Chemical nature and molecular weight distribution of the water-soluble fine and ultrafine PM fractions collected in a rural environment

PM1–2.5, PM0.1–1, and PM<0.1 water-soluble organic carbon (WSOC) fractions of aerosol samples collected during summer and fall 2009 were analyzed by UV/VIS spectroscopy and electrospray ionization–mass spectrometry (ESI/MS). The focus of this study was to compare optical properties, chemical nature, and molecular weight distribution in samples extracted from different size fractions and collected during different seasons (fall vs. summer). Diurnal patterns were also investigated. An increase in absorptivity, aromaticity, and average molecular weight (AMW) in all size fractions found in the fall samples indicates different formation processes for the organic carbon between the summer and fall periods. The fall ultrafine fraction (PM<0.1) demonstrated characteristics different from the other two PM size fractions and more similar to aquatic fulvic acids. It had the highest HULIS/WSOC ratio, molar absorptivity, and AMWs up to about 700 Da when analyzed by the UV/VIS method and about 475 Da by the ESI/MS. Higher concentrations of organosulfate (OS) compounds and polycarboxylic acids were detected in the summer samples while organonitrate (ON) compounds and monocarboxylic acids were higher in the fall samples.