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 NO_X and a source of chlorine radical (Cl^•) 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 NO_X chemistry by efficiently converting NO to NO₂. In addition, it causes a noticeable acceleration of the NO_X-O₃ cycle. When averaged over the campaign, adding ClNO₂ chemistry into the model leads to an increase of O₃ (1.1%), NO_X (3.1%), ^•OH (2.0%), HO₂^• (0.8%), and Cl^• (507.8%) and a decrease of TNO₃ (HNO₃ + aerosol nitrate, 1.7%).