Ground-based aerosol optical depth trends at three high-altitude sites in Switzerland and southern Germany from 1995 to 2010

[1] Ground-based aerosol optical depth (AOD) climatologies at three high-altitude sites in Switzerland (Jungfraujoch and Davos) and Southern Germany (Hohenpeissenberg) are updated and re-calibrated for the period 1995–2010. In addition, AOD time series are augmented with previously unreported data, and are homogenized for the first time. Trend analysis revealed weak AOD trends (λ = 500 nm) at Jungfraujoch (JFJ; +0.007 decade−1), Davos (DAV; +0.002 decade−1) and Hohenpeissenberg (HPB; −0.011 decade−1) where the JFJ and HPB trends were statistically significant at the 95% and 90% confidence levels. However, a linear trend for the JFJ 1995–2005 period was found to be more appropriate than for 1995–2010 due to the influence of stratospheric AOD which gave a trend −0.003 decade−1 (significant at 95% level). When correcting for a recently available stratospheric AOD time series, accounting for Pinatubo (1991) and more recent volcanic eruptions, the 1995–2010 AOD trends decreased slightly at DAV and HPB but remained weak at +0.000 decade−1 and −0.013 decade−1 (significant at 95% level). The JFJ 1995–2005 AOD time series similarly decreased to −0.003 decade−1(significant at 95% level). We conclude that despite a more detailed re-analysis of these three time series, which have been extended by five years to the end of 2010, a significant decrease in AOD at these three high-altitude sites has still not been observed.