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Seasonal variability of carbon in HULIS of size-segregated organic aerosols from an urban background environment

Water-soluble organic fraction of atmospheric aerosols, usually quantified as water-soluble organic carbon (WSOC) represents a highly variable fraction (10 - 80%) of organic carbon in ambient aerosols, depending on season, location, time-of-day, and particle size. Particularly secondary organic aerosols (SOA) are associated with high levels of water-soluble organic fraction, consisting of oxidized and more soluble organic compounds, induced by photooxidation of anthropogenic or biogenic precursors (Pöschl, 2005 ; Ervens et al., 2011). One of the most important classes of water soluble organics extracted from atmospheric aerosols as well as isolated from fog and cloud waters has derived the term HUmic-LIke Substances (HULIS) due to their physico-chemical similarities to that of natural humic matter (Graber and Rudich, 2006). Field experiments have observed HULIS in nearly all types of aerosol ; the carbon fraction of HULIS (HULIS-C) in WSOC can be very variable, but generally constitutes 24-72% of WSOC (Zheng et al., 2013). While size distribution of WSOC has been studied widely, the information on size distribution of HULIS is very limited (Salma et al., 2013). In addition, very little is known on seasonal variations of HULIS in size-segregated ambient aerosols. The present study is focused on the seasonal variations of size-resolved WSOC and HULIS-C in ambient aerosols collected by a ten-stage low-pressure Berner impactor at an urban background environment in Ljubljana, Slovenia. The obtained data were correlated with those of levoglucosan as a primary tracer for biomass burning emissions as well as with aerosol mass (PM) and total carbon (TC). Mass size distribution patterns of PM, WSOC and HULIS-C for all seasons, and also levoglucosan for winter and autumn were obtained as semi-smooth curves with characteristic modes. On the basis of the characteristics of HULIS-C in water soluble organic aerosol the possible sources and formation mechanisms will be discussed. As shown in Figure 1 the mass size distribution of HULIS-C in winter was bimodal, with a clear ultrafine mode and a dominant accumulation mode with 97% contribution to the total HULIS-C concentration. For autumn, the distribution of HULIS-C consisted of three modes, and was nearly the same as for WSOC with almost the same MMADs (mass median aerodynamic diameters) and also the same contributions to each mode, i.e. 7% to ultrafine, 83% to accumulation and 10% to coarse mode. The very similar size distributions of measured species (WSOC, HULIS-C, levoglucosan) indicate their common source, and since levoglucosan is a specific tracer for biomass burning we can conclude that biomass burning during colder seasons was the main contributor to both WSOC and HULIS-C in fine aerosols. This work was supported by the European Commission and the Croatian Ministry of Science, Education and Sports through Marie Curie FP7-PEOPLE-2011-COFUND, project NEWFELPRO and by the Slovenian Research Agency (Contract no. P1-0034-0140).

size segregated organic aerosols, water soluble organic carbon, HULIS

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