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 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
