Optical properties of boreal forest fire smoke derived from Sun photometry

Abstract

Aerosol optical properties derived from S un photometry were investigated in terms of climatological trends at two S un photometer sites significantly affected by western Canadian boreal forest fire smoke and in terms of a 2‐week series of smoke events observed at stations near and distant from boreal forest fires. Aerosol optical depth (τ a ) statistics for Waskesiu, Saskatchewan, and Thompson, Manitoba, were analyzed for summer data acquired between 1994 and 1999. A significant correlation between the geometric mean and the forest fire frequency indices (hot spots) was found; on the average, 80% of summertime optical depth variation in western Canada can be linked to forest fire sources. The average geometric mean and geometric standard deviation at 500 nm was observed to be 0.074 and 1.7 for the clearest, relatively smoke‐free summer and 0.23 and 3.0 for the summer most influenced by smoke. A systematic decrease of fine mode Angstrom exponent (α f ) was noted ( d α f / d log τ a ∼ −0.6). This decrease roughly corresponds to an increase in the fine mode effective radius ( r eff ) from 0.09 to 0.15 μm and an abundance ( A ) to size rate increase near 2.0 ( d log A / d log r eff ). A 1998 series of forest fire events was tracked using TOMS, AVHRR, and GOES imagery, back trajectories, and data from six Sun photometer sites in Canada and eastern United States. The results showed rates of decrease of α f with increasing τ a which were similar to the climatological data. An analysis in terms of source to station distance showed a decrease in α f and an increase in r eff with increasing distance. This observation was coherent with previous observations on the particle growth effects of aging.