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 hyd rocarbon-NOx mixture was generated in a 190 m(3) outdoor Teflon chamber. Th e photooxidation reaction of toluene in the gas phase leads to substituted aromatics (TOL-AR), nonaromatic ring retaining (TOL-R), and ring opening pr oducts (TOL-RO). In this work, the following ring opening oxycarboxylic aci ds were newly identified: glyoxylic acid, methylglyoxylic acid, 4-oxo-2-but enoic acid, oxo-C-5-alkenoic acids, dioxopentenoic acids, oxo-C-7-alkadieno ic acids, dioxo-C-6-alkenoic acids, hydroxydioxo-C-7-alkenoic acids, and hy droxytrioxo-C-6-alkanoic acids. The newly characterized TOL-R and TOL-RO pr oducts 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-pentafluorobenzy l)hydroxylamine hydrochloride (PFBHA) for carbonyls and pentafluorobenzyl b romide (PFBBr) for carboxylic acid and phenol groups and analyzed using a g as chromatograph/mass spectrometry (GC/MS) in an electron impact mode (EI) and a gas chromatograph/ion trap mass spectrometry (GC/ITMS) in both chemic al 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 (FT IR) was used to obtain additional functional group information for SOA prod ucts impacted on a zinc selenide FTIR disk. The major SOA products under th e 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 m ajor SOA products, the experimental partitioning coefficients (K-i(p)) of a ldehyde products were much higher and deviated more from predicted K-i(p) v alues. This is an extremely important result, because it shows that aldehyd e products can further react through heterogeneous processes, which may be a very significant SOA generation mechanism from the oxidation of aromatics in the atmosphere.