Investigating Marine Boundary Layer Aerosol Budgets and Variability

Thesis (Master's)--University of Washington, 2016-08

Marine boundary layer (MBL) aerosol particles are an important feature of the climate system due in major part to their effect on marine boundary layer clouds, but the factors controlling their variability are not fully understood. A better understanding of the budget and controlling factors of these aerosol particles is needed to evaluate the effect of anthropogenic perturbations on aerosol concentrations and ultimately cloud radiative properties. Using data primarily collected during the MAGIC field campaign in the northeast Pacific, complemented by aerosol and meteorological reanalysis, we consider a simplified aerosol budget consisting of advection, precipitation loss, wind-mediated sea-surface generation, and entrainment from the lower free-troposphere. A major feature in MBL aerosol variability is the strong seasonal cycle, with concentrations observed to be approximately 50 cm$^{-3}$ in winter and 100 cm$^{-3}$ in the summer in the focus region (northeast Pacific). We explore seasonal differences in the concentration of accumulation mode aerosol particle number concentration using a steady-state model, which captures approximately two-thirds of the observed summer-winter difference. We find that precipitation differences account for approximately 53\% of the seasonal difference in aerosol particle concentrations, seasonal differences in advection account for 25\%, and wind-driven surface sources and entrainment account for 18\% and 4\% respectively. Secondary particle formation and growth from smaller modes are not considered. The longitudinal gradient in aerosol particle number concentration is well-reproduced in summer, but overestimated in winter. Sensitivity analysis weighted by estimated variable uncertainty show that uncertainty in aerosol particle advection and free-tropospheric aerosol concentrations are the largest contributors to modeled aerosol uncertainty. On subseasonal timescales, high aerosol concentration events (in the top quartile of 6-hourly means) are found to correlate with shallow, non-precipitating boundary layers with high overlying aerosol concentrations, with no correlation with wind speed; this was observed in both summer and winter.