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Issue 27, 2012
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Direct aqueous photochemistry of isoprene high-NOx secondary organic aerosol

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Secondary organic aerosol (SOA) generated from the high-NOx photooxidation of isoprene was dissolved in water and irradiated with λ > 290 nm radiation to simulate direct photolytic processing of organics in atmospheric water droplets. High-resolution mass spectrometry was used to characterize the composition at four time intervals (0, 1, 2, and 4 h). Photolysis resulted in the decomposition of high molecular weight (MW) oligomers, reducing the average length of organics by 2 carbon units. The average molecular composition changed significantly after irradiation (C12H19O9N0.08 + → C10H16O8N0.40). Approximately 65% by count of SOA molecules decomposed during photolysis, accompanied by the formation of new products. An average of 30% of the organic mass was modified after 4 h of direct photolysis. In contrast, only a small fraction of the mass (<2%), belonging primarily to organic nitrates, decomposed in the absence of irradiation by hydrolysis. Furthermore, the concentration of aromatic compounds increased significantly during photolysis. Approximately 10% (lower limit) of photodegraded compounds and 50% (upper limit) of the photoproducts contain nitrogen. Organic nitrates and multifunctional oligomers were identified as compounds degraded by photolysis. Low-MW 0N (compounds with 0 nitrogen atoms in their structure) and 2N compounds were the dominant photoproducts. Fragmentation experiments using tandem mass spectrometry (MSn, n = 2–3) indicate that the 2N products are likely heterocyclic/aromatic and are tentatively identified as furoxans. Although the exact mechanism is unclear, these 2N heterocyclic compounds are produced by reactions between photochemically-formed aqueous NOx species and SOA organics.

Graphical abstract: Direct aqueous photochemistry of isoprene high-NOx secondary organic aerosol

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The article was received on 23 Mar 2012, accepted on 17 May 2012 and first published on 17 May 2012

Article type: Paper
DOI: 10.1039/C2CP40944E
Citation: Phys. Chem. Chem. Phys., 2012,14, 9702-9714
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    Direct aqueous photochemistry of isoprene high-NOx secondary organic aerosol

    T. B. Nguyen, A. Laskin, J. Laskin and S. A. Nizkorodov, Phys. Chem. Chem. Phys., 2012, 14, 9702
    DOI: 10.1039/C2CP40944E

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