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

Abstract. 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 C₄-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, NO₂ enhances SOA formation from methacrolein and two other α, β-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 NO₂/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 α, β-unsaturated and contain an additional methyl group on the α-carbon. Aerosol formation from 2-methyl-3-buten-2-ol (MBO232) is insignificant, even under high-NO₂ conditions, as PAN (peroxy acyl nitrate, RC(O)OONO₂) formation is structurally unfavorable. At atmospherically relevant NO₂/NO ratios (3–8), the SOA yields from isoprene high-NO_x photooxidation are 3 times greater than previously measured at lower NO₂/NO ratios. At sufficiently high NO₂ concentrations, in systems of α, β-unsaturated aldehydes, SOA formation from subsequent oxidation of products from acyl peroxyl radicals+NO₂ can exceed that from RO₂+HO₂ reactions under the same inorganic seed conditions, making RO₂+NO₂ an important channel for SOA formation.