Aerosol black carbon at five background measurement sites over Finland, a gateway to the Arctic

Abstract

Aerosol equivalent black carbon (BCe) was measured at five different background stations in Finland, with the longest data set from Hyytiälä, December 2004–December 2008. Measurements were conducted either with an aethalometer or a Multi-Angle Absorption Photometer, MAAP. Measured black carbon concentrations were highest in Virolahti in southeastern Finland, with annual averages ranging from 385 to 460 ng m−3, followed by Hyytiälä (250–370 ng m−3), Utö (230–270 ng m−3), Puijo (225–230 ng m−3), and Pallastunturi (60–70 ng m−3) in northern Finland. The BCe fractions of measured PM2.5 concentrations were generally between 5 and 10%, with highest fractions at Virolahti close to the Eastern border. At all the stations, the highest concentrations were observed during the spring and the winter, and the lowest concentrations during the summer. The seasonal cycle could generally be attributed to the reaching of long-range-transported black carbon. Additional reasons were increasing domestic wood burning and reduced boundary-layer height during winter, and a more effective vertical mixing during summer. The highest concentrations for each station occurred with southerly winds, and on the basis of trajectory analyses, the source areas of BCe resided mostly in Central and Eastern Europe. Occasionally the long-range-transported BCe concentrations were elevated for short periods to fulfill the characteristics of pollution episodes. From these episodes, about 62% were a result of non-fire anthropogenic sources and 36% due to open biomass burning sources. Episodes from the biomass burning sources were most often observed during the spring.