摘要：Using aerosol loading data from 79 Aerosol Robotic Network (AERONET) stations with
observations from more than six years, changes in aerosol optical depth (AOD) and
Angstrom wavelength exponent (AWE) were studied. A statistical method was developed
to determine whether AOD changes were due to increased background AOD values
and/or an increased number of high AOD events. AOD decreased significantly at
AERONET sites in northeastern North American and in Western Europe, which was
accompanied by decreased AWE. Reduction of AOD there was mainly due to a decreased
frequency of high AOD events and an increased frequency of background AOD
events. In addition, decreased AOD values for high AOD events also accounted for ~ 16–32% of the AOD reduction. This is indicative of significant meteorological effects on AOD
variability. AOD trends in other regions were marginal and most were not significant;
however, AOD increased significantly at one site in the Sahel and another in Saudi Arabia,
predominantly due to the increased frequency of high AOD events and their average
AOD.
