Atmospheric Chemistry and Physics, vol. 13, issue 19 (2013) pp. 10125-10141
During the CalNex study (15 May to 16 June 2010) a large suite of instruments was operated at the Los Angeles area ground supersite to characterize the sources and atmospheric processing of atmospheric pollu-tion. The thermal-desorption proton-transfer-reaction mass-spectrometer (TD-PTR-MS) was deployed to an urban area for the first time and detected 691 organic ions in aerosol samples, the mean total concentration of which was esti-mated as 3.3 µg m −3 . Based on comparison to total organic aerosol (OA) measurements, we estimate that approximately 50 % of the OA mass at this site was directly measured by the TD-PTR-MS. Based on correlations with aerosol mass spectrometer (AMS) OA components, the ions were grouped to represent hydrocarbon-like OA (HOA), local OA (LOA), semi-volatile oxygenated OA (SV-OOA), and low volatil-ity oxygenated OA (LV-OOA). Mass spectra and thermo-grams of the ion groups are mostly consistent with the as-sumed sources and/or photochemical origin of the OA com-ponents. The mass spectra of ions representing the primary components HOA and LOA included the highest m/z, con-sistent with their higher resistance to thermal decomposition, and they were volatilized at lower temperatures (∼ 150 • C). Photochemical ageing weakens C-C bond strengths (also re-sulting in chemical fragmentation), and produces species of lower volatility (through the addition of functional groups). Accordingly the mass spectra of ions representing the ox-idized OA components (SV-OOA, and LV-OOA) lack the highest masses and they are volatilized at higher tempera-tures (250–300 • C). Chemical parameters like mean carbon number (n C), mean carbon oxidation state (OS C), and the atomic ratios O / C and H / C of the ion groups are consistent with the expected sources and photochemical processing of the aerosol components. Our data suggest that chemical frag-mentation gains importance over functionalization as photo-chemical age of OA increases. Surprisingly, the photochem-ical age of OA decreases during the daytime hours, demon-strating the importance of rapid production of new (photo-chemically young) SV-OOA during daytime. The PTR de-tects higher organic N concentrations than the AMS, the rea-sons for which are not well understood and cannot be ex-plained by known artifacts related to PTR or the AMS. The median atomic N / C ratio (6.4 %) of the ion group represent-ing LV-OOA is a factor 2 higher than N / C of any other ion group. This suggests a multiphase chemical source involving ammonium ions is contributing to LV-OOA.
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