Aerosol characteristics at the Alpine site of Innsbruck, Austria

Aerosol optical depth (AOD) is being measured in Innsbruck, Austria. Since 2007 a precision filter radiometer (PFR) is set up at 620 m above sea level in order to characterize the aerosol conditions at this Alpine site. The five year time series is analyzed with respect to AOD, the Angstrom coefficient a and the curvature of the 1n(τ)-1n(λ) relationship γ. Information on the fine mode particle fraction and its modal radius is obtained with a graphical method. For special cases such as Saharan dust events the King inversion algorithm is applied. The PFR measures direct irradiance at 368 nm, 412 nm, 500 nm and 862 nm. It is also employed as a network instrument in the Global Atmosphere Watch (GAW) Program of the World Meteorological Organization. The Innsbruck annual aerosol pattern reveals monthly mean AOD at 500 nm in winter between 0.05 and 0.1 and up to 0.22 in summer. Daily AOD at 500 nm remains below 0.5 for all measurement days. This seasonality in AOD is typical for mid-latitude sites. The overall mean γ is -1.53+/-0.29 and the overall mean γ is -0.53+/-0.26. These parameters indicate submicron aerosols prevailing in Innsbruck. On 93% of the days a fine mode fraction of more than 70% is detected. A fine mode radius between 0.1 μm and 0.15 μm is observed on 90% of the days.