Direct aqueous photochemistry of isoprene high-NOx secondary organic aerosol

Abstract

Secondary organic aerosol (SOA) generated from the high-NO_x 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 (C₁₂H₁₉O₉N_0.08 + hν → C₁₀H₁₆O₈N_0.40). 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 NO_x species and SOA organics.