Volatility parameterization of ambient organic aerosols at a rural site of the North China Plain

Abstract. The volatility of organic aerosols plays a key role in determining their gas-particle partitioning, which subsequently alters the physicochemical properties and atmospheric fates of aerosol particles. Nevertheless, an accurate estimation of the volatility of organic aerosols (OA) remains challenging. Because most standard particulate organic compounds are scarce, on the other hand, their vapor pressures are too low to estimate by most traditional methods. Here, we deployed an iodide-adduct Long Time-of-Flight Chemical Ionization Mass Spectrometer (LToF-CIMS) coupled with a Filter Inlet for Gases and AEROsols (FIGAERO) to probe the relationship between the molecular formulas of atmospheric organic aerosol’s components and their volatilities. A number of Tmax (i.e., the temperature corresponding to the first signal peak of thermogram) were abstracted and validated from the desorption thermograms of mixed organic and inorganic calibrants which were atomized and then collected onto a Teflon filter. Besides, 30 filter samples of ambient air were collected in winter 2019 at Wangdu station in Beijing-Tianjin-Hebei region, and analyzed by FIGAERO-LToF-CIMS, leading to the identification of 1,448 compounds dominated by the CHO (containing carbon, hydrogen, and oxygen atoms) and CHON (containing carbon, hydrogen, oxygen, and nitrogen atoms) species. Among them, 181 organic formulas including 91 CHO and 90 CHON compounds were then selected since their thermograms can be characterized with clear Tmax values in more than 20 out of 30 filter samples and subsequently divided into two groups according to their O / C ratios. The mean O / C of these two groups are 0.56 ± 0.35 (average ± one standard deviation) and 0.18 ± 0.08, respectively. We then obtained the correlation functions between volatility and elemental composition for the two group compounds. Compared with previous volatility parameterizations, our correlation functions provide a better estimation of the volatility of semi-volatility organic compounds (SVOCs) and low-volatility organic compounds (LVOCs) in the ambient organic aerosols. Furthermore, we suggest that there should be specialized volatility parameterizations for different O / C organic compounds.