Abstract
AbstractThe first nitryl chloride (ClNO2) measurements in the UK were made during the summer 2012 ClearfLo campaign with a chemical ionization mass spectrometer, utilizing an I− ionization scheme. Concentrations of ClNO2 exceeded detectable limits (11 ppt) every night with a maximum concentration of 724 ppt. A diurnal profile of ClNO2 peaking between 4 and 5 A.M., decreasing directly after sunrise, was observed. Concentrations of ClNO2 above the detection limit are generally observed between 8 P.M. and 11 A.M. Different ratios of the production of ClNO2:N2O5 were observed throughout with both positive and negative correlations between the two species being reported. The photolysis of ClNO2 and a box model utilizing the Master Chemical Mechanism modified to include chlorine chemistry was used to calculate Cl atom concentrations. Simultaneous measurements of hydroxyl radicals (OH) using low pressure laser‐induced fluorescence and ozone enabled the relative importance of the oxidation of three groups of measured VOCs (alkanes, alkenes, and alkynes) by OH radicals, Cl atoms, and O3 to be compared. For the day with the maximum calculated Cl atom concentration, Cl atoms in the early morning were the dominant oxidant for alkanes and, over the entire day, contributed 15%, 3%, and 26% toward the oxidation of alkanes, alkenes, and alkynes, respectively.
Highlights
The fate of many anthropogenic trace gas pollutants impacting on health and climate is controlled by chemical oxidation in the troposphere [Prinn, 2003]
Laboratory studies have confirmed that heterogeneous uptake of N2O5 on aerosols containing chloride produces ClNO2 (R1) [Finlayson-Pitts et al, 1989; Roberts et al, 2009] and lead to the production of Cl atoms via photolysis (R2) [e.g., Ganske et al, 1992; Osthoff et al, 2008]
Complete time series of ClNO2, N2O5, Na, and Cl are shown in Figure 3, illustrating that ClNO2 was detected every night of the campaign
Summary
The fate of many anthropogenic trace gas pollutants impacting on health and climate is controlled by chemical oxidation in the troposphere [Prinn, 2003]. With numerous studies reporting sources of Cl atoms in locations up to an excess of 900 miles away from a marine source [Thornton et al, 2010; Mielke et al, 2011; Phillips et al, 2012], Cl atom-induced oxidation may have a more significant global influence than initially considered, often enhancing tropospheric ozone production [Simon et al, 2009; Sarwar et al, 2012, 2014] This release of Cl may be important atmospherically as Cl rate constants for reactions with VOCs are almost exclusively an order of magnitude larger than those of OH. Measurements of the hydroxyl radical using laser-induced fluorescence at low pressure [Heard and Pilling, 2003; Whalley et al, 2010], and a comprehensive suite of supporting measurements, for example of speciated VOCs, ozone, and photolysis frequencies, enabled the relative contribution of Cl atoms to the oxidation of VOCs compared with OH and O3 to be evaluated
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