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As a part of the IPY project POLARCAT (Polar Study using Aircraft, RemoteSensing, Surface Measurements and Models, of Climate, Chemistry, Aerosolsand Transport) and building on previous work (Hirdman et al., 2010), thispaper studies the long-term trends of both atmospheric transport as well asequivalent black carbon (EBC) and sulphate for the three Arctic stationsAlert, Barrow and Zeppelin. We find a general downward trend in the measuredEBC concentrations at all three stations, with a decrease of −2.1±0.4 ng m−3 yr−1 (for the years 1989–2008) and −1.4±0.8 ng m−3 yr−1(2002–2009) at Alert and Zeppelin respectively. Thedecrease at Barrow is, however, not statistically significant. The measuredsulphate concentrations show a decreasing trend at Alert and Zeppelin of−15±3 ng m−3 yr−1 (1985–2006) and −1.3±1.2 ng m−3 yr−1(1990–2008) respectively, while there is no trend detectable atBarrow. To reveal the contribution of different source regions on these trends, weused a cluster analysis of the output of the Lagrangian particle dispersionmodel FLEXPART run backward in time from the measurement stations. We haveinvestigated to what extent variations in the atmospheric circulation,expressed as variations in the frequencies of the transport from four sourceregions with different emission rates, can explain the long-term trends inEBC and sulphate measured at these stations. We find that the long-termtrend in the atmospheric circulation can only explain a minor fraction ofthe overall downward trend seen in the measurements of EBC (0.3–7.2%) andsulphate (0.3–5.3%) at the Arctic stations. The changes in emissions aredominant in explaining the trends. We find that the highest EBC and sulphateconcentrations are associated with transport from Northern Eurasia anddecreasing emissions in this region drive the downward trends. NorthernEurasia (cluster: NE, WNE and ENE) is the dominant emission source at allArctic stations for both EBC and sulphate during most seasons. Inwintertime, there are indications that the EBC emissions from the easternparts of Northern Eurasia (ENE cluster) have increased over the last decade

Long-term trends of black carbon and sulphate aerosol in the Arctic: changes in atmospheric transport and source region emissions