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, C 5 H 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-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.

Secondary organic aerosol (SOA) formation from reaction of isoprene with nitrate radicals (NO<sub>3</sub>)

“…SOA components that are either nitrooxy-organic acids or nitrooxy-organosulfates are detected strongly as the [M-H] − ion, consistent with previous work (Surratt et al, 2006;Surratt et al, 2007a,b;Gao et al, 2004a,b;Gao et al, 2006), whereas the hydroxynitrates and nitrooxy-hydroxyperoxides are detected as both the [M-H] − and [M-H + C 2 H 4 O 2 ] − ions, with the latter acetic acid adduct ion, in most cases, being the base peak ion (i.e. dominant ion).…”

“…A Waters ACQUITY UPLC HSS column is selected to separate the SOA components because of its increased retention of water-soluble polar organics; separation is achieved as a result of trifunctionally-bonded (T3) C 18 alkyl residues on this column, which prevent stationary phase collapse when a 100% aqueous mobile phase is used and result in better retention of water-soluble polar organic compounds. In addition to the UPLC/(-)ESI-TOFMS analysis, all remaining Teflon filters are extracted and analyzed for total peroxide content (sum of ROOR and ROOH) by using an iodometricspectroscopic method (Docherty et al, 2005;Surratt et al, 2006).…”

“…The acetic acid adduct ions for the hydroxynitrates and the nitrooxyhydroxyperoxides are formed owing to the presence of acetic acid in the UPLC mobile phase. Previous studies have shown that non-acidic hydroxylated species (such as the 2-methyltetrols) and organic peroxides formed from the photooxidation of isoprene (Claeys et al, 2004;Edney et al, 2005;Surratt et al, 2006) are either undetectable or yield weak negative ions when using (-)ESI-MS techniques. However, it appears that the co-presence of nitrooxy groups in the hydroxylated SOA components allow for these compounds to become acidic enough to be detected by the UPLC/(-)ESI-TOFMS technique, or allow for adduction with acetic acid.…”

“…Owing to its high concentration and reactivity with OH radicals, isoprene plays an important role in the photochemistry occurring within the atmospheric boundary layer. Recently, it has been shown that the photooxidation of isoprene leads to the formation of low volatility species that condense to form SOA (Claeys et al, 2004;Edney et al, 2005;Kroll et al, 2005;Dommen et al, 2006;Kroll et al, 2006;Surratt et al, 2006); SOA yields as high as ∼3% have been observed (Kroll et al, 2005;Kroll et al, 2006). Global SOA production from isoprene photooxidation has been estimated to be about 13 Tg yr −1 (Henze et al, 2007).…”

Mechanism of the hydroxyl radical oxidation of methacryloyl peroxynitrate (MPAN) and its pathway toward secondary organic aerosol formation in the atmosphere

“…When the photooxidation of MACR is performed without inorganic seeds and under dry conditions, long oligomer families of 2MGA that are separated by 102.0317 Da (the exact mass of HMML) are observed, consistent with earlier mass spectrometry observations. 12,14,58 The SOA composition from MPAN + OH appears almost identical to that of MACR + OH + NO 2 . In addition to the 2MGA + HMML oligomers, the MACR high-NO x photooxidation produces an organic nitrogen oligomer family (Fig.…”