Decreasing Production and Potential Urban Explosion of Nighttime Nitrate Radicals amid Emission Reduction Efforts.

Nighttime oxidation by nitrate (NO 3 ) radicals has important ramifications on nocturnal aerosol formation and hence the climate and human health. Nitrate radicals are produced by the reaction of NO 2 and O 3 . Despite large decreases in anthropogenic emissions of nitrogen oxides (NO x = NO + NO 2 ), a previous study found significant increases in NO 3 production (PNO 3 ) from 2014 to 2019 in China, in contrast to decreasing trends in the U.S. and Europe. Using the summer observations from 2014 to 2022, we analyze the interannual variability of nocturnal PNO 3 using a systematic framework, in which PNO 3 is diagnosed as a function of odd oxygen (O x = O 3 + NO 2 ) and the NO 2 /O 3 ratio. We did not find an increase of PNO 3 from 2014 to 2022 in China due to a continuous decrease in the NO 2 /O 3 ratio, although PNO 3 is modulated by the variation in O x . Using in situ observations obtained in Beijing in 2007, we demonstrate the potential for an upsurge resembling an "explosion" in urban nighttime NO 3 radicals amid emission reduction efforts.