Effect of nitryl chloride chemistry on air quality in South Korea during the KORUS-AQ campaign

Nitryl chloride (ClNO2) can play an important role in determining ozone and particulate matter (PM) chemistry as a nighttime reservoir of NOX and a source of chlorine radical (Cl BULL; ) in the daytime in polluted environments. We present a quantitative evaluation of the impact of tropospheric ClNO2 chemistry on air quality in South Korea using extensive observations during the Korea-United States Air Quality (KORUS-AQ) field study and a 3-D chemical transport model (CTM). We first estimated anthropogenic emissions of reactive chlorine in South Korea to implement it along with the latest Chinese emissions in the model. A comparison between the simulated and observed ClNO2 concentrations during the KORUS-AQ campaign shows that the model captures the observed spatial and temporal variations, including local and transboundary transport of ClNO2 and its precursors. The model simulation highlights that ClNO2 plays a significant role in nighttime NOX chemistry by efficiently converting NO to NO2. In addition, it causes a noticeable acceleration of the NOX-O3 cycle. When averaged over the campaign, adding ClNO2 chemistry into the model leads to an increase of O3 (1.1%), NOX (3.1%), BULL; OH (2.0%), HO2 BULL; (0.8%), and Cl BULL; (507.8%) and a decrease of TNO3 (HNO3 + aerosol nitrate, 1.7%).