Modeling the Global Impact of Chlorine Chemistry on Secondary Organic Aerosols.

Simulation of secondary organic aerosol (SOA) in models has been an uncertain component in determining the impacts of atmospheric aerosols on air quality and climate. Recent studies have shown that reactive chlorine can rapidly oxidize volatile organic compounds (VOCs), trigger SOA formation, and alter other oxidants, thus having a potentially significant effect on SOA, which has not been thoroughly investigated at the global scale. Here, we developed a chlorine-SOA simulation within a global chemical transport model along with updated anthropogenic continental chlorine emissions. Our simulations demonstrate that chlorine chemistry increases the annual mean boundary layer SOA by 5-12% over most continents while decreasing SOA by 5-11% over northern Atlantic and Pacific oceans, which are in the right direction to narrow existing discrepancies between models and observations. Notably, sensitivity simulations in China with observed high chlorine levels capture the temporal variations of both observed fine Cl and organic aerosol, showing an increase in SOA by more than 100%. Our study also reveals that polluted regions, which have ample emissions of both chlorine species and VOCs, exhibit potential chlorinated SOA, which are commonly toxic, contributing up to 15% to total SOA.