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

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 has been not 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 (1σ)) 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 the mean possible fraction of 56 − 97 %. For δ15N(NO3−), we found that a combined effect of variability in NOX sources and isotopic exchange between NO and NO2 is likely to be most responsible for its variations. Our results illustrate the potentiality of isotope in tracing NOX sources and nitrate formation pathways, future modelling work with the constraint of isotope data reported here may further improve our understanding of nitrogen cycle during haze.