AMS and LC/MS analyses of SOA from the photooxidation of benzene and 1,3,5-trimethylbenzene in the presence of NO<sub>x</sub>: effects of chemical structure on SOA aging

Abstract

Abstract. Oxygenated organic aerosol (OOA) observed in remote areas is believed to comprise aged secondary organic aerosol (SOA); however, the reaction processes relevant to SOA chemical aging have hitherto been unclear. We recently measured the mass spectra of SOA formed from the photooxidation of aromatic hydrocarbons using an Aerodyne aerosol mass spectrometer (AMS) and reported that SOA aging is slowed with increasing number of alkyl groups in the precursor molecule. In this study, we selected benzene and 1,3,5-trimethylbenzene (TMB) as precursors to analyze SOA formed from the photooxidation of aromatic hydrocarbons in the presence of NOx using high-resolution time-of-flight AMS (H-ToF-AMS) and liquid chromatography/time-of-flight mass spectrometry (LC/TOF-MS). A van Krevelen diagram was studied using the O/C and H/C ratios obtained by H-ToF-AMS for organics present in SOA. The results showed these organics to be rich in carboxylic acids or hydroxycarbonyls and the O/C ratio of SOA formed by the reaction of 1,3,5-TMB to be lower than that for benzene. Analytical results from LC/TOF-MS showed the particulate products formed by the reaction of 1,3,5-TMB to be richer in ketocarboxylic acids than for benzene. These results indicate that SOA aging proceeds mainly by formation of carboxylic acids and that the rate of SOA aging in laboratory chambers is limited by the oxidation of ketone groups. SOA formed in laboratory chamber experiments is less oxidized than for ambient OOA, not only because the experimental duration is insufficient or the SOA mass loading in the chamber is higher than that of the atmosphere. The laboratory chamber experiments under dry conditions are not able to simulate ketocarboxylic acid photochemical oxidation in the aqueous phase. The fractions of organic peroxides to the total SOA mass were determined by iodometric spectrophotometry to be 12 ± 8% (1,3,5-TMB) and <39% (benzene). Further, it was newly found that, unlike the reaction of benzene, only very small amounts of nitrophenols are produced by the reaction of 1,3,5-TMB.

Online measurements of water-soluble organic acids in the gas and aerosol phase from the photooxidation of 1,3,5-trimethylbenzene

“…It is possible that the IC/MS instrumentation used in this study either is not sensitive enough to detect the other species, or that these cannot be separated by the chromatographic method. However, the results presented here suggests that at least one of the compounds identified by Sato et al (2012) should be a dicarboxylic acid. If the other compounds cannot be separated, it could be that all three are dicarboxylic acids.…”

“…Sato et al (2012) report three compounds with this mass and the chemical formula C 9 H 14 O 7 , while we could identify only one peak with this mass and assigned it to a dicarboxylic acid, based on its retention time (18.6-20.1 min). It is possible that the IC/MS instrumentation used in this study either is not sensitive enough to detect the other species, or that these cannot be separated by the chromatographic method.…”

“…If the other compounds cannot be separated, it could be that all three are dicarboxylic acids. It is also possible that two of the compounds detected by Sato et al (2012) do not contain a carboxylic functional group and cannot be detected by the IC/MS method used. However, if they do have at least one such functional group, their concentration could be below the detection limit of our instrumentation.…”

“…Seven compounds could be unambiguously identified: formic acid, acetic acid, glycolic acid, butanoic acid, pyruvic acid, lactic acid and methylmaleic acid. While several possible elemental compositions are reported in Table 2 for these compounds, as it was not possible to unambiguously identify which peaks in the highresolution spectra corresponded to the unit mass observed with IC/MS, Sato et al (2012) report unique elemental composition for each reported mass. They also report other masses, which were not found in this work, either because they do not have a carboxylic acid functional group, or because the sensitivity of our method was not sufficient to detect them.…”

Dithiothreitol activity by particulate oxidizers of SOA produced from photooxidation of hydrocarbons under varied NO<sub><i>x</i></sub> levels

“…First, non-catalytic particulate oxidizers in SOA, such as organic hydroperoxides (alkyl hydroperoxides and acyl hydroperoxides) and peroxy acyl nitrates (RC(O)OONO 2 ; PANs), can oxidize sulfhydryl groups in DTT to form disulfides, sulfenic acids (RSOH), sulfinic acids (RSO 2 H), or sulfonic acids (RSO 3 H; Grek et al, 2013;Mudd, 1966). These non-catalytic particulate oxidizers are abundant in SOA sourced from various hydrocarbons (Docherty et al, 2005;Sato et al, 2012). Second, catalytic particulate oxidizers, such as quinoid substances, can oxidize sulfhydryl groups through a redox cycle (Cho et al, 2005;Kumagai et al, 2002).…”