Analysis of tropospheric aerosol number density for aerosols of 0.2‐ to 3‐μm diameter: Central and northeastern Canada

NASA's Atmospheric Boundary Layer Experiment conducted during the summer of 1990 focused on the distribution of trace species in central and northeastern Canada (altitudes <6 km) and the importance of surface sources/sinks, local emissions, distant transport, tropospheric/stratospheric exchange. Aircraft flights were based from North Bay, Ontario, and Goose Bay, Labrador, Canada. As part of the aircraft measurements, aerosol number density (0.2‐ to 3‐μ diameter) was measured using an optical laser technique. Results show that summertime aerosol budgets of central and northeastern Canada can be significantly impacted by the transport of pollutants from distant source regions. Biomass burning in Alaska and western and central Canada exerts major influences on regional aerosol budgets. Urban emissions transported from the U.S./Canadian border regions are also important. Aerosol enhancements (mixed layer and free troposphere) were most prevalent in air with carbon monoxide mixing ratios >110 parts per billion by volume (ppbv). When data were grouped as to the source of the air (5‐day back trajectories) either north or south of the polar jet, aerosol number density in the mixed layer showed a tendency to be enhanced for air south of the jet relative to north of the jet. However, this difference was not observed for measurements at the higher altitudes (4 to 6 km). For some flights, mixed layer aerosol number densities were >100 higher than free‐tropospheric values (3‐ to 6‐km altitude). The majority of the observed mixed layer enhancement was associated with transport of effluent‐rich air into the Canadian regions. Aerosol emissions from natural Canadian ecosystems were relatively small when compared to transport.