ATMOSPHERIC AEROSOL AND WATER-VAPOR CHARACTERISTICS OVER NORTH CENTRAL CANADA DURING BOREAS

A network of five automated and two handheld solar radiometers was ope rated during the 1993-1996 Boreal Ecosystem-Atmosphere Study (BOREAS) in northern Saskatchewan and Manitoba, Canada, in order to characteriz e the atmospheric aerosol properties. Direct solar measurements were u sed to measure atmospheric transmission and infer aerosol optical thic kness and water vapor column abundance. Near-Sun sky radiance measurem ents (solar aureole) were used to estimate the aerosol size distributi on. Aerosol conditions were heavily influenced by the presence or abse nce of forest fires. In 1996, when few fires occurred, conditions were uniform across the region with median aerosol optical thickness (AOT) at 500 nm of 0.12 and 90th percentile values of 0.27 for the May-Octo ber period. During the 1994 and 1995 seasons, numerous fires occurred in the vicinity of the sites. The median AOT values were comparable wi th the 1996 values, though the 90th percentile values were larger, in general measuring 0.85 (southern 1994 season was 0.43). Median column water vapor measurements for the same 7 month period were in the range from 1.32 to 1.58 cm at both sites, with 1995 being the driest year o f observation. Winter median values of AOT and water vapor were typica lly 0.09 and 0.34 cm, respectively. Size distributions derived from so lar almucantar measurements show the predominance of small particles d uring smoke episodes when compared to that for background conditions. Spectral dependence of the AOT as characterized by the wavelength expo nent, alpha, asymptotes at 1.8 for high optical depths for a 7 month s eason of cloud-screened data at the northern young jack pine site. Thi s observed wavelength exponent for boreal biomass burning conditions i s within the range of values found during the burning season in a stud y in Brazil.