Chemical nature and molecular weight distribution of the water-soluble fine and ultrafine PM fractions collected in a rural environment J. Pavlovic, P.K. Hopke * Center for Air Resources Engineering and Science, Clarkson University, Potsdam, NY 13699-5708, USA highlights < Water-soluble organic carbon (WSOC) in different size PM fractions. < WSOC in the PM 1e2.5 fraction ranged from 37% to 47%. < PM <0.1 contribute the least to the WSOC content (19%e31%). < Average molecular weights ranged from 560 Da to 700 Da. < Tandem MS found carboxylic, polycarboxylic acids, organosulfates, and -nitrates. article info Article history: Received 19 October 2011 Received in revised form 20 April 2012 Accepted 27 April 2012 Keywords: Water-soluble organic carbon HULIS Organosulfates Organonitrates Ultrafine PM abstract PM 1e2.5 , PM 0.1e1 , 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 ionizationemass 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. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction During the last decade, many studies have reported that macromolecular species make significant contributions to the organic matter mass present in ambient atmospheric particulate matter (PM). The largest fraction of these species showed signifi- cant similarities in structural properties with terrestrial and aquatic humic and fulvic acids and were termed humic-like substances (HULIS). HULIS can affect many aerosol properties including hygroscopicity, cloud condensation nuclei (CCN) activity, surface tension, and optical properties (Hoffer et al., 2006). The estimated fraction of HULIS in the fine PM water-soluble organic carbon (WSOC) fraction can be variable, ranging from a low of 15e36% in Amazon biomass burning aerosol (Mayol-Bracero et al., 2002) to a high of 55e60% in European fine aerosol (Krivacsy et al., 2001). Duarte et al. (2005) reported seasonal differences and showed that that the relative aromaticity of HULIS was greater in autumn than in summer. This difference was explained by wood combustion during the colder periods and contributions from lignin breakdown. Samburova et al. (2005) observed seasonal differences in urban aerosol particle composition (Zurich, Switzerland), and also attributed it to wood combustion during the winter and photo- chemistry during the summer months. Samburova et al. (2005) measured an upper limit of 700 Da for HULIS molecular weight distribution while Kiss et al. (2003) found a maximum value of 500 Da with an average between 200 and 300 Da. Stone et al. (2009) analyzed high MW compounds from WSOC fractions and identified and quantified functional groups such as aliphatic, aromatic, and bulk carboxylic acids, organo- sulfates, and carbohydrate-like substances with MW from 200 to 600 Da. A number of products from non-oxidative (accretion) * Corresponding author. E-mail address: hopkepk@clarkson.edu (P.K. Hopke). Contents lists available at SciVerse ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2012.04.053 Atmospheric Environment 59 (2012) 264e271