Carbonaceous aerosol at a forested and an urban background sites in Southern Finland

Submicrometer organic carbon (OC) and elemental carbon (EC) concentrations were studied for one year (2007–2008) in Finland at an urban background site (Helsinki) and a forested site (Hyytiälä). Particles were collected on quartz filters that were analyzed by a thermal-optical carbon analyzer in the laboratory. Results were calculated by taking into account the positive sampling artefact and the optical correction for the pyrolysis during the analysis. Typically OC and EC concentrations were higher in Helsinki than in Hyytiälä. OC was highly correlated between the sites, whereas EC had much weaker correlation. There were no clear seasonal variations for OC either in Hyytiälä or in Helsinki, whereas both sites had the lowest EC concentrations in summer. There was a clear summer maximum in OC to EC ratios in Hyytiälä reflecting that biogenic emissions had a considerable influence to OC in summer. Same phenomena could not be seen in Helsinki as there were other major sources (e.g. traffic), which had no seasonal dependency. Traffic had a clear contribution to EC concentrations in Helsinki but in winter there were also other major sources, like residential wood combustion, that affected EC concentrations. Three cases with the high OC concentrations and one case with the low OC concentration in Hyytiälä were studied in detail.