Controlling particulate nitrate pollution in China
Journal
Nature Geoscience
Date
2021.04.26
Abstract
Stringent emission controls have decreased fine particulate matter (PM2.5) pollution in China by 30-50％ over the 2013-2018 period. However, the nitrate component of PM2.5 has not responded effectively to emission controls on nitrogen oxides (NOx). This is particularly the case over the North China Plain in winter where pollution is at its worst, and where nitrate levels during winter haze pollution events have actually increased. Simulations with the GEOS-Chem model reproduce the distribution and trends of nitrate in China over 2013-2017, and further show a 30％ increase of nitrate during winter haze pollution days consistent with observations. The 2013-2017 nitrate trend would have been worse were it not for favorable meteorology. The lack of response of nitrate to NOx emission controls is explained in the model by a combination of chemical factors including faster nighttime NO？x oxidation in winter due to decreased ozone titration, and slower deposition of total nitrate (gas + particle) as the particulate fraction of nitrate approaches to unity. Although ammonia (NH3) is thermodynamically in excess for formation of nitrate PM2.5, we find that NH3 emission controls are most effective for controlling nitrate because they decrease the lifetime of total nitrate against deposition. This calls for the need to better understand the sources of NH3 in China in winter as targets for emission controls.