Global inorganic nitrate production mechanisms: comparison of a global model with nitrate isotope observations

Abstract

Abstract. The formation of inorganic nitrate is the main sink for nitrogen oxides (NOx = NO + NO2). Due to the importance of NOx for the formation of tropospheric oxidants such as the hydroxyl radical (OH) and ozone, understanding the mechanisms and rates of nitrate formation is paramount for our ability to predict the atmospheric lifetimes of most reduced trace gases in the atmosphere. The oxygen isotopic composition of nitrate (Δ17O(nitrate)) is determined by the relative importance of NOx sinks and thus can provide an observational constraint for NOx chemistry. Until recently, the ability to utilize Δ17O(nitrate) observations for this purpose was hindered by our lack of knowledge about the oxygen isotopic composition of ozone (Δ17O(O3)). Recent and spatially widespread observations of Δ17O(O3) motivate an updated comparison of modeled and observed Δ17O(nitrate) and a reassessment of modeled nitrate formation pathways. Model updates based on recent laboratory studies of heterogeneous reactions render dinitrogen pentoxide (N2O5) hydrolysis as important as NO2 + OH (both 41 %) for global inorganic nitrate production near the surface (below 1 km altitude). All other nitrate production mechanisms individually represent less than 6 % of global nitrate production near the surface but can be dominant locally. Updated reaction rates for aerosol uptake of NO2 result in significant reduction of nitrate and nitrous acid (HONO) formed through this pathway in the model and render NO2 hydrolysis a negligible pathway for nitrate formation globally. Although photolysis of aerosol nitrate may have implications for NOx, HONO, and oxidant abundances, it does not significantly impact the relative importance of nitrate formation pathways. Modeled Δ17O(nitrate) (28.6±4.5 ‰) compares well with the average of a global compilation of observations (27.6±5.0 ‰) when assuming Δ17O(O3) = 26 ‰, giving confidence in the model's representation of the relative importance of ozone versus HOx (= OH + HO2 + RO2) in NOx cycling and nitrate formation on the global scale.

Differentiation Between Nitrate Aerosol Formation Pathways in a Southeast Chinese City by Dual Isotope and Modeling Studies

“…And then we can calculate the source apportionment (Zong et al, 2017 Previous studies suggested that the oxidation pathway of NO 3 + HC/DMS (DMS is a major in coastal environments) to form HNO 3 (3) is also very important (Alexander et al, 2009;He et al, 2018;Walters & Michalski, 2016;Wang et al, 2019). However, few studies have quantitatively calculated its contribution (Alexander et al, 2009(Alexander et al, , 2020Wang et al, 2019). In our second model, we used a δ 15 N and δ 18 O array of NO 3 − in PM 2.5 to determine the contributions of all three major oxidation pathways using the MixSIAR model.…”

“…Nitrate (NO 3 − ) is an important component of PM 2.5 (aerodynamic diameters <2.5 μm) in many Chinese urban atmospheres (Guo et al, 2020;Huang et al, 2014;Luo et al, 2019;Wang et al, 2019). As a precursor of NO 3 − , NO x (NO and NO 2 ) is a major component of reactive nitrogen in the atmosphere, mainly derived from anthropogenic activities, such as combustion of fossil fuels (Alexander et al, 2009(Alexander et al, , 2020Galloway et al, 2008;Xiao et al, 2015). Tropospheric ozone (O 3 ), formed in the photochemical cycle of NO x , and its byproduct, the hydroxyl radical (OH), largely determine the oxidizing capacity of the atmosphere and the lifetimes of most reduced trace gases (Thompson, 1992).…”

A Complete Isotope (δ15N, δ18O, Δ17O) Investigation of Atmospherically Deposited Nitrate in Glacial‐Hydrologic Systems Across the Third Pole Region

“…Low Δ 17 O values in atmospheric nitrate were reported by Brothers et al (2008) in Ecuador (13-22‰) and were also observed in Taiwan (3-31‰) (Guha et al, 2017) and the Central TP (7-24‰) (Xia et al, 2019). The extremely low Δ 17 O values in Taiwan have not yet been simulated by a state-of-the-art global chemical transport model (Goddard Earth Observing System-Chem) (Alexander et al, 2009(Alexander et al, , 2020. Such low nitrate Δ 17 O values were also observed in surface snow samples collected from Greenland (~5‰) (Fibiger et al, 2013) and inland Antarctica (~13‰) (Noro et al, 2018) and in an ice core drilled from the Quelccaya Ice Cap at Peru (~6‰) (Buffen et al, 2014), which were interpreted as preserved atmospheric signals.…”

Model bias in simulating major chemical components of PM&lt;sub&gt;2.5&lt;/sub&gt; in China

“…2b-c). Nighttime production of nitrate by the heterogeneous uptake of N2O5 and NO2 is a major pathway of nitrate production in northern China (Wang et al, 2018;Alexander et al, 2020). The 2-5 times greater concentrations of simulated nitrate at night suggest overpredicted nighttime production, underestimated boundary-layer dilution, or underestimated removal of nitrate.…”

“…Moreover, laboratory and field measurements show that the NO2 uptake coefficient (γNO2) on the aerosol surface ranges from 10 -8 to 10 -4 (Spataro and Ianniello, 2014 and references therein; M. Li et al, 2019 and references therein). The default GEOS-Chem model uses a relatively high γNO2 of 10 -4 , which may cause the underestimated NO2 concentrations as well as the overestimated concentrations of HNO3, HONO, and nitrate and needs further evaluation (Alexander et al, 2020). For NH3, the model underestimates its monthly mean concentrations in Beijing (Fig.…”