Intercomparison of Measurement Techniques for Black or Elemental Carbon Under Urban Background Conditions in Wintertime: Influence of Biomass Combustion P. REISINGER, † A. WONASCHÜTZ, † R. HITZENBERGER,* ,† A. PETZOLD, ‡ H. BAUER, § N. JANKOWSKI, § H. PUXBAUM, § X. CHI, 4 AND W. MAENHAUT 4 Faculty of Physics, University of Vienna, Vienna, Austria, DLR, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany, Institute for Chemical Technologies and Analytics, Vienna University of Technology, Vienna, Austria, and Department of Analytical Chemistry, Institute for Nuclear Sciences, Ghent University, Gent, Belgium Received June 20, 2007. Revised manuscript received September 25, 2007. Accepted October 25, 2007. A generally accepted method to measure black carbon (BC) or elemental carbon (EC) still does not exist. An earlier study in the Vienna area comparing practically all measurement methods in use in Europe gave comparable BC and EC concentrations under summer conditions (Hitzenberger et al., 2006a). Under summer conditions, Diesel traffic is the major source for EC or BC in Vienna. Under winter conditions, space heating (also with biomass as fuel) is another important source (Caseiro et al., 2007). The present study compares the response of thermal methods (a modified Cachier method, Cachier et al., 1989; a thermal-optical method, Schmid et al., 2001; and two thermal-optical (TOT) methods using Sunset instruments, Birch and Cary, 1996 and Schauer et al., 2003) and optical methods (a light transmission method, Hansen et al., 1984; the integrating sphere method, Hitzenberger et al., 1996; and the multiangle absorption photometer MAAP, Petzold and Schönlinner, 2004). Significant differences were found between the TOT methods on the one hand and all other methods on the other. The TOT methods yielded EC concentrations that were lower by 44 and 17% than the average of all measured concentrations (including the TOT data). The largest discrepancy was found when the contribution of brown carbon (measured with the integrating sphere method) was largest. 1. Introduction Black carbon (BC) or elemental carbon (EC), which is produced by incomplete combustion of carbonaceous fuels, is an important component of the atmospheric aerosol because of its light absorbing characteristics (1) and possible health effects (2). Despite intensive efforts over the last three decades, no widely accepted standard measurement method exists. Over the years, numerous method intercomparison studies were performed (see review in ref 3 and intercom- parison studies in refs 4–6). Usually, thermal methods differ little in total carbon (TC) concentrations. The major problem is the OC (organic carbon)/EC split. EC (measured with thermal methods) or BC (measured with optical methods) can differ in intercomparisons by factors of three or four depending on aerosol characteristics. In recent years, humic like substances (HULIS) were found in atmospheric aerosols (7, 8). This organic material weakly absorbs visible light with stronger absorption at short wavelengths. HULIS are therefore often called “brown” carbon (9), although brown carbon seems to include also other compounds (33). Brown carbon originates mainly from biomass fires (10) and chemical reactions in the atmosphere (11, 12). Samples containing biomass smoke present more analytical difficulties than, e.g., traffic emissions because Na and K can lower the combustion temperature of EC (13). HULIS, however, are thermally rather stable, so they evolve at higher temperatures than other OC (9, 10) leading to interferences with the EC signal. The study by Hitzenberger et al. (14) compared practically all methods for EC and BC determination in use in Europe under summer conditions in the urban area of Vienna. No significant differences were found, so at least under the conditions of the study (urban background aerosol, strong Diesel traffic source) all methods performed equally well. We decided to perform another study under wintertime conditions, when space heating contributes an important fraction to the EC or BC aerosol in Vienna (15). The goal of the present study was to investigate wheth- er the different source contributions in wintertime influence the relative performance of the measurement methods. The newly developed extension of the integrating sphere tech- nique to separate black and brown carbon is used to examine the effect of brown carbon on the comparability of the optical and thermal methods. We show here that the presence of specific wintertime sources, such as domestic heating, leads to the occurrence of brown particulate material that can interfere considerably with current EC/OC or BC determi- nation techniques. 2. Experimental Section 2.1. Site Description. Measurements were performed during the heating season from February 7 to March 15, 2006, at the roof laboratory of the Physics building of the University of Vienna, Austria, at 35 m above ground under late winter conditions. Daily average temperatures ranged from -7.7 to 6.2 °C, and average humidities were between 56.5 and 90.5%. During this period, winds were mostly from the north to west sector. The sampling site (48°13′17” N, 16°21′19”E) is situated in central Vienna (population 1.8 million) in a densely built upon area heavily impacted by traffic. The site, however, does not receive direct traffic emissions because it is separated from the nearest road by ca. 100 m of interconnected buildings and courtyards. The street directly below is used mainly for parking. Roof heights in this part of the city are rather homogeneous. Sampling was performed during the heating season, but only little direct influence from nearby chimneys is expected, because most of the buildings in the area are heated by district heating or natural gas. * Corresponding author fax: +43 1 4277 9511; e-mail: regina.hitzenberger@univie.ac.at. † University of Vienna. ‡ Institut für Physik der Atmosphäre. § Vienna University of Technology. 4 Institute for Nuclear Sciences. Environ. Sci. Technol. 2008, 42, 884–889 884 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 3, 2008 10.1021/es0715041 CCC: $40.75  2008 American Chemical Society Published on Web 01/03/2008