Atmospheric Δ 17 O(NO 3 − ) 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 have not been well constrained from field observations. Here we present the first observations of the oxygen-17 excess of atmospheric nitrate ( Δ 17 O ( NO 3 - ) ) collected in Beijing haze to reveal the relative importance of different nitrate formation pathways, and we also present the simultaneously observed δ 15 N ( NO 3 - ) . During our sampling period, 12 h averaged mass concentrations of PM 2.5 varied from 16 to 323 µ g m −3 with a mean of ( 141±88 (1SD)) µ g m −3 , with nitrate ranging from 0.3 to 106.7 µ g m −3 . The observed Δ 17 O ( NO 3 - ) ranged from 27.5 ‰ to 33.9 ‰ with a mean of ( 30.6±1.8 ) ‰, while δ 15 N ( NO 3 - ) ranged from −2.5 ‰ to 19.2 ‰ with a mean of ( 7.4±6.8 ) ‰. Δ 17 O ( NO 3 - ) -constrained calculations suggest nocturnal pathways ( N 2 O 5 + H 2 O / Cl - and NO 3 +HC ) dominated nitrate production during polluted days ( PM 2.5 ≥75 µ g m −3 ), with a mean possible fraction of 56–97 %. Our results illustrate the potentiality of Δ 17 O 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.