Ambient Quantification and Size Distributions for Organic Aerosol in Aerosol...

Hu, W., Campuzano Jost, D. A. Day, B. Nault, T. Park, T. Lee, A. Pajunoja, A. Virtanen, P. Croteau, M. R. Canagaratna, J. T. Jayne, D. Worsnop, and J. Jimenez-Palacios (2020), Ambient Quantification and Size Distributions for Organic Aerosol in Aerosol Mass Spectrometers with the New Capture Vaporizer, Anal. Chem., 676, 676−689, doi:10.1021/acsearthspacechem.9b00310.

Aerodyne aerosol mass spectrometers (AMSs) or aerosol chemical speciation monitors (ACSMs) are widely deployed to quantify organic aerosol (OA) mass concentration and size distribution in various field and laboratory studies across the world. A nonunity collection efficiency (CE, usually 0.45−1), resulting from particle bounce (PB) on a standard vaporizer (SV), depends on the chemical composition and phase of the aerosol. The estimation of CE contributes a significant fraction of the total quantification uncertainty for these instruments. To address this uncertainty, a capture vaporizer (CV) was recently designed to reduce or eliminate PB. Here, we evaluate the quantification of ambient submicron OA with the CV, including multiple biogenic- and anthropogenic-influenced field studies. Good agreement of OA between the SV and CV has been found (slopes = 0.84−1, R > 0.9), consistent with both CE ≈ 1 for ambient OA with the CV and with the chemical composition-dependent CE (CDCE) previously developed for ambient SV data. The effects of oxidation and thermal denuding of aerosols on quantification using both vaporizers are also examined. No effect of the oxidation state of OA on quantification of SV and CV AMSs is observed. Our results show that the SV CDCE, which works well for ambient aerosols, overestimates the CE of OA after thermal denuding because of nominally increasing inorganic acidity upon heating. Size distributions of three laboratory-generated OA species have similarly delayed detection and broadened ratios in the CV versus SV as for (NH4)2SO4. The CV cannot measure size distributions at a lower vaporizer temperature (<500 °C for oleic acid and squalene and <350 °C for citric acid) because of too slow vaporization of OA. Finally, we summarize all the relevant pros and cons for using the CV versus SV in AMS and ACSM studies, which at present, point to the need to decide on the best vaporizer depending on the main objectives of a given instrument and study.

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Tropospheric Composition Program (TCP)