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. The rapid mass increase of atmospheric nitrate is a critical driving forcefor the occurrence of fine-particle pollution (referred to as haze hereafter)in Beijing. However, the exact mechanisms for this rapid increase of nitratemass have not been well constrained from field observations. Here we presentthe first observations of the oxygen-17 excess of atmospheric nitrate ( Δ 17 O ( NO 3 - ) ) collected in Beijing haze to reveal therelative importance of different nitrate formation pathways, and we alsopresent the simultaneously observed δ 15 N ( NO 3 - ) . Duringour sampling period, 12 h averaged mass concentrations of PM2.5 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 from27.5 ‰ 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 amean of ( 7.4±6.8 ) ‰. Δ 17 O ( NO 3 - ) -constrained calculations suggest nocturnal pathways( N 2 O 5 + H 2 O / Cl - and NO3+HC ) dominatednitrate production during polluted days ( PM2.5≥75 µ g m −3 ), with a mean possible fraction of 56–97 %.Our results illustrate the potentiality of Δ17O in tracing nitrateformation pathways; future modeling work with the constraint of isotope datareported here may further improve our understanding of the nitrogen cycle duringhaze.

atmospheric chemistry and physics

Atmospheric Δ&amp;lt;sup&amp;gt;17&amp;lt;/sup&amp;gt;O(NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;&amp;lt;sup&amp;gt;−&amp;lt;/sup&amp;gt;) reveals nocturnal chemistry dominates nitrate production in Beijing haze