Molecular characteristics of the water soluble organic matter in size-fractionated aerosols collected over the North Atlantic Ocean

Size-fractionated aerosol impactor samples (n = 3) representing North American, North African, and marine air mass influences were collected over the North Atlantic Ocean as part of the 2011 US GEOTRACES cruise (GEOTRACES GA03), and a molecular-level chemical characterization of the water soluble organic matter (WSOM) was obtained using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Remarkable differences were found between the molecular characteristics of WSOM from the fine (< 0.49 μm) and coarse size fractions demonstrating the importance of collecting size-fractionated aerosols in order to obtain a full molecular characterization of aerosol WSOM. The greatest molecular complexity, as defined by the number of formulas in each fraction, was found in the < 0.49 μm fraction of North American and North African-influenced aerosols, and in the > 7.2 μm fraction of marine-influenced aerosols. Principal component analysis (PCA) showed the < 0.49 μm fraction of the North American-influenced WSOM to be differentiated from all other size fractions by the prevalence of highly oxidized compounds, an indicator of atmospheric aging. The coarse fractions of the North American-influenced sample showed evidence of important contributions from low O/C (< 0.2) and condensed aromatic compounds indicative of primary combustion-influenced organic matter (OM). The fine fractions of the marine and North African-influenced samples showed characteristics of a primary marine biological source including peptide-like, phospholipid-like, and carbohydrate-like molecular formulas. The coarse fractions of the marine sample and the 0.49–3.0 μm fraction of the North African-influenced sample were defined by molecular formulas at intermediate O/C (0.2 to 0.6) and high H/C (> 1.5) ratios including many heteroatom (N, S, P) containing formulas. Condensed aromatic compounds, which originate from anthropogenic sources and can affect the global climate by absorbing solar radiation, as well as compounds that can be attributed to secondary organic aerosols (SOA), were found in all 17 WSOM fractions analyzed. This indicates the ubiquitous nature of these types of compounds across aerosol size fractions and air mass influences, even in aerosols collected over the middle of the ocean. These results provide an indication of size-fractionated aerosol composition, and future molecular characterization work should be paired with physical characteristics to inform size-fractionated and source-resolved models of aerosol OM impacts in the atmosphere.