Organic Solvents-Based Offline Aerosol Mass Spectrometry (SOff-AMS) for Comprehensive Chemical Characterization of Ambient Organic Aerosol.

The chemical composition of atmospheric aerosols is frequently investigated offline via mass spectrometry techniques. The established offline aerosol mass spectrometry (Off-AMS) uses water for extracting airborne particulate filter samples, efficiently detecting water-soluble constituents of organic aerosols (OA), which are prevalent in secondary OA (SOA), oxygenated primary OA (POA), and aged POA. Consequently, sources with substantial water-insoluble fraction may be undetectable via Off-AMS or be subject to increased uncertainties due to the absence of useful markers (e.g., polycyclic aromatic and aliphatic hydrocarbons). This potentially compromises the investigation of primary and less-aged OA from diverse anthropogenic sources (e.g., traffic, coal combustion, waste burning, tire wear, among others). Here, we present a new analytical method that combines Off-AMS with organic solvent-based sample extraction (termed: SOff-AMS) to extract and quantify both aged and fresh aerosols simultaneously. Ultrahigh-purity methanol and high-purity acetone were used, alongside water as a reference, and the extracts were reaerosolized to be analyzed via a high-resolution time-of-flight aerosol mass spectrometer (ToF-AMS). Multiseason airborne particulate matter (PM) filter samples collected in urban and rural environments were used in these tests. The organic solvents extracted substantially higher fractions of organic carbon, which for winter samples ranged from 45 to 85% of the total organic carbon in comparison to 12-40% in water alone. The AMS spectra of samples extracted in organic solvents showed significantly increased contributions from OA fragments that are known tracers of fresh and aged emissions. These included small and polycyclic aromatic hydrocarbons and oxygenated and reduced nitrogen-containing fragments that were enhanced over a broad range of factors (1.2-50). A comparison with an online quadrupole aerosol chemical speciation mass spectrometer (Q-ACSM) in Krakow showed highly similar spectra, demonstrating that SOff-AMS-based offline measurements can provide very similar information as the online data. Future SOff-AMS-based source apportionment could identify air pollution sources more comprehensively regardless of sampling locations, particle sizes, and seasonal conditions, especially in complex urban areas with both primary and secondary source contributors.