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

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. Key Points AOD trends at 3 European high‐alpine sites appear to be flat Our observed trends appear to be at odds with dimming/brightening models Establishing three homogenized and re‐calibrated 15‐year AOD data‐sets