Characterization of secondary aerosol from the photooxidation of toluene in the presence of NOx and 1-propene

Secondary organic aerosol (SOA) from the photooxidation of toluene in a hydrocarbon-NOx mixture was generated in a 190 m(3) outdoor Teflon chamber. The photooxidation reaction of toluene in the gas phase leads to substituted aromatics (TOL-AR), nonaromatic ring retaining (TOL-R), and ring opening products (TOL-RO). In this work, the following ring opening oxycarboxylic acids were newly identified: glyoxylic acid, methylglyoxylic acid, 4-oxo-2-butenoic acid, oxo-C-5-alkenoic acids, dioxopentenoic acids, oxo-C-7-alkadienoic acids, dioxo-C-6-alkenoic acids, hydroxydioxo-C-7-alkenoic acids, and hydroxytrioxo-C-6-alkanoic acids. The newly characterized TOL-R and TOL-RO products included methylcyclohexenetriones, hydroxymethylcyclohexentriones , 2-hydroxy-3-penten-1,5-dial, hydroxyoxo-C-6-alkenals, hydroxy-C-5-triones, hydroxydioxo-C-7-alkenals, and hydroxy-C-6-tetranones. Products in both the gas and aerosol phases were derivatized with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride (PFBHA) for carbonyls and pentafluorobenzyl bromide (PFBBr) for carboxylic acid and phenol groups and analyzed using a gas chromatograph/mass spectrometry (GC/MS) in an electron impact mode (EI) and a gas chromatograph/ion trap mass spectrometry (GC/ITMS) in both chemical impact and EI modes. To confirm different isomers, the PFBHA-derivatives of products were rederivatized by silylation using N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA). The Fourier transform infrared spectroscope (FTIR) was used to obtain additional functional group information for SOA products impacted on a zinc selenide FTIR disk. The major SOA products under the high NOx conditions of the above experiment included methylnitrophenols, methyldinitrophenols, methylbenzoquinones, methylcyclohexenetriones, 4-oxo-2-butenoic acid, oxo-C-5-alkenoic acids, hydroxy-C-3-diones, hydroxyoxo-C-5-alkenals, hydroxyoxo-C-6-alkenals, and hydroxydioxo-C-7-alkenals. Of the major SOA products, the experimental partitioning coefficients (K-i(p)) of aldehyde products were much higher and deviated more from predicted K-i(p) values. This is an extremely important result, because it shows that aldehyde products can further react through heterogeneous processes, which may be a very significant SOA generation mechanism from the oxidation of aromatics in the atmosphere.