Volatility Parametrization of Low-Volatile Components of Ambient Organic Aerosols Based on Molecular Formulas

Evaluating the volatility of organic compounds basedsolely ontheir molecular formulas would avoid tough demands in deriving molecularstructures. Here, we deployed an iodide-adduct Long Time-of-FlightChemical Ionization Mass Spectrometry (LToF-CIMS) combined with aFilter Inlet for Gases and AEROsols (FIGAERO) to investigate molecularformulas and thermograms of organic compounds on ambient particulatesamples collected in the summer of 2021 in a suburban site of Shanghaiand to estimate saturation vapor pressures of low- and semivolatilecomponents of ambient organic aerosols. Then, a hierarchical clusteranalysis and a subsequent classification of obtained clusters by similaritycalculation were applied to the measured data set of molecular formulasand saturation vapor pressures of organic aerosols with at least a2/3 appearance frequency, together with a similar data set collectedat a rural site in the Beijing-Tianjin-Hebei regionduring the winter of 2018 (Ren et al., 2018), to classify all compoundsinto multiple groups. For each group of compounds, parametrizationsbetween volatility and elemental composition were derived, and thenrelationships between each group of parameters and the mean O:C wereestablished to achieve a volatility-molecular formula parametrizationwith the O:C as a key input. Statistical comparison of estimated volatilitiesof low-volatile organic compounds shows a much better performanceof our parametrization than previous molecular formula-based ones. Previous studies on volatility parametrizationoverlookedO:C. This study developed a parametrization with O:C as one of inputsand with a better predictive performance for low-volatile organiccompounds.

Automated summary

In this study, a Long Time-of-Flight Chemical Ionization Mass Spectrometry (LToF-CIMS) combined with a Filter Inlet for Gases and AEROsols (FIGAERO) was used to analyze ambient particulate samples collected in Shanghai in the summer of 2021. The molecular formulas and thermograms of organic compounds were investigated, and the saturation vapor pressures of low- and semivolatile components of ambient organic aerosols were estimated. A hierarchical cluster analysis and classification were conducted to classify all compounds into multiple groups based on similarity calculation. Parametrizations between volatility and elemental composition were derived for each group, and relationships between the parameters and the mean O:C were established to achieve a volatility-molecular formula parametrization with the O:C as a key input. The study showed a better predictive performance for low-volatile organic compounds compared to previous molecular formula-based methods.