Insights into source origins and formation mechanisms of nitrate during winter haze episodes in the Yangtze River Delta.

Nitrate became the most significant component of secondary inorganic aerosols (SNA, the sum of sulfate, nitrate and ammonium ions) as the emissions of sulfur dioxide (SO2) have been greatly reduced in China in recent years. In the Yangtze River Delta (YRD), nitrate could contribute 56% of SNA and 34% of total PM2.5 during haze episodes. In this study, a modified Community Multiscale Air Quality (CMAQ) model was used to provide a comprehensive understanding of nitrate source and formation under severe pollution during winter 2015 and 2016. Three haze episodes (HEP1, HEP2 and HEP3) and one clean episode (CEP) were selected to investigate the emission sector and regional contributions to nitrate at six environmental monitoring sites in the YRD. Source apportionment results showed that industry (35%), transportation (32%) and power (28%) sectors were the important sources of nitrate during haze episodes. Regional transport (60-98%) was responsible for the high nitrate concentrations in the YRD. During haze episodes, the high ozone production (PO3) rate (up to 700 ppb/h) and hydroxyl radicals (OH) removal rate (up to 9 ppb/h) were observed in the daytime indicating the important atmospheric oxidation capacity in the YRD. Also, the nitrogen oxidation ratio (NOR) analysis elucidated that daytime photochemistry played an important role in nitrate formation and the heterogeneous chemistry enhanced the high nitrate at night. Results from emission scenario analysis demonstrated that averaged nitrate concentration in Shanghai decreased by 18% during haze episodes under emission reductions of 20% NOx, NH3 and VOC in the YRD, and Shandong, Shanxi, Henan and Hebei provinces. Emission reduction on the regional scale (one city and its surrounding areas) is an efficient strategy to reduce nitrate concentration in the YRD.