Long-term trend analysis of aerosol variables at the high-alpine site Jungfraujoch

This study reports the first long-term trend analysis of aerosol optical measurements at the high-alpine site Jungfraujoch, which started 10.5 years ago. Since the aerosol variables are approximately lognormally distributed, the seasonal Kendall test and Sen's slope estimator were applied as nonparametric methods to detect the long-term trends for each month. The yearly trend was estimated by a least-mean-square fit, and the number of years necessary to detect this trend was calculated. The most significant trend is the increase (4-7% yr(-1)) in light-scattering coefficients during the September to December period. The light absorption and backscattering coefficients and the aerosol number concentration also show a positive trend during this time of the year. The hemispheric backscattering fraction and the scattering exponent calculated with the smaller wavelengths (450 and 550 nm), which relate to the small aerosol size fraction, decrease except during the summer, whereas the scattering exponent calculated with the larger wavelengths (550 and 700 nm) remains constant. Generally, the summer months at the Jungfraujoch, which are strongly influenced by planetary boundary layer air masses, do not show any long-term trend. The trends determined by least-mean-square fits of the scattering and backscattering coefficients, the hemispheric backscattering fractions, and the scattering exponent are significant, and the number of years necessary to detect them is shorter than 10 years. For these variables, the trends and the slopes estimated by the seasonal Kendall test are therefore confirmed by the least-mean-square fit results.