Abstract. In this study, we present results from 12 years of black
carbon (BC) measurements at 14 sites around the Helsinki metropolitan area (HMA)
and at one background site outside the HMA. The main local sources of BC in
the HMA are traffic and residential wood combustion in fireplaces and sauna
stoves. All BC measurements were conducted optically, and therefore we refer
to the measured BC as equivalent BC (eBC). Measurement stations were located
in different environments that represented traffic environment, detached
housing area, urban background, and regional background. The measurements of
eBC were conducted from 2007 through 2018; however, the times and the
lengths of the time series varied at each site. The largest annual mean eBC
concentrations were measured at the traffic sites (from 0.67 to 2.64 µg m−3) and the lowest at the regional background sites (from 0.16 to
0.48 µg m−3). The annual mean eBC concentrations at the detached
housing and urban background sites varied from 0.64 to 0.80 µg m−3 and from 0.42 to 0.68 µg m−3, respectively. The
clearest seasonal variation was observed at the detached housing sites
where residential wood combustion increased the eBC concentrations during
the cold season. Diurnal variation in eBC concentration in different urban
environments depended clearly on the local sources that were traffic and
residential wood combustion. The dependency was not as clear for the typically measured air quality parameters, which were here NOx concentration and mass concentration of particles smaller that 2.5 µm in diameter (PM2.5). At four sites which had at least a 4-year-long time series available, the
eBC concentrations had statistically significant decreasing trends that
varied from −10.4 % yr−1 to −5.9 % yr−1. Compared to trends determined at
urban and regional background sites, the absolute trends decreased fastest
at traffic sites, especially during the morning rush hour. Relative
long-term trends in eBC and NOx were similar, and their concentrations
decreased more rapidly than that of PM2.5. The results indicated that
especially emissions from traffic have decreased in the HMA during the last
decade. This shows that air pollution control, new emission standards, and a
newer fleet of vehicles had an effect on air quality.
Spatiotemporal variation and trends in equivalent black carbon in the Helsinki metropolitan area in Finland