Black carbon (soot) aerosol in the lower stratosphere and upper troposphere

As determined by impactor samplers flown on ER‐2 and DC‐8 aircraft, black carbon aerosol (BCA) mass loadings in the stratosphere average 0.6 nanograms per standard cubic meter, or 0.01% of the total aerosol. Upper tropospheric BCA increases to 0.3%. Low stratospheric concentration is commensurate with present commercial air traffic fuel consumption, given the following assumptions: the BCA emissions are 0.1 grams per kilogram of fuel consumed, 10% of route mileage is above the tropopause, and average BCA stratospheric residence time is about one year. Taking BCA into account, the stratospheric single scatter albedo is ≈0.99. Using parameters for planned supersonic commercial aircraft, whose emissions will be predominantly in the stratosphere, we show that such traffic will double stratospheric BCA concentration. This would reduce the aerosol single scattering albedo by one percent, and double the BCA surface area that is available for heterogeneous chemistry.