Composition of water-soluble organic carbon in non-urban atmospheric aerosol collected at the Storm Peak Laboratory

Water-soluble organic constituents of PM2.5 aerosol (particulate matter with an aerodynamic diameter <= 2.5 mu m) have not been well characterised so far. The goal of this work was to perform quantitative analysis of individual water-soluble organic species in aerosol samples collected in July of 2010 at the Storm Peak Laboratory (3210 m above sea level) located in the Colorado Park Range (Steamboat Springs, CO, USA). Aqueous extracts were combined into six composites and analysed for organic carbon (OC), water-soluble organic carbon (WSOC), water-insoluble OC, inorganic ions, organic acids, lignin derivatives, sugar-alcohols, sugars and sugar-anhydrates. Analysis of higher molecular weight water-soluble organics was done using ultrahigh resolution mass spectrometry. Approximately 2400 positive and 4000 negative ions were detected and assigned to monoisotopic molecular formulae in the mass range of 100-800 Da. The higher number of negative ions reflects a predominant presence of highly oxidised organic compounds. Individual identified organic species represented up to 30% of the water-soluble organic mass (WSOM). The WSOM fractions of the low molecular weight organic acids, sugars and sugar alcohols were 3-12 %, 1.0-16% and 0.4-1.9 %. Significant amounts of arabitol, mannitol and oxalic acid are most likely associated with airborne fungal spores and conidia that were observed on the filters using high resolution electron microscopy. Overall, higher concentrations of sugars (glucose, sucrose, fructose etc.) in comparison with biomass burning tracer levoglucosan indicate that a significant mass fraction of WSOC is related to airborne biological species.