Have aerosols affected trends in visibility and precipitation in Europe?

Abstract

[1] Here we investigate whether the large variations in anthropogenic aerosol concentrations over the last decades have had a notable effect on precipitation in Europe. Our main focus is on the heavily industrialized region formerly known as the Black Triangle (BT), where pollution levels increased until the late 1980s and then decreased substantially. Precipitation changes in this area are compared to changes in a clean coastal region in western Europe with minor pollution trends but with potentially higher aerosol-precipitation susceptibility. We find that pollution reductions caused a substantial increase in horizontal visibility of 15 km from 1983 to 2008 in the BT, contrasting a 2.5 km increase in the clean region. Over the same period total precipitation trends show no sign of aerosol influence, neither in years of emission increases nor emission reductions, and a circulation-based precipitation estimate indicates that the observed trends may be entirely explained by atmospheric circulation changes. The annual frequency of light precipitation events, however, increased significantly in the clean region, and in the BT we find significant changes in both total and light precipitation frequency for wind directions at which sulphate trends were largest. The trends were most pronounced in the summer season. Altogether, we find no verification that changes in pollution have caused measurable changes in total precipitation in Europe, but changes in light precipitation types bear a potential aerosol signal.