Dust emission reduction enhanced gas-to-particle conversion of ammonia in the North China Plain.

Ammonium salt is an important component of particulate matter with aerodynamic diameter less than 2.5 µm (PM 2.5 ) and has significant impacts on air quality, climate, and natural ecosystems. However, a fundamental understanding of the conversion kinetics from ammonia to ammonium in unique environments of high aerosol loading is lacking. Here, we report the uptake coefficient of ammonia (γ NH3 ) on ambient PM 2.5 varying from 2.2 × 10 -4 to 6.0 × 10 -4 in the North China Plain. It is significantly lower than those on the model particles under simple conditions reported in the literature. The probability-weighted γ NH3 increases obviously, which is well explained by the annual decrease in aerosol pH due to the significant decline in alkali and alkali earth metal contents from the emission source of dust. Our results elaborate on the complex interactions between primary emissions and the secondary formation of aerosols and the important role of dust in atmospheric chemistry.