Interpretation of the effects of anthropogenic chlorine on nitrate formation over northeast Asia during KORUS-AQ 2016.

The Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model, implemented with anthropogenic chlorine (Cl) emissions, was evaluated against ground and NASA DC-8 aircraft measurements during the Korea-United States Air Quality (KORUS-AQ) 2016 campaign. The latest anthropogenic Cl emissions, including gaseous HCl and particulate chloride (pCl) emissions from the Anthropogenic Chlorine Emissions Inventory of China (ACEIC-2014) (over China) and a global emissions inventory (Zhang et al., 2022) (over outer China), were used to examine the impacts of Cl emissions and the role of nitryl chloride (ClNO) chemistry in NO heterogeneous reactions on secondary nitrate (NO) formation across the Korean Peninsula. The model results against aircraft measurements clearly showed significant Cl- underestimations due mainly to the high gas-particle (G/P) partitioning ratios at aircraft measurement altitudes such as 700-850 hPa, but the ClNO simulations were reasonable. Several simulations of CMAQ-based sensitivity experiments against ground measurements indicated that although addition of Cl emission did not significantly alter NO formation, the activated ClNO chemistry with Cl emissions showed the best model performance with the reduced normalized mean bias (NMB) of 18.7 % compared to a value of 21.1 % for the Cl emissions-free case. In our model evaluation, ClNO accumulated during the night but quickly produced Cl radical due to ClNO photolysis at sunrise, which modulated other oxidation radicals (e.g., ozone [O] and hydrogen oxide radicals [HO]) in the early morning. In the morning hours (0800-1000 LST), the HO were the dominant oxidants, contributing 86.6 % of the total oxidation capacity (sum of major oxidants such as O and HO species), while oxidability was enhanced by up to ∼6.4 % (increase in 1 h HO average of 2.89 × 10 molecules·cm) in the early morning mainly due to the changes in OH (+7.2 %), hydroperoxyl radical (HO)(+10.0 %), and O (+4.2 %) over the Seoul Metropolitan Area, during the KORUS-AQ campaign. Our results improve understanding of the atmospheric changes in the PM formation pathway caused by ClNO chemistry and Cl emissions over northeast Asia.