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

• Published in: Atmospheric chemistry and physics Vol. 20; no. 6; pp. 3859 - 3877
• Main Authors: Alexander, Becky, Sherwen, Tomás, Holmes, Christopher D, Fisher, Jenny A, Chen, Qianjie, Evans, Mat J, Kasibhatla, Prasad
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 31-03-2020; Copernicus Publications
• Subjects: Atmosphere; Atmospheric chemistry; Biodiesel fuels; Chemical composition; Chemistry; Comparative analysis; Composition; Gases; Hydrolysis; Hydroxides; Hydroxyl radicals; Isotope composition; Isotopes; Laboratories; Nitrates; Nitric acids; Nitrogen (Chemical element); Nitrogen compounds; Nitrogen dioxide; Nitrogen oxides; Nitrous acid; Oxidants; Oxides; Oxidizing agents; Oxygen; Ozone; Photochemicals; Photolysis; Trace gases; Uptake
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-20-3859-2020

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.