Controlling particulate nitrate pollution in China

Abstract:

Stringent emission controls to improve air quality have decreased fine particulate matter (PM_2.5) concentrations in China by 30-50% over the 2013-2018 period. However, the nitrate component of PM_2.5 has not responded effectively to emission controls on nitrogen oxides (NO_x), and has actually increased during winter haze pollution events in the North China Plain. Recent studies have suggested that this could be caused by increased nitrate partitioning to the particulate phase as the sulfate component of PM_2.5 decreases, or by faster NO_x oxidation as oxidants increase. However, we find that these two processes are inadequate to explain the observed nitrate trends, and that nitrate would have increased even more over 2013-2018 were it not for favorable meteorology. We present simulations with the GEOS-Chem model that can successfully reproduce the observed nitrate trends, including the increase under winter haze conditions, and show that slower deposition of total nitrate (gas + particle) as the particulate fraction approaches unity is the dominant explanatory factor. Although ammonia is thermodynamically in excess for formation of PM_2.5 nitrate, we find that ammonia emission controls are more effective than NO_x for controlling nitrate because they decrease the lifetime of total nitrate against deposition. This calls for better understanding of the sources of wintertime ammonia in China as targets for emission controls.