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