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

Nighttime oxidation by nitrate (NO3) radicals has important ramifications on nocturnal aerosol formation and hence the climate and human health. Nitrate radicals are produced by the reaction of NO2 and O-3. Despite large decreases in anthropogenic emissions of nitrogen oxides (NOx = NO + NO2), a previous study found significant increases in NO3 production (PNO3) 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 PNO3 using a systematic framework, in which PNO3 is diagnosed as a function of odd oxygen (O-x = O-3 + NO2) and the NO2/O-3 ratio. We did not find an increase of PNO3 from 2014 to 2022 in China due to a continuous decrease in the NO2/O-3 ratio, although PNO3 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 NO3 radicals amid emission reduction efforts.