Long-term characterization of organic and elemental carbon at three different background areas in northern Europe

Abstract

Elemental carbon (EC) and organic carbon (OC) are major components of atmospheric PM2.5. In this article we represent the results of long-term measurements (8–12 years) of EC and OC at three different background sites in Finland: in a rural area (Virolahti) since the summer 2010, in a marine environment (Utö) since the summer 2011, and in a clean arctic environment (Pallas/Matorova) since 2014. The concentrations of OC and EC were measured with a semi-continuous organic and elemental carbon analyser (SC-OCEC) in all the sites. The yearly average concentrations of OC varied between 0.96–3.1, 0.76–1.6 and 0.30–0.69 μg m−3 at the rural (Virolahti), marine (Utö) and arctic (Pallas/Matorova) sites, respectively. Similarly, the corresponding yearly average concentrations of EC ranged between 0.095–0.48, 0.090–0.2 and 0.010–0.086 μg m−3 at those sites. A clear seasonal variation in OC and EC concentrations was observed at each measurement site. OC concentrations were highest during summertime whereas EC concentrations were highest in wintertime. The seasonality of OC was clearest at the Arctic site that had also the largest temperature variation and shortest growing season resulting in a sharp increase in OC concentrations from June to August. At all the measurement sites, OC concentrations gradually increased when the temperature rose over sub-zero temperatures whereas the daily average EC concentrations did not show as apparent temperature dependence as OC. Based on the cluster analysis, highest OC and EC concentrations, and the highest total load for EC (23–32%), at all the sites were detected with the air mass origin of southeast. In the marine environment, the effect of black carbon from ship plumes was investigated. The limit for ship fuel sulfur content changed during the measurement period (in January 2015), but it was not observed to influence the Optical EC concentrations. Overall, long-term, continuous measurements are crucial when the time trends in air quality and the effect of emission mitigation actions are investigated. In this study, a slight decrease in OC was observed at the Marine site, however, a decrease for EC was seen both at the rural and marine sites suggesting that the emission mitigation actions like EURO limits for light vehicles or improved after-treatment systems developed for industry and energy production have already decreased the background concentrations in rural areas.