Chemical Composition of Secondary Organic Aerosol Formed from the Photooxidation of Isoprene

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 110; no. 31; pp. 9665 - 9690
• Main Authors: Surratt, Jason D, Murphy, Shane M, Kroll, Jesse H, Ng, Nga L, Hildebrandt, Lea, Sorooshian, Armin, Szmigielski, Rafal, Vermeylen, Reinhilde, Maenhaut, Willy, Claeys, Magda, Flagan, Richard C, Seinfeld, John H
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 10.08.2006
• Subjects: Aerosols - chemistry; Butadienes - chemistry; Hemiterpenes - chemistry; Mass Spectrometry; Molecular Structure; Nitric Oxide - chemistry; Oxidation-Reduction; Pentanes - chemistry; Photochemistry
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp061734m

Recent work in our laboratory has shown that the photooxidation of isoprene (2-methyl-1,3-butadiene, C5H8) leads to the formation of secondary organic aerosol (SOA). In the current study, the chemical composition of SOA from the photooxidation of isoprene over the full range of NO x conditions is investigated through a series of controlled laboratory chamber experiments. SOA composition is studied using a wide range of experimental techniques:  electrospray ionization−mass spectrometry, matrix-assisted laser desorption ionization−mass spectrometry, high-resolution mass spectrometry, online aerosol mass spectrometry, gas chromatography/mass spectrometry, and an iodometric-spectroscopic method. Oligomerization was observed to be an important SOA formation pathway in all cases; however, the nature of the oligomers depends strongly on the NO x level, with acidic products formed under high-NO x conditions only. We present, to our knowledge, the first evidence of particle-phase esterification reactions in SOA, where the further oxidation of the isoprene oxidation product methacrolein under high-NO x conditions produces polyesters involving 2-methylglyceric acid as a key monomeric unit. These oligomers comprise ∼22−34% of the high-NO x SOA mass. Under low-NO x conditions, organic peroxides contribute significantly to the low-NO x SOA mass (∼61% when SOA forms by nucleation and ∼25−30% in the presence of seed particles). The contribution of organic peroxides in the SOA decreases with time, indicating photochemical aging. Hemiacetal dimers are found to form from C5 alkene triols and 2-methyltetrols under low-NO x conditions; these compounds are also found in aerosol collected from the Amazonian rainforest, demonstrating the atmospheric relevance of these low-NO x chamber experiments.