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

Abstract

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 NOₓ 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 NOₓ level, with acidic products formed under high-NOₓ 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-NOₓ conditions produces polyesters involving 2-methylglyceric acid as a key monomeric unit. These oligomers comprise ∼22−34% of the high-NOₓ SOA mass. Under low-NOₓ 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-NOₓ conditions; these compounds are also found in aerosol collected from the Amazonian rainforest, demonstrating the atmospheric relevance of these low-NOₓ chamber experiments.