Atmospheric Δ<sup>17</sup>O(NO<sub>3</sub><sup>−</sup>) reveals nocturnal chemistry dominates nitrate production in Beijing haze

Abstract

Abstract. The rapid mass increase of atmospheric nitrate is a critical driving force for the occurrence of fine-particle pollution (referred to as haze hereafter) in Beijing. However, the exact mechanisms for this rapid increase of nitrate mass have not been well constrained from field observations. Here we present the first observations of the oxygen-17 excess of atmospheric nitrate (Δ17O(NO3-)) collected in Beijing haze to reveal the relative importance of different nitrate formation pathways, and we also present the simultaneously observed δ15N(NO3-). During our sampling period, 12 h averaged mass concentrations of PM2.5 varied from 16 to 323 µg m−3 with a mean of (141±88(1SD)) µg m−3, with nitrate ranging from 0.3 to 106.7 µg m−3. The observed Δ17O(NO3-) ranged from 27.5 ‰ to 33.9 ‰ with a mean of (30.6±1.8) ‰, while δ15N(NO3-) ranged from −2.5 ‰ to 19.2 ‰ with a mean of (7.4±6.8) ‰. Δ17O(NO3-)-constrained calculations suggest nocturnal pathways (N2O5+H2O/Cl- and NO3+HC) dominated nitrate production during polluted days (PM2.5≥75 µg m−3), with a mean possible fraction of 56–97 %. Our results illustrate the potentiality of Δ17O in tracing nitrate formation pathways; future modeling work with the constraint of isotope data reported here may further improve our understanding of the nitrogen cycle during haze.

Influences of Atmospheric Pollution on the Contributions of Major Oxidation Pathways to PM2.5 Nitrate Formation in Beijing

“…Lulin in central Taiwan (2.7–31.4‰, mean = 17‰; Guha et al, ). In this study, Δ 17 O‐NO 3 − values of PM 2.5 in winter (27.2 ± 3.7‰) were similar with those previously observed between October 2014 to January 2015 in Beijing (29.1 ± 1.3‰; He et al, ), and both were significantly higher than those in summer (24.2 ± 1.32‰; Figure ). Such seasonal pattern is consistent with that of Δ 17 O‐NO 3 − records in ice cores (Geng et al, ; Kunasek et al, ).…”

“…This pathway (termed as the OH· pathway in this study) is also most prevalent in summer when the concentrations of photochemically produced OH· are at their highest (Calvert et al, 1985). Meanwhile, NO 2 can also react with O 3 to form NO 3 radicals (R6; Alexander et al, 2009;He et al, 2018).…”

Formation mechanisms of atmospheric nitrate and sulfate during the winter haze pollution periods in Beijing: gas-phase, heterogeneous and aqueous-phase chemistry

“…Forward mode calculates the equilibrium partitioning given the total concentrations of gas and aerosol species, whereas reverse mode involves predicting the thermodynamic compositions based only on the concentrations of aerosol components. Forward mode was adopted in this study because reverse mode calculations have been verified to be not suitable to characterize aerosol acidity (Guo et al, 2015;Hennigan et al, 2015;Murphy et al, 2017;Pathak et al, 2004;Weber et al, 2016). The ISORROPIA II model is available in "metastable" or "solid + liquid" state solutions.…”

“…Atmospheric nitrate is considered to be mainly from NO 2 oxidation by OH radicals in the gas phase, heterogeneous uptake of NO 2 on the surface of particles, and heterogeneous hydrolysis of N 2 O 5 on wet aerosols or chloride-containing aerosols (He et al, 2014(He et al, , 2018Nie et al, 2014;Ravishankara, 1997;Wang et al, 2018a). Atmospheric N 2 O 5 is usually produced by the reaction of NO 3 radicals with NO 2 .…”

“…Atmospheric N 2 O 5 is usually produced by the reaction of NO 3 radicals with NO 2 . Since both NO 3 radicals and N 2 O 5 are easily photolytic during the daytime, the heterogeneous hydrolysis of N 2 O 5 is a nighttime pathway for the formation of atmospheric nitrate (He et al, 2018;Wang et al, 2018a). As shown in Fig.…”