Elevated Formation of Particulate Nitrate From N 2 O 5 Hydrolysis in the Yangtze River Delta Region From 2011 to 2019

Aerosol nitrate has become the most abundant compound during aerosol pollution in eastern China. The Chinese government implemented a stringent policy during 2013–2017 to tackle aerosol pollution. However, the response of nitrate to nitrogen oxides (NO x ) reduction is unclear owing to the limitation of long‐term measurement. Here, we performed a 9‐year continuous measurement of aerosol compositions in Shanghai and confirmed a decrease in most species except nitrate. The contribution of nitrate to fine particulate matter (PM 2.5 ) increased significantly, reaching up to 35% in pollution episodes after 2017. This is in contrast to the evident reduction in NO x emissions. We found that the elevated dinitrogen pentoxide (N 2 O 5 ) hydrolysis is responsible for the observed nitrate trend. Increased ozone and decreased nitrogen dioxide (NO) facilitated the formation of N 2 O 5 , and increased nitrate proportion promoted the uptake of N 2 O 5 and eventually enhanced the conversion efficiency of NO 2 to nitrate. Our results highlight the importance of synergic control of aerosol and ozone pollution.