Molecular Characterization of Water-Soluble Aerosol Particle Extracts by Ultrahigh-Resolution Mass Spectrometry: Observation of Industrial Emissions and an Atmospherically Aged Wildfire Plume at Lake Baikal

Air pollution over Lake Baikal originating from anthropogenic emissions as well as wildfire events and secondary organic aerosol (SOA) formation poses a threat to this world heritage ecosystem. During a ship expedition in 2018, ambient aerosol particles were sampled on filters at different locations of Lake Baikal and extracted to obtain the fraction of water-soluble organic aerosol (WSOA). Subsequent analysis by Fourier transform ion cyclotron resonance (FTICR) mass spectrometry with direct-infusion electrospray ionization (ESI) in both ionization modes provides insights into the molecular composition of polar and high-molecular-weight species in WSOA. Light absorption spectral dependence determined by absorption Ångstrom exponent (AAE) is correlated to summed compound class abundancies to identify light-absorbing compounds. Most detected species are heavily oxidized, associated with intense atmospheric aging, and contain significant amounts of nitrogen or sulfur, with the most abundant compound classes being CHO, CHON, and CHOS. Nitrogen-containing species are more frequently found in positive ESI and correlate well with AAE. Oxygen-to-nitrogen ratios (O/N) larger than 3 indicate organic nitrates, and also less oxidized nitrogen species can be linked to brown carbon (BrC). ESI in both ionization modes provides complementary molecular information with, on the one hand, more sensitive detection of high average carbon oxidation state (OS_C) and low-volatility oxidized organic aerosol species in negative mode, and on the other hand, lower OS_C (OS_C < 0) species as well as oligomers and potential biomass burning organic aerosol in positive mode. Additionally, a great overlap of sum formulae was found at all sampling sites along the expedition route, regardless of the potential primary emission sources, indicating strong atmospheric aging and mixing, leading to a similar oxidized organic aerosol (OOA) from different primary sources.