Chlorine oxidation of VOCs at a semi-rural site in Beijing: Significant chlorine liberation from ClNO<sub>2</sub> and subsequent gas and particle phase Cl-VOC production

Abstract. Nitryl Chloride (ClNO2) accumulation at night-time acts as a significant reservoir for active chlorine and impacts the following day's photochemistry when the chlorine atom is liberated at sunrise. Here, we report simultaneous measurements of N2O5, a suite of inorganic halogens including ClNO2 and Cl-VOCs in the gas and particle phase utilizing the FIGAERO-ToF-CIMS during an intensive measurement campaign 40 km Northwest of Beijing in May and June 2016. A maximum concentration of 2900 ppt of ClNO2 was observed with a mean campaign night-time concentration of 487 ppt, appearing to have an anthropogenic source supported by correlation with SO2, CO and benzene, which often persisted at high levels after sunrise until midday. This was attributed to such high concentrations persisting after numerous e-folding times of the photolytic lifetime enabling the chlorine atom production to reach 2.3 × 105 molecules cm−3 from ClNO2 alone, peaking at 9:30 am and up to 8.4 × 105 molecules cm−3 when including the supporting inorganic halogen measurements. Cl-VOCs were measured in the particle and gas phase for the first time at high time resolution and illustrate how the iodide ToF-CIMS can detect unique markers of chlorine atom chemistry in ambient air from both biogenic and anthropogenic sources. Their presence and abundance can be explained via time series of their measured and steady state calculated precursors, enabling the assessment of competing OH and chlorine atom oxidation via measurements of products from both of these mechanisms and their relative contribution to SOA formation.