Vertical distribution of the spectral aerosol optical depth in the Arctic from 1993 to 1996

During the period from summer 1993 to spring 1996, 51 profiles of spectral aerosol optical depth were measured with airborne Sun photometers throughout the Arctic. This period was influenced by volcanic aerosols after the Pinatubo eruption and the removal of volcanic material from the stratosphere into the troposphere. By 1996, stratospheric aerosol concentration has decreased to pre-Pinatubo levels. Mean values of aerosol optical depth have changed during the period from 1993 to 1996 from 0.09 to 0.02 at 403 nm and from 0.065 to 0.01 at 1041 nm/1057 nm, respectively. Size spectra of stratospheric aerosol also show the influence of volcanic aerosols. A bimodal distribution was found with main radius modes at 0.1 mu m to 0.3 mu m and 0.75 mu m to 0.9 mu m The typical vertical distribution of tropospheric aerosols in the Arctic in this time period and inferred size spectra are presented. The seasonal change in the spectral aerosol optical depth with a minimum in summer and high values in spring is shown, as well as the high extinction and distinct layering of Arctic haze up into the free troposphere. Size spectra of Arctic haze also change with height and show, in addition to particles with radii smaller than 0.1 mu m, main radius modes between 0.1 mu m and 0.2 mu m and between 1 mu m and 2 mu m. Possible influence of tropospheric aerosols on the radiation balance is also discussed for the spring and summer season. While in summer the influence on the radiation balance of the very small amount of tropo-spheric aerosols is negligible, in spring, haze layers with high extinction can result in a slight warming.