Chlorine oxidation of VOCs at a semi-rural site in Beijing: significant chlorine liberation from ClNOsub2/sub and subsequent gas- and particle-phase Cl–VOC production

摘要

Nitryl chloride (ClNO2) accumulation at nightacts as a significant reservoir for active chlorine and impacts the followingday's photochemistry when the chlorine atom is liberated at sunrise. Here, wereport simultaneous measurements of N2O5 and a suite ofinorganic halogens including ClNO2 and reactions of chloridewith volatile organic compounds (Cl–VOCs) in the gas andparticle phases utilising the Filter Inlet for Gas and AEROsols time-of-flight chemical ionisation massspectrometer (FIGAERO-ToF-CIMS) during an intensive measurementcampaign 40 km northwest of Beijing in May and June 2016. A maximum mixingratio of 2900 ppt of ClNO2 was observed with a mean campaignnighttime mixing ratio of 487 ppt, appearing to have an anthropogenicsource supported by correlation with SO2, CO and benzene, whichoften persisted at high levels after sunrise until midday. This wasattributed to such high mixing ratios persisting after numerous e-foldingtimes of the photolytic lifetime enabling the chlorine atom production toreach 2.3  ×  105 molecules cm−3 from ClNO2alone, peaking at 09:30 LTand up to 8.4  ×  105 molecules cm−3 when including thesupporting inorganic halogen measurements.Cl–VOCs were observed in the particle and gas phases for the first time athigh time resolution and illustrate how the iodide ToF-CIMS can detect uniquemarkers of chlorine atom chemistry in ambient air from both biogenic andanthropogenic sources. Their presence and abundance can be explained via timeseries of their measured and steady-state calculated precursors, enabling theassessment of competing OH and chlorine atom oxidation via measurements ofproducts from both of these mechanisms and their relative contribution tosecondary organic aerosol (SOA) formation.