Quantification and source apportionment of atmospheric oxidation capacity in urban atmosphere by considering reactive chlorine species and photochemical loss of VOCs.

Abstract

Atmospheric oxidation capacity (AOC) reflects the potential of the atmosphere in converting primary pollutants into secondary aerosols and ozone (O₃). In this study, the AOC at an urban supersite in Wuhan, a megacity in central China, was quantified by considering the reactions of volatile organic compounds (VOCs) and carbon monoxide (CO) with atmospheric oxidants (OH, NO₃, O₃, and Cl). Photochemical loss of total VOCs (13.7-23.7 %) during transport was accounted for by monitoring the concentration ratio of o-xylene and ethylbenzene, a VOC pair with diverse reaction rate constants with OH. AOC would be underestimated by 9.0-25.2 % in the 4 seasons if being estimated from the observed VOCs instead of photochemical-loss corrected VOCs. The atmospheric oxidants were measured or indirectly estimated using well-established parameterizations. In particular, hourly reactive chlorine species were measured using an iodide-based chemical ionization mass spectrometer (I-CIMS). Cl radical concentration was calculated by assuming a steady-state between the production from reactive chlorine species and the removal by O₃ and VOCs. The result showed that AOC would be underestimated by 14.5 % and 1.9 % in winter and spring if Cl radicals were neglected. Based on the above quantification, the composition of AOC was further apportioned to atmospheric oxidants, VOC categories, as well as the sources of VOC and CO. In winter, CNG combustion exhaust, electronics industry, and secondary formation were critical for regulating the AOC. In spring, secondary formation was the most important AOC source, followed by electronics industry and biogenic sources.