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Nitryl chloride (ClNO2) has been reported as a critical species of chlorine chemistry. Its chemistry interrupts a nighttime sink of NOX and emits chlorine radical (Cl•) in the daytime, consequently altering gaseous and aerosol chemistry. Recent field studies have also measured considerable concentrations of ClNO2 in South Korea under the influences of natural and anthropogenic chlorine sources. However, the impacts of ClNO2 chemistry on air quality in South Korea have yet to be evaluated. We validate simulated ClNO2 and its chemistry in South Korea using observations and a 3-D chemical transport model (CTM) during the Korea-United States Air Quality field study. We implemented the latest Chinese and Korean anthropogenic chlorine emissions in the model. We found that the model reproduces the observed spatial and temporal variations of ClNO2, including its local and transboundary transport and precursors. We found that ClNO2 chemistry results in a more efficient conversion of NO to NO2 at night and daytime acceleration of the NOX-O3 cycle. It results in an increase of O3 (1.1%), NOX (3.1%), OH (2.0%), HO2 (0.8%), and Cl• (507.8%) and a decrease of TNO3(HNO3 + aerosol nitrate, 1.7%) on the campaign mean basis.

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.

Photolysis of nitrous acid (HONO) has long been recognized as an early morning source of OH radicals in urban air, but the detailed mechanism of its formation is still unclear. During the Korea-US Air Quality (KORUS-AQ) campaign, HONO was measured using Quantum Cascade Tunable Diode Laser Absorption Spectroscopy (QC-TDLAS) at Olympic Park in Seoul from 17 May to 10 June, 2016. HONO concentrations ranged from 0.07 ppbv to 3.46 ppbv with an average of 0.93 ppbv. HONO remained high at night from 1 am to 5 am, during which the mean concentration was higher in high-O3 episodes (1.82 ppbv) than non-episode (1.20 ppbv). In the morning, OH budget due to HONO photolysis were higher by 50 % (0.95 pptv) during high-O3 episodes compared to non-episode. Diurnal variations of HOx and O3 simulated by the F0AM model demonstrated a difference of ~ 20 ppbv in daily maximum O3 between the two periods. The HONO concentration increased with relative humidity (RH) until 80 %, of which the highest HONO was associated with the top 10 % NOx, confirming that NOx is a crucial precursor of HONO and its formation is facilitated by humidity. The conversion ratio of NOx to HONO was estimated to be 0.86 × 10−2 h−1 at night and also increased with RH. As surrogate for the catalyst surface, the mass concentrations of black carbon (eBC) and the surface areas of particles smaller than 120 nm showed a tendency for RH similar to conversion ratio. Using an Artificial Neuron Network (ANN) model, HONO concentrations were successfully simulated with measured variables (r = 0.8 for the best suite), among which NOx, surface area, and RH were found to be main factors affecting ambient HONO concentrations with weigh values of 26.2 %, 11.9 %, and 10.6 %, respectively. This study demonstrates the coupling of HONO with HOx-VOCs-O3 cycle in Seoul Metropolitan Areas (SMA) and provides practical evidence for heterogeneous formation of HONO by employing the ANN model to atmospheric chemistry.