Role of aldehyde chemistry and NO<sub>x</sub> 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

Secondary organic aerosol formation from the photooxidation of isoprene, 1,3-butadiene, and 2,3-dimethyl-1,3-butadiene under high NO<sub>x</sub> conditions

“…For product identification, all detected ions in this study were assumed to be deprotonated molecules. All ion formulae suggested for oligomer series 1A-4A in the present study agreed with those suggested in a recent study by Chan et al (2010). The products of series 1A, 2A, 3A, and 4A were identified as 2-methylglyceric acid oligoesters and their mononitrates, monoformates, and monoacetates, respectively.…”

“…Similar oligomer signals were found at m/z 266, 368, 470, 572, 674, 776 (series 2A); m/z 249, 351, 453, 555, 657, 759 (series 3A); and m/z 263, 365, 467, 569, 671, 773 (series 4A). The mass numbers of these oligomer signals agreed with those measured in previous studies (Surratt et al, 2006;Chan et al, 2010). In this study, a new series of oligomer signals with a regular mass difference of 102 amu was observed at m/z 327, 429, 531, 633, and 735 (series 5A).…”

“…The major particle-phase products formed in the presence of NO x from isoprene photooxidation are oligoesters; these oligoesters are produced by the aerosol-phase oligomerization of 2-methylglyceric acid (Surratt et al, 2006Szmigielski et al, 2007;Chan et al, 2010). 2-Methyltetrols and C 5 -alkenetriols are formed as particle-phase products under low NO x conditions (Surratt et al, 2006;Kleindienst et al, 2009;Paulot et al, 2009;Surratt et al, 2010) but are not produced under high NO x conditions (Surratt et al, 2006).…”

Mechanism of the hydroxyl radical oxidation of methacryloyl peroxynitrate (MPAN) and its pathway toward secondary organic aerosol formation in the atmosphere

“…2). Chan et al, 13 in a series of photochemical chamber experiments, proposed that the main fate of A* is collisional stabilization (producing A) followed by reaction with molecular oxygen to form an alkylperoxyl radical (B), where the beta-peroxyl moiety attacks at the carbonyl carbon to form a di-oxoketone (DOK) and NO 2 as a co-product (Fig. 2, mechanism 2).…”

“…For example, earlier studies of the MPAN + OH system 20,21 did not measure SOA formation and were conducted with high mixing ratios of NO x (blind to the formation of nitrogen products). More-recent laboratory studies of Chan et al 13 and Lin et al 18 were performed with methacrolein, such that products from MPAN oxidation and from the OH addition route of methacrolein (B55% probability) were formed simultaneously, complicating the analysis. 8 The theoretical study of Kjaergaard et al 17 has yet to be experimentally validated.…”