Abstract
[1] Ethylene is one of the most abundant anthropogenic volatile organic compounds (VOCs) in urban atmospheres. The potential of forming secondary organic aerosol (SOA) by this smallest alkene in urban environments has not previously been considered because of the general lack of attention to SOA formation through in-cloud processing of soluble oxidation products. Ethylene reacts with OH radicals to form glycolaldehyde as one of its major products. Glycolaldehyde partitions into cloud water where it is oxidized to glyoxylic acid and oxalic acid and thereby contributing to SOA forming. A chemistry box model incorporating explicit multiphase reactions of relevant C2 oxygenated species is used to evaluate the SOA mass that can be formed from ethylene under the typical conditions of Hong Kong ambient atmosphere. In an idealized scenario involving five cloud cycles in a period of 5 d, the model predicts 59 ng m−3 aerosol phase organic acids from ethylene, corresponding to an aerosol yield of 0.3%. Sensitivity tests show that the SOA mass formed in the model depends strongly on the cloud liquid water content (LWC), increasing from 59 ng m−3 at a LWC of 0.3 g m−3 in the base case to 208 ng m−3 at a LWC of 1.0 g m−3. The SOA mass predicted increases with increasing cloud period length and is affected by the time of the day when the cloud events occur; however, it is little affected by the initial pH value of cloud droplets. The modeling results indicate that in-cloud processing is a viable pathway for SOA formation from ethylene. A more general implication is that in-cloud processing is a potentially important pathway to generate SOA, especially for places such as Hong Kong, where the annual cloud coverage is over 50%.
Published Version
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