongs to China's most important agricultural and industrial production areas, and the composition of the atmosphere near the mountain can be considered representative with regards to the quality and levels of atmospheric pollution in the region (Li et al., 2017;Liu et al., 2018).
to each sampling using high-purity deionized water.After sampling, cloud samples were filtered immediately using disposable syringe filters (0.45 µm) to remove any suspended particulate matter, and then stored in a freezer at -80 o C until further analysis.More details on the monitoring site and 115 sampling procedures can be found elsewhere (Li et al., 2017).

Bayesian Mixing Model Analysis
By taking the uncertainty associated with the N isotopic signatures of multiple sources and 145 associated isotope fractionation during (trans-)formations into account, the Bayesian method is more appropriate than simple linear mixing modeling to yield estimates on the source partitioning of a mixture like air pollutants (Chang et al., 2016;Chang et al., 2018).The relative contribution of each source in Bayesian theorem is expressed as: where θ(data|fq) and P(fq) represent the likelihood of the given mixed isotope signature, and the pre-determined probability of the given state of nature, based on prior information, respectively.The denominator represents the numerical approximation of the marginal probability of the data.Here the Bayesian mixing model MixSIR (stable isotope mixing models using sampling-importance-resampling) was used to disentangle the various potential NH3 and NOx 155 sources contributing to the cloudwater NH4 + and NO3 -pools, respectively, by forming the true probability distributions through generating 10000 solutions of source apportionment.Details on the model approach can be found in SI Text S1.
The measured δ 15 N-NO3 -values of cloudwater samples depend on the δ 15 N signatures of the original NOx sources (δ 15 N-NOx), the N isotope fractionation between nitrogen oxides (i.e., 160 NO and NO2 (Walters et al., 2016)), and the N isotope enrichment factor (εN) associated with the kinetic transformation of NOx to HNO3 (Walters and Michalski, 2015).εN is considered a hybrid of two dominant processes: one is the reaction of NO2 and OH radicals to form NO3 -, the other is the heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) with water to form NO3 -.We recently developed a quantum chemistry computation module to quantify the N fractionation during nitrate 165 formation, which had been validated by field measurements (Chang et al., 2018).Here this module was adopted to calculate the N isotope fractionation during NO3 -formation, and in turn to correct the raw δ 15 N-NO3 -values of cloudwater samples.
While the N isotopic source signatures of NOx are relatively well constrained (Table S1), this is not the case for NH3.We recently established a pool of isotopic source signatures of NH3 in 170 eastern China, in which livestock breeding and fertilizer application were identified to produce NH3 with a δ 15 N of -29.1 ± 1.7‰ and -50.0 ± 1.8‰, respectively (Chang et al., 2016).Although fossil-fuel combustion, urban waste, and natural soils also represent potential sources of NH3, their emission factors and relatively high concentrations in the ambient aerosols (Hoffmann et al., 2010).
In our study, the concentrations of NO3 -(r 2 = 0.55) and NH4 + (r 2 = 0.66) are strongly correlated with that of levoglucosan, suggesting that the pronounced increase of NO3 -and NH4 + levels observed here can at least be partly attributed to BB activities during the study period.Globally, BB accounts for around 10% of NH3 and NOx emissions (Benkovitz et al., 1996;Bouwman et al., 200 1997;Olivier et al., 1998;Schlesinger and Hartley, 1992).The N (and O) isotopic composition of cloudwater nitrogenous species was more (NH4 + ) or less (NO3 -) variant (Table 1), with the average δ 15 N values of 6.53‰ and -2.35‰ for NH4 + and NO3 -, respectively.The average δ 18 O-NO3 -value was 57.80‰.These values are generally different from gas, rainwater, and aerosol values measured worldwide (Fig. 2).Various atmospheric processes can influence the isotopic composition of atmospheric nitrogenous species including: during transport, partitioning between wet and dry components, and spatial gradients in atmospheric chemistry (Elliott et al., 2007;Hastings et al., 2003).These aspects may affect the ) and NH4 + in precipitation (Fang et al., 2011;Leng et al., 2018;Yang et al., 2014;Zhang et al., 2008), cloudwater (this study), PM2.5 (particulate matter with aerodynamic diameter less than 2.5 μm; (Lin et al., 2016;Park et al., 2018;Proemse et al., 2012;Smirnoff et al., 2012), and TSP (particulate matter with aerodynamic diameter less than 100 μm (Kundu et al., 2010;Savard et al., 2018;Yeatman et al., 2001) are shown.ε   ; e.g., Deng et al., 2018;Li et al., 2012).Dependent of the choice for it is evident that biomass burning represents an important NH3 source, validating our isotope approach.Our sampling site was located in the North China Plain, also known as the granary of China.Although non-agricultural NH3 emissions like on-road traffic are important in the urban atmosphere (Chang et al., 2016), their contribution must be considered insignificant with respect to fertilizer application and livestock breeding in this region (Kang et al., 2016).Besides, coal-260 based heating in China is suspended during summertime, and coal combustion has been demonstrated to be a minor contributor of total NH3 emissions (Li et al., 2016a).Hence the partitioning between the three main NH3 sources appears plausible.Moreover, existing emission inventory data confirm that the ratio of NH3 emissions in North China Plain from livestock breeding (1658 kt) and fertilizer application (1413 kt) was 1.17 (Zhang et al., 2010), which is very 265 close to our estimate (between 0.98 and 1.14) when range that we consider most plausible (i.e. between 25‰ and 33‰), the relative cloudwater NH4 + source partitioning between biomass burning, fertilizer application, and livestock is 32.9 ± 4.6%, 32.9 ± 3.0%, and 34.2 ± 1.6%, respectively (indicated as red square in Fig. 3a)., the overall contribution of various NOx sources to NO3 -in cloudwater can be estimated (Fig. 3c).As was expected, biomass burning was the largest contributor (28.2 ± 2.7%), followed by on-road traffic (27.1 ± 2.2%), coal combustion (26.8 ± 285 3.4%), and biogenic soil (17.9 ± 3.9%).The fundamental importance of biomass burning-emitted NOx to NO3 -in cloudwater is supported by the observed correlation between the concentrations of levoglucosan and biomass burning-derived NO3 -(r 2 = 0.66).The average contribution ratio of coal combustion and on-road transportation to NOx emissions in our study (0.99) is slightly lower than that calculated from regional emission inventories (9.0 Tg/7.4 Tg = 1.22) (Zhao et al., 2013).The ) involves two oxidation pathways where γ/(1-γ) represents the contribution ratio of the isotope fractionation associated with the formation of HNO3/NO3 -through the "OH+NO2" pathway (   As for the δ 18 O value of HNO3 formed during hydrolysis of N2O5, 5/6 of the O atoms is derived from O3 and 1/6 from OH (Hastings et al., 2003): f vary between 0.2 and 0.95 (Walters and Michalski, 2015).δ 18 O-X is the O isotopic composition of X.
The range of δ 18 O-H2O can be approximated using an estimated tropospheric water vapor δ 18 O range of -25‰-0‰ (Zong et al., 2017).The δ 18 O of NO2 and N2O5 varies between 90‰ and 122‰ 315 (Zong et al., 2017).Using the measured δ 18 O and equations 1-4 (and the assumptions above), we can calculate γ, and the relative importance of the two oxidation pathways of NO3 -formation (Fig. 3d).On average, 57% NO3 -formation can be attributed to the "NO2 + OH" pathway, and 43% to the "N2O5 325 + H2O" pathway.In the low-latitude regions, where atmospheric OH concentrations are highest, particulate -production via the "NO2 + OH" pathway predominates (up to 87%) (Alexander et al., 2009).Sampling during summertime, oxidation of NO2 through OH was expected to be the dominant pathway of nitrate formation, in accordance with observations from the subtropics (Hastings et al., 2003).However, our results highlight that N2O5 hydrolysis can be an almost 330 equally important process as the oxidization of NO2 with OH with regards to the NO3 -formation in cloudwater (Wang et al., 2017b).

Figure 1 .
Figure 1.(a) Location of Mt.Tai (triangle) and twenty-four 48-h back trajectories (black lines) of air masses arriving at Mt. Tai on 1 August 2015 (4:00 UTC) at an altitude 1500-m a.s.l (http://ready.arl.noaa.gov/HYSPLIT.php).The base map of land use in China was modified from 210 the original emission source of NOx, seasonality of oxidation pathways, isotope fractionation Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1196Manuscript under review for journal Atmos.Chem.Phys.This is just a preview and not the published paper.c Author(s) 2018.CC BY 4.0 License.

235 Figure 2 .
Figure 2. (a) Observed range of typical δ 18 O and δ 15 N values of NO3 -and NOx for different sources (adapted from Fenech et al. (2012)).The red squares represent nitrate isotope data in cloudwater (this study).(b) The 25 th percentiles, median and 75 th percentiles for the δ 15 N values of the ambient NH3(Chang et al., 2016; Felix et al., 2013;Savard et al., 2018;Smirnoff et al., assessment of the sources and formation of nitrogenous species in clouds Using the MixSIR model, the relative contribution of four NH3 sources to NH4 + can be calculated, based on the isotope data of ambient δ 15 N-NH4 + , and considering the N fractionation and prior information on the site.As upper limit for the N isotope enrichment factor associated with the conversion of NH3 to NH4 + ( 4 3 NH NH ε   ), we assumed 33‰ when using MixSIR, but also 250 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1196Manuscript under review for journal Atmos.Chem.Phys.This is just a preview and not the published paper.c Author(s) 2018.CC BY 4.0 License.

Figure 3 .
Figure 3. (a) Source partitioning estimates for NH4 + in cloudwater as a function of Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1196Manuscript under review for journal Atmos.Chem.Phys.This is just a preview and not the published paper.c Author(s) 2018.CC BY 4.0 License.
290apparent difference is likely real, and reflects the fact that NOx emissions by anthropogenic activities changed significantly since 2010: a 17% total emission decrease between 2010 and 2017 can primarily be attributed to upgraded emission standards and new "ultra-low emission" techniques in the coal-fired power plant sector, given that traffic-emitted NOx likely increased as a consequence of the continuous expansion of auto trade market during the last decade(Chang et   295   al., 2018).In turn, our source partitioning estimate probably reflects the most updated status of NOx emissions in China, where transportation-related NOx emissions have reached levels that are comparable to NOx emissions by coal combustion.In this regard, our study demonstrates that Bayesian-based isotopic mixing modeling can be an effective and timely approach to track rapid emissions changes of NOx in a fast developing country like China.300TheO isotope fractionation during the conversion of NOx to HNO3/NO3 - . Phys.Discuss., https://doi.org/10.5194/acp-2018-1196Manuscript under review for journal Atmos.Chem.Phys.This is just a preview and not the published paper.c Author(s) 2018.CC BY 4.0 License. .The δ 18 O value of HNO3 produced by the former process reflects the O atom partitioning of 2/3 O3 and 1/3 OH: fraction of NO2 in the total NOx pool.Values for 2 NO B, C, and D are experimental constants over the temperature range of 150-450 K. Based on Equations 1-4 and measured values for δ 18 O-NO3 -of cloudwater, a Monte Carlo simulation was performed to generate 10000 feasible solutions.The error between predicted and measured δ 18 O was less than 0.5‰.

4
Conclusionsthe first time, we measured the isotopic composition of nitrogenous species in cloudwater at the summit of Mt.Tai during a long-lasting biomass burning event, in order to 335 investigate the sources and process involved in cloudwater NO3 -/NH4 + formation, and in turn to test our isotope-balance approach to constrain N source partitioning in cloudwater.Using a Bayesian isotope mixing model, the δ 15 N-based estimates confirm that at least transiently biomass burning related NH3 and NOx emissions is a major source of cloudwater N.Moreover, our data are in accordance with regional emission inventories for both NH3 and NOx, validating the Bayesian 340 isotope mixing model approach.Based on cloud water nitrate δ 18 O measurements, the reaction of NO2 with OH turned out to be the dominant pathway to form cloud nitrate, yet the contribution from the heterogeneous hydrolysis of N2O5 to NO3 -is almost equally important.Our study Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1196Manuscript under review for journal Atmos.Chem.Phys.This is just a preview and not the published paper.c Author(s) 2018.CC BY 4.0 License.underscores the value of cloud-water dissolved inorganic nitrogen isotopes as carrier of quantitative information on regional NOx emissions.It sheds light on the origin and production 345 pathways of nitrogenous species in clouds and emphasizes the importance of BB-derived nitrogenous species as cloud condensation nuclei in China's troposphere.Moreover, it highlights the rapid evolution of NOx emissions in China.Despite an overall reduction in total anthropogenic NOx emission due to effective emission control actions and stricter emission standards for vehicles, the relative contribution of transportation to total NOx emissions has increased over the last decade 350 and may already have exceeded emissions from the power sector.

Table 1 .
Sampling details and results of chemical and isotopic analysis for collected cloudwater samples.