Source Attribution of Aerosol Size Distributions at Whistler Mountain

Remote and free tropospheric aerosols represent a large fraction of climate relevant aerosols; however, aerosol in these regions is less characterized than that in polluted boundary layers. The difficulty is compounded at mountain peaks, where the free troposphere can experience influence from the local boundary layer due to upslope flow. Here we evaluate predictions of aerosol number and size distribution by GEOS-Chem-TOMAS, a global chemical transport model with online aerosol microphysics, using measurements from the peak of Whistler Mountain, BC, Canada (2182 m a.s.l). By replacing the high elevation model layer with the surface layer when measured temperatures exceed 3ºC and restricting the comparison to time points with less than 90% relative humidity, we increase the correlation between simulated and measured aerosol number and size distribution. Using this method to select model layers, we characterize contributions from known aerosol sources by comparing the base simulation to model runs without Asian anthropogenic emissions and without biomass burning. We find that Asian contributions travel in the free troposphere, suppress new particles < 40 nm while enhancing CCN size particles > 80 nm, and fluctuate at low levels throughout the year. Biomass burning particles are carried in the boundary layer, enhance both new particles < 10 nm and CCN size particles > 80 nm, and contribute almost exclusively during late spring and early summer. HYSPLIT back trajectories suggest contribution from biogenic secondary organic aerosols during low Asian transport and biomass burning days. The comparison of measurements and modeling gives us insight into the sources and processes that shape the aerosol at Whistler. 4-->