Effect of nitryl chloride chemistry on oxidants concentrations during the KORUS-AQ campaign

Nitryl chloride (ClNO2) plays an important role as a night-time reservoir of NOX and the source of Cl radical during the daytime, which consequently affects the ozone photochemistry. Its impacts on regional air quality in East Asia, however, are not fully understood so far. We here use extensive observations during the international KORea-US cooperative Air Quality field study in Korea (KORUS-AQ), which occurred in May-June 2016, with a 3-D chemistry transport model to examine the impacts of ClNO2 chemistry on radical species and total nitrate concentrations in East Asia. We first update the model by implementing chlorine chemistry and latest anthropogenic chlorine emissions of China and South Korea. We conduct model simulations for May-June, 2016 and validate the model by comparing against the observations from the KORUS-AQ campaign. We find that the ClNO2 chemistry in the model results in an increase of ozone by ~1.4 ppbv (~2.5%), Cl radical by ~ 4.6x103 molec cm-3 (~3600%), OH ~8.2x104 molec cm-3 (~5.3%), HO2 ~6.6 molec cm-3 (~3.0%), a decrease of TNO3 (HNO3 + nitrate aerosol) concentrations by ~2 μg m-3 on a daily mean basis during the campaign. Overall, the enhanced conversion of NO to NO2 driven by ClNO2 chemistry contributes to higher oxidant concentrations in the model. As a result, the updated model shows a better agreement with the observations in Korea during the KORUS-AQ campaign.