Elevated Formation of Particulate Nitrate From N₂O₅ Hydrolysis in the Yangtze River Delta Region From 2011 to 2019

Abstract

Abstract 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.