Role of aldehyde chemistry and NO sub(x) 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 sub(4)-unsaturated aldehyde) under urban high-NO sub(x) conditions. Previously, we have identified peroxy methacryloyl nitrate (MPAN) as the important intermediate to isoprene and methacrolein SOA in this NO sub(x) regime. Here we show that as a result of this chemistry, NO sub(2) enhances SOA formation from methacrolein and two other a, b-unsaturated aldehydes, specifically acrolein and crotonaldehyde, a NO sub(x) effect on SOA formation previously unrecognized. Oligoesters of dihydroxycarboxylic acids and hydroxynitrooxycarboxylic acids are observed to increase with increasing NO sub(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 a, b-unsaturated and contain an additional methyl group on the a-carbon. Aerosol formation from 2-methyl-3-buten-2-ol (MBO232) is insignificant, even under high-NO sub(2) conditions, as PAN (peroxy acyl nitrate, RC(O)OONO sub(2)) formation is structurally unfavorable. At atmospherically relevant NO sub(2)/NO ratios (3-8), the SOA yields from isoprene high-NO sub(x) photooxidation are 3 times greater than previously measured at lower NO sub(2)/NO ratios. At sufficiently high NO sub(2) concentrations, in systems of a, b-unsaturated aldehydes, SOA formation from subsequent oxidation of products from acyl peroxyl radicals+NO sub(2) can exceed that from RO sub(2)+HO sub(2) reactions under the same inorganic seed conditions, making RO sub(2)+NO sub(2) an important channel for SOA formation.