Chlorine oxidation of VOCs at a semi-rural site in Beijing: significant chlorine liberation from ClNO2 and subsequent gas- and particle-phase Cl–VOC production
- 1Department of Chemistry and Molecular Biology, University of Gothenburg, Gothenburg, Sweden
- 2IVL Swedish Environmental Research Institute, Gothenburg, Sweden
- 3Earth and Space Sciences, Chalmers University of Technology, Gothenburg, Sweden
- 4Norwegian Meteorological Institute, Oslo, Norway
- 5State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, China
- 6Division of Environment and Sustainability, Hong Kong University of Science and Technology, Clearwater Bay, Kowloon, Hong Kong
- 7School of Energy and Environment, City University of Hong Kong, Hong Kong
- 8Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, Hong Kong
- 9Centre for Atmospheric Science, School of Earth, Atmospheric and Environmental Science, University of Manchester, Manchester, UK
- 10School of Chemistry, University of Bristol, Cantock's Close, Bristol, UK
- 11Department of Chemistry, University of the Western Cape, Bellville, Cape Town, South Africa
- anow at: Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, USA
Abstract. Nitryl chloride (ClNO2) accumulation at night 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 and a suite of inorganic halogens including ClNO2 and reactions of chloride with volatile organic compounds (Cl–VOCs) in the gas and particle phases utilising the Filter Inlet for Gas and AEROsols time-of-flight chemical ionisation mass spectrometer (FIGAERO-ToF-CIMS) during an intensive measurement campaign 40 km northwest of Beijing in May and June 2016. A maximum mixing ratio of 2900 ppt of ClNO2 was observed with a mean campaign nighttime mixing ratio 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 mixing ratios 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 09:30 LT and up to 8.4 × 105 molecules cm−3 when including the supporting inorganic halogen measurements.
Cl–VOCs were observed in the particle and gas phases 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 secondary organic aerosol (SOA) formation.