Atmos. Chem. Phys., 20, 3859–3877, 2020 https://doi.org/10.5194/acp-20-3859-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Global inorganic nitrate production mechanisms: comparison of a global model with nitrate isotope observations Becky Alexander1, Tomás Sherwen2,3, Christopher D. Holmes4, Jenny A. Fisher5, Qianjie Chen1,a, Mat J. Evans2,3, and Prasad Kasibhatla6 1Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA 2Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York YO10 5DD, UK 3National Center for Atmospheric Science, University of York, York YO10 5DD, UK 4Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA 5Centre for Atmospheric Chemistry, University of Wollongong, Wollongong, New South Wales 2522, Australia 6Nicholas School of the Environment, Duke University, Durham, NC 27708, USA anow at: Department of Chemistry, University of Michigan, Ann Arbor, MI 48109, USA Correspondence: Becky Alexander ([email protected]) Received: 2 May 2019 – Discussion started: 8 May 2019 Revised: 15 February 2020 – Accepted: 17 February 2020 – Published: 31 March 2020 Abstract. The formation of inorganic nitrate is the main sink pathway for nitrate formation globally. Although photolysis for nitrogen oxides (NOx D NO C NO2). Due to the impor- of aerosol nitrate may have implications for NOx, HONO, tance of NOx for the formation of tropospheric oxidants such and oxidant abundances, it does not significantly impact the as the hydroxyl radical (OH) and ozone, understanding the relative importance of nitrate formation pathways. Modeled mechanisms and rates of nitrate formation is paramount for 117O(nitrate) (28:6 ± 4:5 ‰) compares well with the aver- our ability to predict the atmospheric lifetimes of most re- age of a global compilation of observations (27:6 ± 5:0 ‰) 17 duced trace gases in the atmosphere. The oxygen isotopic when assuming 1 O.O3/ D 26 ‰, giving confidence in the composition of nitrate (117O(nitrate)) is determined by the model’s representation of the relative importance of ozone relative importance of NOx sinks and thus can provide an versus HOx (D OH C HO2 C RO2) in NOx cycling and ni- observational constraint for NOx chemistry. Until recently, trate formation on the global scale. the ability to utilize 117O(nitrate) observations for this pur- pose was hindered by our lack of knowledge about the oxy- 17 gen isotopic composition of ozone (1 O.O3/). Recent and 17 spatially widespread observations of 1 O.O3/ motivate an 1 Introduction updated comparison of modeled and observed 117O(nitrate) and a reassessment of modeled nitrate formation pathways. Nitrogen oxides (NOx D NO C NO2) are a critical ingredient Model updates based on recent laboratory studies of hetero- for the formation of tropospheric ozone (O3). Tropospheric geneous reactions render dinitrogen pentoxide (N2O5) hy- ozone is a greenhouse gas, is a major precursor for the hy- drolysis as important as NO2 C OH (both 41 %) for global droxyl radical (OH), and is considered an air pollutant due to inorganic nitrate production near the surface (below 1 km al- its negative impacts on human health. The atmospheric life- titude). All other nitrate production mechanisms individually time of NOx is determined by its oxidation to inorganic and organic nitrate. The formation of inorganic nitrate (HNO3(g) represent less than 6 % of global nitrate production near the − surface but can be dominant locally. Updated reaction rates and particulate NO3 ) is the dominant sink for NOx globally, for aerosol uptake of NO2 result in significant reduction of while formation of organic nitrate may be significant in rural nitrate and nitrous acid (HONO) formed through this path- and remote continental locations (Browne and Cohen, 2014). way in the model and render NO2 hydrolysis a negligible Organic nitrate as a sink for NOx may be becoming more im- portant in regions in North America and Europe where NOx Published by Copernicus Publications on behalf of the European Geosciences Union. 3860 B. Alexander et al.: Global inorganic nitrate isotopes emissions have declined (Zare et al., 2018). Uncertainties in 2018; Chen et al., 2019) boundary layer, with implications the rate of oxidation of NOx to nitrate have been shown to for ozone and OH (Kasibhatla et al., 2018). represent a significant source of uncertainty for ozone and Organic nitrates form during reaction of NOx and NO3 OH formation in models (e.g., Newsome and Evans, 2017), with biogenic volatile organic compounds (BVOCs) and with implications for our understanding of the atmospheric their oxidation products (organic peroxy radicals, RO2) lifetime of species such as methane, whose main sink is re- (Browne and Cohen, 2014; Liang et al., 1998). Products of action with OH. these reactions include peroxy nitrates (RO2NO2) and alkyl NOx is emitted to the atmosphere primarily as NO by and multifunctional nitrates (RONO2) (O’Brien et al., 1995). fossil fuel and biomass/biofuel burning, soil microbes, and Peroxy nitrates are thermally unstable and decompose back lightning. Anthropogenic sources from fossil fuel and biofuel to NOx on the order of minutes to days at warm temperatures. burning and from the application of fertilizers to soil for agri- Decomposition of longer-lived peroxy nitrates such as perox- culture currently dominate NOx sources to the atmosphere yacetyl nitrate (PAN) can provide a source of NOx to remote (Jaeglé et al., 2005). After emission, NO is rapidly oxidized environments (Singh et al., 1992). The fate of RONO2 is un- to NO2 by ozone (O3), peroxy (HO2) and hydroperoxy radi- certain. First-generation RONO2 is oxidized to form second- cals (RO2), and halogen oxides (e.g., BrO). During the day- generation RONO2 species with a lifetime of about a week time, NO2 is rapidly photolyzed to NO C O at wavelengths for the first-generation species with ≥ 4 carbon atoms and (λ) < 398 nm. NOx cycling between NO and NO2 proceeds up to several weeks for species with fewer carbon atoms several orders of magnitude faster than oxidation of NOx to (e.g., days to weeks for methyl nitrate) (Fisher et al., 2018). nitrate during the daytime (Michalski et al., 2003). Subsequent photolysis and oxidation of second-generation Formation of inorganic nitrate is dominated by oxida- RONO2 species can lead to the recycling of NOx (Müller tion of NO2 by OH during the day and by the hydrolysis et al., 2014), although recycling efficiencies are highly un- of dinitrogen pentoxide (N2O5) at night (Alexander et al., certain (Horowitz et al., 2007;Paulot et al., 2009). RONO2 2009). Recent implementations of reactive halogen chem- can also partition to the particle phase (pRONO2) contribut- istry in models of tropospheric chemistry show that forma- ing to organic aerosol formation (Xu et al., 2015). pRONO2 tion of nitrate from the hydrolysis of halogen nitrates (XNO3, is removed from the atmosphere by deposition to the surface where X D Br, Cl, or I) is also a sink for NOx with im- or through hydrolysis to form inorganic nitrate and alcohols plications for tropospheric ozone, OH, reactive halogens, (Rindelaub et al., 2015; Jacobs et al., 2014). and aerosol formation (Schmidt et al., 2016; Sherwen et The oxygen isotopic composition (117O D δ17 − 0:52 × al., 2016; Saiz-Lopez et al., 2012; Long et al., 2014; Par- δ18O) of nitrate is determined by the relative importance of rella et al., 2012; von Glasow and Crutzen, 2004; Yang et oxidants leading to nitrate formation from the oxidation of al., 2005). Other inorganic nitrate formation pathways in- NOx (Michalski et al., 2003). Observations of the oxygen clude hydrogen abstraction of hydrocarbons by the nitrate isotopic composition of nitrate (117O(nitrate)) have been radical (NO3), heterogeneous reaction of N2O5 with particu- used to quantify the relative importance of different nitrate − late chloride (Cl ), heterogeneous uptake of NO2 and NO3, formation pathways and to assess model representation of the direct oxidation of NO to HNO3 by HO2, and hydrolysis chemistry of nitrate formation in the present day (Alexan- of organic nitrate (Atkinson, 2000). Inorganic nitrate parti- der et al., 2009; Michalski et al., 2003; Costa et al., 2011; − tions between the gas (HNO3(g)) and particle (NO3 ) phases, Ishino et al., 2017; Morin et al., 2009, 2008, 2007; Savarino with its relative partitioning dependent upon aerosol abun- et al., 2007, 2013; Kunasek et al., 2008; McCabe et al., dance, aerosol liquid water content, aerosol chemical com- 2007; Hastings et al., 2003; Kaiser et al., 2007; Brothers position, and temperature. Inorganic nitrate is lost from the et al., 2008; Ewing et al., 2007) and in the past from ni- atmosphere through wet or dry deposition to the Earth’s sur- trate archived in ice cores (Sofen et al., 2014; Alexander et face with a global lifetime against deposition on the order of al., 2004; Geng et al., 2014, 2017). Ozone-influenced reac- 17 3–4 d (Park et al., 2004). tions in NOx oxidation lead to high 1 O(nitrate) values 17 Formation of inorganic nitrate was thought to be a perma- while HOx-influenced reactions lead to 1 O(nitrate) near nent sink for NOx in the troposphere due to the slow photol- zero. Oxidation by XO (where X D Br, Cl, or I) leads to ysis of nitrate compared to deposition. However, laboratory 117O(nitrate) values similar to reactions with ozone because − C and field studies have shown that NO3 adsorbed on surfaces the oxygen atom in XO is derived from the reaction X O3. 17 is photolyzed at rates much higher than HNO3(g) (Ye et al., Therefore, 1 O(nitrate) is determined by the relative impor- − C D C C 2016). For example, the photolysis of NO3 in snow grains tance of O3 XO versus HOx ( OH HO2 RO2) in both on ice sheets has a profound impact on the oxidizing capacity NOx cycling and oxidation to nitrate.