Characteristics of black carbon aerosol mass concentration over the East Baltic region from two-year measurements

Continuous measurements of black carbon (BC) aerosol mass concentration were performed at a background site Preila (55°55′N, 21°00′E, 5 m a.s.l., Lithuania) during the period 2008–2009. The data were used to characterize the BC mass concentration distribution over the East Baltic region. High increase in aerosol BC concentration was associated with the change in air mass characteristics and biomass burning during the winter heating season and spring wildfires. Monthly means of BC concentration ranged from 212 to 1268 ng m⁻³ and the highest hourly means of concentration were from 4800 to 6300 ng m⁻³, predominantly in spring and winter months. During the October–April period the BC mass concentrations were about twice as high as those in the summertime. The BC diurnal pattern in winter was typically different from that in spring indicating the seasonal variation of the atmospheric boundary layer height. The weekday/weekend difference was not strongly pronounced because the BC concentrations in Preila are mainly affected by long-range transport or local sources. Typical periodicities caused by anthropogenic and meteorological influences have been identified using Fourier analysis. It was shown that domestic heating appears as a 365 day periodicity; traffic slightly contributes 5–7 day peaks in the spectrum and elevated long-range BC can be identified as characteristic peaks with periodicities in the range from 16 to 29 days. Temporal evolution and transport of BC aerosols were interpreted by the air mass backward trajectory analysis in conjunction with the examination of the wavelength dependence on the aethalometer data. Air masses originated from the North Atlantic Ocean and Scandinavia were favourable for lower BC concentrations (350 ng m⁻³), while the BC level associated with the Western Europe airflows was significantly higher (970 ng m⁻³). The mean values of Ångström exponent of the absorption coefficient (monthly means 1.45 ± 0.25 and 0.84 ± 0.50 over January and June, respectively) revealed that the BC concentration observed over the East Baltic is influenced by submicron sized particles as a result of incomplete biomass combustion during the winter season.