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 NO_X 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.