Mechanism of Secondary Organic Aerosol Formation from Aldehyde Photooxidation: Role of Molecular Structure and NOx Concentrations

ARTHUR W. H. CHAN (1), Man Nin Chan (1), Jason D. Surratt (1), Puneet S. Chhabra (1), Christine L. Loza (1), John D. Crounse (1), Lindsay D. Yee (1), Richard C. Flagan (1), Paul O. Wennberg (1), John H. Seinfeld (1)

(1) California Institute of Technology

     Abstract Number: 250
     Last modified: May 1, 2010

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-NOx conditions. Previously, we have identified peroxy methacryloyl nitrate (MPAN) as the important intermediate to isoprene and methacrolein SOA in this NOx regime. Here we show that as a result of this chemistry, SOA formation from methacrolein and two other alpha,beta-unsaturated aldehydes, specifically acrolein and crotonaldehyde, depend strongly on the relative concentrations of NO and NO2. Oligoesters of dihydroxycarboxylic acids and hydroxynitrooxycarboxylic acids are observed to increase with increasing NO2/NO ratio. Studies of other unsaturated aldehydes reveal that molecular structure also determines the amount of SOA formation. Possible mechanisms of MPAN oxidation leading to low-volatility products are explored. 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, the SOA yields from isoprene high-NOx photooxidation are significantly greater than previously measured at lower NO2/NO ratios, and can be greater than those under RO2 + HO2 dominated conditions, making RO2 + NO2 an important channel for SOA formation.