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[1] Brown carbon is a ubiquitous and unidentified component of organic aerosol which has recently come into the forefront of atmospheric research. This component is strongly linked to the class of humic-like substances (HULIS) in aerosol whose ultimate origin is still being debated. Using a simplified spectroscopic method the concentrations of brown carbon have been determined in aqueous extracts of fine aerosol collected during the CARBOSOL project. On the basis of the results of 2-year measurements of several aerosol constituents at six European sites, possible sources of brown carbon are inferred. Biomass burning (possibly domestic wood burning) is shown to be a major source of brown carbon in winter. At elevated sites in spring, smoke from agricultural fires may be an additional source. Direct comparison of measured brown carbon concentrations with HULIS determined by an independent method reveals that the two quantities correlate well at low-elevation sites throughout the year. At high-elevation sites the correlation is still high for winter but becomes markedly lower in summer, implying different sources and/or atmospheric sinks of brown carbon and HULIS. The results shed some light on the relationships between atmospheric brown carbon and HULIS, two illdefined and overlapping components of organic aerosol.

[1] With a newly developed method based on the combination of two separation steps (by polarity and by acidity) with a universal detector for organic carbon, efficient isolation of humic-like substances (HULIS) from the matrix and quantitative determination of the isolated organic carbon is achieved. This new method was applied to determine the water extractable (HULIS WS ) and, in sequence, the 0.1 M NaOH alkaline extractable HULIS (HULIS AS ) fractions in aerosol from six sites situated at a transect from west to east across Europe. The sum of the two HULIS fractions is here defined as total HULIS (HULIS T ). The lowest 12-month average concentrations of HULIS T ranged from 0.075 mgC/m 3 the Azores (Portugal) to 1.7 mgC/m 3 at the continental background site K-puszta (Hungary). On the continent, the HULIS T concentration decreases exponentially with elevation. The relative amounts of water extractable and alkaline soluble HULIS were relatively similar at the six sites. Dramatic differences were observed for the seasonal variations of the HULIS fractions at the different sites. At the Azores, as well as at the higher mountain sites (1450 and 3100 m), a summer maximum of the HULIS T concentration was observed, while at the continental low-level sites (Aveiro and K-puszta), winter maxima dominated the seasonal variation. The summer/winter ratio of the HULIS T concentration varied from 7.1 at Sonnblick to 0.36 at Aveiro. The seasonal variation at the two continental lower-level sites with winter maxima might be explained by overlapping of a weaker summer source and a stronger winter source.

No abstract

A new approach for the carbon-specific determination of humic-like substances (HULIS) in atmospheric aerosols is presented. The method is based on a two-step isolation procedure of HULIS and the determination of HULIS carbon with a dissolved organic carbon analyzer. In the first step, a C18 solid-phase extraction is performed to separate HULIS from inorganic and hydrophilic organic sample constituents in aqueous sample solutions. The second isolation step is conducted on a strong anion exchanger to separate HULIS from remaining carbonaceous compounds. This ion chromatographic separation step including the subsequent on-line detection of HULIS carbon was performed fully automated to avoid the risk of sample contamination and to enhance the reproducibility of the method. With a 5-mL sample volume, a limit of detection of 1.0 mg C/L was obtained; this corresponds to an absolute amount of 5 microg of HULIS carbon. The reproducibility of the method given as the relative standard deviation was 4.3% (n = 10). The method was applied for the determination of water-soluble HULIS in airborne particulate matter. PM10 concentrations at an urban site in Vienna, Austria, ranged from around 0.1 to 1.8 microg of C/m(3) (n = 49); the fraction of water-soluble HULIS in OC was 12.1 +/- 7.2% (n = 49).