|dc.description.abstract||Biomass-burning aerosols contribute to aerosol radiative forcing on the climate system. The magnitude of this effect is partially determined by aerosol size distributions, which are functions of source fire characteristics (e.g. fuel type, MCE) and in-plume microphysical processing (occurring on a GCM sub-grid scale). The uncertainties in biomass-burning emission number size-distributions in climate model inventories lead to uncertainties in the CCN concentrations and forcing estimates derived from these models. This emphasizes the need for observational and modelling studies to better represent effective biomass-burning size-distributions in larger-gridbox models. The BORTAS-B measurement campaign was designed to sample boreal biomass-burning outflow over Eastern Canada in the summer of 2011. Using these BORTAS-B data, we implement plume criteria to isolate the characteristic size-distribution of aged biomass-burning emissions (aged ~ 1 - 2 days) from boreal wildfires in Northwestern Ontario. The composite median size-distribution yields a single dominant accumulation mode with Dpm = 230 nm (number-median diameter), σ = 1.5, which are comparable to literature values of other aged plumes of a similar type. The organic aerosol enhancement ratios (ΔOA/ΔCO) along the path of Flight b622 show values of 0.05-0.18 μg m⁻³ ppbv⁻¹ with no significant trend with distance from the source. This lack of enhancement ratio increase/decrease with distance suggests no detectable net OA production/evaporation within the aged plume over the sampling period. A Lagrangian microphysical model was used to determine an estimate of the freshly emitted size distribution and flux corresponding to the BORTAS-B aged size-distributions. The model was restricted to coagulation and dilution processes only based on the insignificant net OA production/evaporation derived from the ΔOA/ΔCO enhancement ratios. We estimate that the fresh-plume median diameter was in the range of 59-94 nm with modal widths in the range of 1.7-2.8 (the ranges are due to uncertainty in the entrainment rate). Thus, the size of the freshly emitted particles is relatively unconstrained due to the uncertainties in the plume dilution rates. Expanding on the fresh-plume coagulational modelling of the BORTAS-B plumes, a coagulation-only parameterization for effective biomass-burning size-distributions was developed using the SAM-TOMAS plume model and a gaussian emulator. Under a range of biomass-burning conditions, the SAM-TOMAS simulations showed increasing Dpm and decreasing σ (converging to 1.2) with distance from the emission source. Final Dpm also shows a strong dependence on dM/dx (Mass flux x Fire area/vg), with larger values resulting in more rapid coagulation and faster dDpm/dt. The SAM-TOMAS simulations were used to train the Gaussian Emulation Machine for Sensitivity Analysis (GEM-SA) to build a Dpm and σ parameterization based on seven inputs. The seven inputs are: emission Dpm0, emission σ0, mass flux, fire area, mean boundary layer wind (vg), time, and plume mixing depth (dmixing). These inputs are estimated to account for 81% of the total variance in the final size distribution Dpm, and 87% of the total variance in the final σ. The parameterization performs very well against non-training modelled SAM-TOMAS size-di stributions in both final Dpm (slope = 0.92, R² = 0.83, NMBE=-0.06) and final σ (slope = 0.91, R² = 0.93, NMBE = 0.01). These final size distribution parameters are meant to be inserted as effective biomass-burning aerosol size-distributions (single lognormal mode) into larger-scale atmospheric models.