Observations of nitryl chloride and modeling its source and effect on ozone in the planetary boundary layer of southern China

Nitryl chloride (ClNO 2 ) plays potentially important roles in atmospheric chemistry, but its abundance and effect are not fully understood due to the small number of ambient observations of ClNO 2 to date. In late autumn 2013, ClNO 2 was measured with a chemical ionization mass spectrometer (CIMS) at a mountain top (957 m above sea level) in Hong Kong. During 12 nights with continuous CIMS data, elevated mixing ratios of ClNO 2 (>400 parts per trillion by volume) or its precursor N 2 O 5 (>1000 pptv) were observed on six nights, with the highest ever reported ClNO 2 (4.7 ppbv, 1 min average) and N 2 O 5 (7.7 ppbv, 1 min average) in one case. Backward particle dispersion calculations driven by winds simulated with a mesoscale meteorological model show that the ClNO 2 /N 2 O 5 ‐laden air at the high‐elevation site was due to transport of urban/industrial pollution north of the site. The highest ClNO 2 /N 2 O 5 case was observed in a later period of the night and was characterized with extensively processed air and with the presence of nonoceanic chloride. A chemical box model with detailed chlorine chemistry was used to assess the possible impact of the ClNO 2 in the well‐processed regional plume on next day ozone, as the air mass continued to downwind locations. The results show that the ClNO 2 could enhance ozone by 5–16% at the ozone peak or 11–41% daytime ozone production in the following day. This study highlights varying importance of the ClNO 2 chemistry in polluted environments and the need to consider this process in photochemical models for prediction of ground‐level ozone and haze.