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 (OAs) remains challenging because most standards for particulate organic compounds are not available, and even for those with standards, their vapor pressures are too low to be measured 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 formulae of atmospheric organic aerosols' components and their volatilities. Tmax (i.e., the temperature corresponding to the first signal peak of thermogram) for calibrants was abstracted and validated from the desorption thermograms of mixed organic and inorganic calibrants that were atomized and then collected on a PTFE filter, leading to a linear correlation between Tmax and volatility. In addition, 30 ambient filter samples were collected in winter 2019 at Wangdu station in the Beijing-Tianjin-Hebei region and analyzed by FIGAERO-LToF-CIMS, leading to the identification of 1448 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 formulae 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 and different thermal desorption behavior. The mean O/C of these two groups is 0.56±0.35 (average±1 standard deviation) and 0.18±0.08, respectively. Then the parameterizations between volatility and elemental composition for the two group compounds were obtained. Compared with previous volatility parameterizations, our functions provide a better estimation for the volatility of low-volatility organic compounds (LVOCs) in ambient organic aerosols. Furthermore, our results suggest that volatility parameterizations should be specialized for organic compounds with different O/C ratios.