Dimming and brightening trends in direct solar irradiance from 1909 to 2010 over Davos, Switzerland: Proportions of aerosol and gaseous transmission

Abstract

Apparent atmospheric transmission has been calculated from the longest stationary pyrheliometer record worldwide located at the Physikalisch- Meteorologisches Observatorium Davos covering the years 1909 to 2010. Periods of early brightening up to 1929, dimming from 1930 to 1973, and brightening from 1974 onward were localized in this transmission record and found to be highly significant. Total transmission was partitioned into factors accounting for water vapor and aerosols. By modeling the apparent transmission of the gaseous components, aerosol transmission was obtained as a residual factor. Compared to the trend results found in total transmission, the dimming and brightening periods are more pronounced by a factor of 2; the early brightening period was found to be insignificant in aerosol transmission. The transmission records of water vapor, pure Rayleigh scattering, and ozone have shown only small trends over these periods, which are considered negligible. ©2013. American Geophysical Union. All Rights Reserved.