Atmospheric black carbon in Helsinki

Atmospheric fine particle black carbon (BC) was measured close to downtown of Helsinki, during November 1996–June 1997. The average BC concentrations were 1.5 μg m−3 for working days, 1.2 μg m−3 for Saturdays, and about 1 μg m−3 for Sundays and public holidays. The overall average BC concentration was equal to 1.38 μg m−3 and its average contribution to fine particle mass equal to 19%. On working days BC concentrations showed a clear diurnal variation with the highest hourly average values occurring during the morning (2.7 μg m−3) and evening (1.9 μg m−3) rush hours. Contribution of the long-range transport to the BC was estimated to be on average about 0.4 μg m−3, which is believed to represent the average regional background over southern Finland. However, this transport was very different from different geographical regions: from the densely populated areas of East- and Central-Europe it was about 0.8 μg m−3, while in northerly and northwesterly air masses it was only about 0.15 μg m−3. Local traffic was by far the most important local BC source contributing about 63, 54 and 44% on working days, Saturdays and Sundays, respectively. Other local sources were largely masked by the traffic and their contribution was estimated roughly to be of the order of 10%. The rest of BC was attributed to long-range transport.