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

D Hirdman,J Ström,J F Burkhart,P K Quinn,S Eckhardt,H Sodemann,S Sharma,A Jefferson,A Stohl

doi:10.5194/acp-10-9351-2010

Abstract

Abstract. As a part of the IPY project POLARCAT (Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols and Transport) and building on previous work (Hirdman et al., 2010), this paper studies the long-term trends of both atmospheric transport as well as equivalent black carbon (EBC) and sulphate for the three Arctic stations Alert, Barrow and Zeppelin. We find a general downward trend in the measured EBC 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. The decrease at Barrow is, however, not statistically significant. The measured sulphate 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 at Barrow. To reveal the contribution of different source regions on these trends, we used a cluster analysis of the output of the Lagrangian particle dispersion model FLEXPART run backward in time from the measurement stations. We have investigated to what extent variations in the atmospheric circulation, expressed as variations in the frequencies of the transport from four source regions with different emission rates, can explain the long-term trends in EBC and sulphate measured at these stations. We find that the long-term trend in the atmospheric circulation can only explain a minor fraction of the overall downward trend seen in the measurements of EBC (0.3–7.2%) and sulphate (0.3–5.3%) at the Arctic stations. The changes in emissions are dominant in explaining the trends. We find that the highest EBC and sulphate concentrations are associated with transport from Northern Eurasia and decreasing emissions in this region drive the downward trends. Northern Eurasia (cluster: NE, WNE and ENE) is the dominant emission source at all Arctic stations for both EBC and sulphate during most seasons. In wintertime, there are indications that the EBC emissions from the eastern parts of Northern Eurasia (ENE cluster) have increased over the last decade.

Highlights

Short-lived pollutants have recently received much attention as climate forcers, in the Arctic (Quinn et al, 2008)
We find that the long-term trend in the atmospheric circulation can only explain a minor fraction of the overall downward trend seen in the measurements of equivalent black carbon (EBC) (0.3–7.2%) and sulphate (0.3–5.3%) at the Arctic stations
We find that the highest EBC and sulphate concentrations are associated with transport from Northern Eurasia and decreasing emissions in this region drive the downward trends

Summary

Introduction

Short-lived pollutants have recently received much attention as climate forcers, in the Arctic (Quinn et al, 2008). Continuous measurements of aerosol light absorption (which can be converted to equivalent BC (EBC) concentrations) at Alert and Barrow started in the late 1980s (see Table 1). These records are long enough for meaningful trend analysis. Previous studies using these data sets have shown – in agreement with the ice-core study of McConnell et al (2007) – a general decrease of EBC since the start of the measurements (Sharma et al, 2004, 2006; Quinn et al, 2007). The two parallel EBC measurement time series available from Zeppelin (Table 1) are both still relatively short but for one a decreasing trend over the last decade was reported (Eleftheriadis et al, 2009)

Methods

Results

Conclusion