The influence of residential wood combustion on the concentrations of PM2.5 in four Nordic cities

Residential wood combustion (RWC) is an important contributor to air quality in numerous regions worldwide. This study is the first extensive evaluation of the influence of RWC on ambient air quality in several Nordic cities. We have analysed the emissions and concentrations of PM2.5 in cities within four Nordic countries: in the metropolitan areas of Copenhagen, Oslo, and Helsinki and in the city of Umea. We have evaluated the emissions for the relevant urban source categories and modelled atmospheric dispersion on regional and urban scales. The emission inventories for RWC were based on local surveys, the amount of wood combusted, combustion technologies and other relevant factors. The accuracy of the predicted concentrations was evaluated based on urban concentration measurements. The predicted annual average concentrations ranged spatially from 4 to 7 mu g m(-3) (2011), from 6 to 10 mu g m(-3) (2013), from 4 to more than 13 mu g m(-3) (2013) and from 9 to more than 13 mu g m(-3) (2014), in Umea, Helsinki, Oslo and Copenhagen, respectively. The higher concentrations in Copenhagen were mainly caused by the relatively high regionally and continentally transported background contributions. The annual av-erage fractions of PM2.5 concentrations attributed to RWC within the considered urban regions ranged spatially from 0% to 15 %, from 0% to 20 %, from 8% to 22% and from 0% to 60% in Helsinki, Copenhagen, Umea and Oslo, respectively. In particular, the contributions of RWC in central Oslo were larger than 40% as annual averages. In Oslo, wood combustion was used mainly for the heating of larger blocks of flats. In contrast, in Helsinki, RWC was solely used in smaller detached houses. In Copenhagen and Helsinki, the highest fractions occurred outside the city centre in the suburban areas. In Umea, the highest fractions occurred both in the city centre and its surroundings.