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Journal article

Role of aldehyde chemistry and NOx concentrations in secondary organic aerosol formation

Chan A, Chan M, Surratt J, Chhabra P, Loza C, Crounse J, Yee L, Flagan R, Wennberg P, Seinfeld J...(+10 more)

Atmospheric Chemistry and Physics, vol. 10, issue 15 (2010) pp. 7169-7188

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Aldehydes are an important class of products from atmospheric oxidation of hydrocarbons. Isoprene (2-methyl-1,3-butadiene), the most abundantly emitted atmospheric non-methane hydrocarbon, produces a significant amount of secondary organic aerosol (SOA) via methacrolein (a C(4)-unsaturated aldehyde) under urban high-NO(x) conditions. Previously, we have identified peroxy methacryloyl nitrate (MPAN) as the important intermediate to isoprene and methacrolein SOA in this NO(x) regime. Here we show that as a result of this chemistry, NO(2) enhances SOA formation from methacrolein and two other alpha, beta-unsaturated aldehydes, specifically acrolein and crotonaldehyde, a NO(x) effect on SOA formation previously unrecognized. Oligoesters of dihydroxycarboxylic acids and hydroxynitrooxycarboxylic acids are observed to increase with increasing NO(2)/NO ratio, and previous characterizations are confirmed by both online and offline high-resolution mass spectrometry techniques. Molecular structure also determines the amount of SOA formation, as the SOA mass yields are the highest for aldehydes that are alpha, beta-unsaturated and contain an additional methyl group on the alpha-carbon. Aerosol formation from 2-methyl-3-buten-2-ol (MBO232) is insignificant, even under high-NO(2) conditions, as PAN (peroxy acyl nitrate, RC(O)OONO(2)) formation is structurally unfavorable. At atmospherically relevant NO(2)/NO ratios (3-8), the SOA yields from isoprene high-NO(x) photooxidation are 3 times greater than previously measured at lower NO(2)/NO ratios. At sufficiently high NO(2) concentrations, in systems of alpha, beta-unsaturated aldehydes, SOA formation from subsequent oxidation of products from acyl peroxyl radicals+NO(2) can exceed that from RO(2)+HO(2) reactions under the same inorganic seed conditions, making RO(2)+NO(2) an important channel for SOA formation.

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