In-cloud processes of methacrolein under simulated conditions – Part 2: Formation of secondary organic aerosol
The fate of methacrolein in cloud evapo-condensation cycles was\nexperimentally investigated. To this end, aqueous-phase reactions of\nmethacrolein with OH radicals were performed (as described in Liu et\nal., 2009), and the obtained solutions were then nebulized and dried\ninto a mixing chamber. ESI-MS and ESI-MS/MS analyses of the aqueous\nphase composition denoted the formation of high molecular weight\nmultifunctional products containing hydroxyl, carbonyl and carboxylic\nacid moieties. The time profiles of these products suggest that their\nformation can imply radical pathways. These high molecular weight\norganic products are certainly responsible for the formation of\nsecondary organic aerosol (SOA) observed during the nebulization\nexperiments. The size, number and mass concentration of these particles\nincreased significantly with the reaction time: after 22 h of reaction,\nthe aerosol mass concentration was about three orders of magnitude\nhigher than the initial aerosol quantity. The evaluated SOA yield ranged\nfrom 2 to 12%. These yields were confirmed by another estimation method\nbased on the hygroscopic and volatility properties of the obtained SOA\nmeasured and reported by Michaud et al. (2009). These results provide,\nfor the first time to our knowledge, strong experimental evidence that\ncloud processes can act, through photooxidation reactions, as important\ncontributors to secondary organic aerosol formation in the troposphere.