ARTICLE OPEN A diurnal story of Δ 17 O(NO  3 ) in urban Nanjing and its implication for nitrate aerosol formation Yan-Lin Zhang 1,2,3 ✉ , Wenqi Zhang 1,2,3,4 , Mei-Yi Fan 1,2,3 , Jianghanyang Li 5 , Huan Fang 5 , Fang Cao 1,2,3 , Yu-Chi Lin 1,2,3 , Benjamin Paul Wilkins 5 , Xiaoyan Liu 1,2,3 , Mengying Bao 1,2,3 , Yihang Hong 1,2,3 and Greg Michalski 5 Inorganic nitrate production is critical in atmospheric chemistry that reflects the oxidation capacity and the acidity of the atmosphere. Here we use the oxygen anomaly of nitrate (Δ 17 O(NO  3 )) in high-time-resolved (3 h) aerosols to explore the chemical mechanisms of nitrate evolution in fine particles during the winter in Nanjing, a megacity of China. The continuous Δ 17 O(NO  3 ) observation suggested the dominance of nocturnal chemistry (NO 3 + HC/H 2 O and N 2 O 5 + H 2 O/Cl - ) in nitrate formation in the wintertime. Significant diurnal variations of nitrate formation pathways were found. The contribution of nocturnal chemistry increased at night and peaked (72%) at midnight. Particularly, nocturnal pathways became more important for the formation of nitrate in the process of air pollution aggravation. In contrast, the contribution of daytime chemistry (NO 2 + OH/H 2 O) increased with the sunrise and showed a highest fraction (48%) around noon. The hydrolysis of N 2 O 5 on particle surfaces played an important role in the daytime nitrate production on haze days. In addition, the reaction of NO 2 with OH radicals was found to dominate the nitrate production after nitrate chemistry was reset by the precipitation events. These results suggest the importance of high-time- resolved observations of Δ 17 O(NO  3 ) for exploring dynamic variations in reactive nitrogen chemistry. npj Climate and Atmospheric Science (2022)5:50 ; https://doi.org/10.1038/s41612-022-00273-3 INTRODUCTION Nitrate (NO  3 ) and its precursor NOx (NOx = NO + NO 2 ) play a crucial role in atmospheric chemical processes and the formation of PM 2.5 , fine particles with diameter less than 2.5 μm 1,2 . Tropo- spheric NOx oxidation drives the formation of ozone (O 3 ) and recycle of hydroxyl radicals (OH) that control the atmospheric self- cleansing capacity 3 . Majority of NOx emitted from various sources are finally converted into nitric acid (HNO 3 ) and organic nitrate (e.g., RONO 2 ) through atmospheric oxidation processes by oxidants (e.g., O 3 , OH, HO 2 , and RO 2 ) 4 . HNO 3 lowers the pH of the precipitation and increases the risk of forming acid rain 5 . Furthermore, HNO 3 easily be transformed into nitrate particles through atmospheric reactions with alkaline ammonia that in turn influence the chemical composition and the size of existing particles, affecting the formation of clouds and precipitation as well 3,6 . RONO 2 can partition into the particle phase (RONO 2(p) ) and then is removed from the atmosphere by deposition to the surface or through hydrolysis to form inorganic nitrate and alcohols 7,8 . Atmospheric nitrate in gas phase (HNO 3(g) ), liquid phase (HNO 3(aq) ) and particulates (NO  3 (p) ) are eventually removed through wet/dry deposition. Thus investigating the mechanism of NOx-NO  3 conversion is important to the study of atmospheric chemistry. The conversion of NOx to NO  3 is a combination of the NOx cycle (Supplementary Note 1) and nitrate production processes. During the day, OH radical is easily generated under the strong sunlight and HNO 3(g) is then formed through the NO 2 + OH reaction 9 . NO 2 can be hydrolyzed on surfaces to produce HNO 3(aq) 10 , which was found to be a weak source of nitrate formation on severe haze days in winter in the North China Plain (NCP) 11,12 . In addition, NO 2 can also react with O 3 to form NO 3 radicals and then NO 3 directly reacts with hydrocarbon (HC) and dimethylsulfide (DMS) or be hydrolyzed on surfaces to produce HNO 3 13–15 . This reaction occurred at night because NO 3 radical is easily photolyzed to NO 2 under sunlight 16 . And the contribution of NO 3 + DMS is small in non-coastal areas due to the low mixing ratio of DMS 17 . Dinitrogen pentoxide (N 2 O 5 ), a nocturnal NOx reservoir, can react on airborne particle surface to produce only HNO 3(aq) or both NO  3 (p) and nitryl chloride (ClNO 2 ) 18 . Other potential formation mechanisms of nitrate particles, like the hydrolysis of organic nitrates (RONO 2 ) and halogen nitrates (XNO 3 ), might be important in coastal regions or the rainforest areas like Amazonia 13 . NO 2 þ OH þ M ! HNO 3 þ M (1) 2NO 2 þ H 2 O surface ð Þ! HNO 3 liquid ð Þþ HONO (2) NO 2 þ O 3 ! NO 3 þ O 2 (3) NO 3 þ HC=DMS ! HNO 3 þ Others (4) NO 3 þ H 2 O surface ð Þ! HNO 3 liquid ð Þþ OH (5) NO 2 þ NO 3 $ N 2 O 5 (6) N 2 O 5 þ H 2 O surface ð Þ! 2HNO 3 liquid ð Þ (7) N 2 O 5 þ Cl  surface ð Þ! NO  3 particle ð Þþ ClNO 2 (8) 1 Atmospheric Environment Center, International Joint Laboratory on Climate and Environment Change (ILCEC), Nanjing University of Information Science & Technology, Nanjing 210044, China. 2 School of Applied Meteorology, Nanjing University of Information Science & Technology, Nanjing 210044, China. 3 Key Laboratory of Meteorological Disaster, Ministry of Education (KLME)/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science & Technology, Nanjing 210044, China. 4 Ningbo Meteorological Bureau of Zhejiang Province, Ningbo 315012, China. 5 Department of Earth, Atmospheric and Planetary Sciences and Department of Chemistry, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907, USA. ✉ email: zhangyanlin@nuist.edu.cn www.nature.com/npjclimatsci Published in partnership with CECCR at King Abdulaziz University 1234567890():,;