Aerosol optical depth in the UVB and visible wavelength range from Brewer spectrophotometer direct irradiance measurements: 1991–2002

Abstract

Direct irradiance measurements from Brewer spectrophotometer #66 at Ispra, Italy, were reanalyzed for the period 1991–2002. Two calibration methodologies, one adapted to the ultraviolet (UV) and one adapted to the visible wavelength range, have been developed to retrieve the aerosol optical depth (AOD) from these measurements. The measurements in the UV were calibrated in absolute units using laboratory lamp calibrations of the global entrance port of the same instrument, while in the visible wavelength range a zero air mass Langley extrapolation methodology was implemented. The retrieved aerosol optical depths were validated with a colocated CIMEL Sun photometer operating within the Aerosol Robotic Network (AERONET) since 1997. AOD values at Ispra show a strong short‐term variability between 0.05 and 2 at 320 nm as well as a pronounced seasonal variability. The highest monthly mean values of around 0.6 (at 320 nm) are usually found in spring and summer while the lowest values, of about 0.3, are found in the winter months. An aerosol characterization by means of the Ångström power law yields a mean wavelength exponent of 1.6. The influence of Föhn events on the aerosol climatology at Ispra was investigated. The AOD measurements at Ispra show the strong influence of the Mount Pinatubo eruption in June 1991 and the following decay to background levels in late 1997.