AEROSOL BLACK CARBON AT FIVE BACKGROUND MEASUREMENT SITES IN NORTHERN EUROPE. Keywords: BLACK CARBON; AETHALOMETER; MULTI ANGLE ABSORPTION PHOTOMETER.

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1 AEROSOL BLACK CARBON AT FIVE BACKGROUND MEASUREMENT SITES IN NORTHERN EUROPE A.-P. HYVÄRINEN 1, P. KOLMONEN 1, V.-M. KERMINEN 1, 2, A. VIRKKULA 2, A. LESKINEN 3, M. KOMPPULA 3, P. AALTO 2, M. KULMALA 2, K. E. J. LEHTINEN 3, Y. VIISANEN 1, and H. LIHAVAINEN 1 1 Finnish Meteorological Institute, Helsinki, FI-00101, Helsinki, Finland. 2 Department of Physics, University of Helsinki, P.O. Box 64, Helsinki, Finland. 3 Department of Physics, University of Eastern Finland, Kuopio, FI-70210, Kuopio, Finland. Keywords: BLACK CARBON; AETHALOMETER; MULTI ANGLE ABSORPTION PHOTOMETER. INTRODUCTION Black carbon (BC), a by-product of fossil fuel combustion and biomass burning (e.g. Bond et al., 2004), is an important component of atmospheric particulate matter. Being a strong absorber of solar radiation, BC has potentially large effects on climate and the hydrological cycle. Atmospheric heating by BC causes a strong positive radiative forcing at the top of the global atmosphere, offsetting a large fraction of the cooling by other aerosol components (Haywood et al. 1997; Jacobson, 2004; Ramanathan and Carmichael, 2008). Deposited BC reduces the reflectance of snow (e.g. Warren and Wiscombe 1980), which might significantly influence on local and regional climate (e.g. Flanner et al. 2009). It was recently estimated that BC may have contributed to more than a half of the observed arctic warming since 1890, most of this occurring during the last three decades (Shindell and Faluvegi 2009). There is a large uncertainty in the emission estimates of atmospheric BC. For a better understanding of the climatic impacts of atmospheric BC, a more accurate and up-to-date knowledge of global distribution is needed. Measurement data can corroborate satellite and model results, as well as serve as useful information for public health assessments. Concerning remote, northern parts of Europe, little information on the concentration levels and temporal variability of BC is available (Ricard et al., 2002; Eleftheriadis et al., 2009; Yttri et al. 2007). Such data would be of great value when studying the effects of BC on Arctic climate and when evaluating atmospheric models that simulate the long-range atmospheric transport of BC. This paper provides the first overview on the temporal and spatial variability of BC in atmospheric aerosols over Finland in Northern Europe. The analysis is based on measurements made at five Finnish background stations until the end of METHODS The measurements were conducted at five different locations in Finland (Figure 1), including the Global Atmosphere Watch (GAW) station at Pallastunturi, SMEAR II station in Hyytiälä, Puijo tower in Kuopio, and EMEP stations in Utö and Virolahti. The equivalent BC measurements were conducted with two different instruments: the AE 31 Aethalometer (Magee Scientific) and the multi angle absorption photometer (MAAP) (Thermo Scientific). Backward trajectories were also calculated with the FLEXTRA model (Stohl 1995) for each station for a source region analysis.

2 Figure 1. Locations of the measurement stations. RESULTS AND DISCUSSION In general, the Pallastunturi station had the lowest BC concentrations due to its remote, Arctic location, with a grand average value of about 70 ng m -3. BC concentrations measured in Virolahti were the highest (grand average 450 ng m -3 ), probably due to its close proximity to Central and Eastern Europe. Interestingly, BC concentrations measured in Hyytiälä (grand average 320 ng m -3 ) were higher than those in Utö (grand average 250 ng m -3 ), especially during winter, even though the southerly location of Utö would suggest it to be more affected by Central European sources. The BC concentrations in Utö resembled more those measured further north in Puijo (grand average 225 ng m -3 ). The BC concentrations exhibit a clear seasonal trend (Fig 2). At all the stations, BC concentrations were at their highest during the spring and winter, and lowest during the summer. This can be attributed to a reduced boundary layer height during winter and more effective vertical mixing during summer, as well as stronger BC sources in Central and Southern Europe during winter.

3 Figure 2. Aerosol black carbon at the background stations in Finland. 10 % percentile (error bar), 25 % percentile (box bottom), median (horizontal line), mean (rectangle), 75 % percentile (box top), and 90 % percentile (error bar) calculated from hourly averages are presented for each month. Only months with data coverage > 50 % are considered. SOURCE REGIONS A source region analysis based on backward trajectories was performed for all the stations. After checking that the analysis did not differ significantly for any individual station, the full data set for all the stations

4 was used. A statistical representation of the trajectory directions can be seen in Figure 3, in which the percent time of trajectories spent in each sector is shown. At each station the prevailing trajectory directions were degrees. This is generally the clean sector from the Atlantic Ocean, representing the polar nature of measured air masses. During winter, the portion of southerly trajectories increases due to the Arctic front moving to more southern latitudes. Even though the increase of the southerly sectors was only 4 % on average, this is important, as the sources of long range transported BC are in the south. Figure 3. Distribution of trajectory sectors by seasons. Each direction corresponds to the upper value of a 30 degree sector. The black carbon concentrations of the source regions were determined for each of the four seasons: spring, summer, autumn, and winter. On the basis of this analysis the source areas of BC seem to mostly reside in Central and Eastern Europe (Figure 4). During the spring and summer, increased source contribution can be seen in Eastern Europe. This is mostly due to the fire season. During spring and early summer the land in the region is cultivated by agricultural fires and forest fires occur often, too. Due to evolution of the Arctic front, the source region emphasis moves towards south during the winter, allowing the strong emission from Southern Europe to be transported all the way to Finland. When compared to known anthropogenic emission inventories (Schaap et at. 2004), we find a higher fraction of source areas in Eastern Europe. This indicates the importance of fires as well as the general pathways of air masses.

5 Figure 4. Source region maps according to different seasons. Color scale in a.u. Top left: spring, top right: summer, bottom left: autumn, bottom right: winter. White color means there was an insufficient number of trajectories from the area. The source region analysis made in this paper can be assumed to be only qualitative, as there are no deposition schemes taken into account. Furthermore, it emphasizes the sources that are important from the point of view of the stations where the measurements were conducted at, and thus may not represent the actual emission strengths. Such an analysis could be conducted if measurements were obtained from a uniform distribution around the sources, rather than from one part of Europe. CONCLUSIONS This study demonstrates the importance of long-range transport in establishing background BC concentrations in northern Europe, with source regions extending down to southern Europe during the summertime. Furthermore, over this region, the average BC concentration field was shown to display a strong decrease (a factor of 4-5) in the south-north direction and less so (factor of about 2) in the east-west direction. Our findings and data are useful for evaluating how regional and large-scale atmospheric models are able to simulate the transport of BC to Arctic areas, which is essential when assessing the role of BC in Arctic climate change. ACKNOWLEDGEMENTS The financial support by the Academy of Finland and the Maj & Tor Nessling foundation are gratefully acknowledged.

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