Abstract
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.
Highlights
NO and NO2 (NOx) are important catalysts in the photochemical production of ozone (O3) playing a significant role in the oxidation of volatile organic compounds (VOCs) and subsequently have an adverse effect on air quality
A ToF-CIMS utilising the Filter Inlet for Gas and AEROsols (FIGAERO) is deployed at a site in semirural Beijing, China, to measure the gas- and particle-phase precursors (ClNO2, N2O5) and selective halogen-containing species at high time frequency and resolving power to further our understanding of the chlorine atom budget in this region and its potential fate
FIGAERO was operated in a cyclic pattern; 25 min of gas-phase measurement and simultaneous particle collection, followed by a 20 min period during which the filter was shifted into position over the ion-molecule reaction (IMR) inlet and the collected particle mass was desorbed
Summary
NO and NO2 (NOx) are important catalysts in the photochemical production of ozone (O3) playing a significant role in the oxidation of volatile organic compounds (VOCs) and subsequently have an adverse effect on air quality. Wang et al (2017) revealed chlorine-initiated oxidation of isoprene can produce SOA yields up to 36 %, with products similar to that of OH isoprene oxidation, compared with the 15 % yield from standard oxidation calculated by Liu et al (2016), this is known to be a factor of 2 higher than utilised in standard climate models This SOA formation from chlorine-initiated oxidation presents a large knowledge gap in the literature, which to date is limited by measurement capabilities. A ToF-CIMS utilising the Filter Inlet for Gas and AEROsols (FIGAERO) is deployed at a site in semirural Beijing, China, to measure the gas- and particle-phase precursors (ClNO2, N2O5) and selective halogen-containing species at high time frequency and resolving power to further our understanding of the chlorine atom budget in this region and its potential fate
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