Chemical composition of secondary organic aerosol formed from the photooxidation of isoprene

Recent work in our laboratory has shown that the photooxidation of isoprene (2-methyl-1,3-butadiene, C ₅H ₈) 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-methylgryceric 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 C ₅ 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.