Physical and optical properties of aged biomass burning aerosol from wildfires in Siberia and the Western USA at the Mt. Bachelor Observatory

Abstract. The summer of 2015 was an extreme forest fire year in the Pacific Northwest. Our sample site at the Mt. Bachelor Observatory (MBO, 2.7 km a.s.l.) in central Oregon observed biomass burning (BB) events more than 50 % of the time during August. In this paper we characterize the aerosol physical and optical properties of 19 aged BB events during August 2015. Six of the 19 events were influenced by Siberian fires originating near Lake Baikal that were transported to MBO over 4–10 days. The remainder of the events resulted from wildfires in Northern California and Southwestern Oregon with transport times to MBO ranging from 3 to 35 h. Fine particulate matter (PM₁), carbon monoxide (CO), aerosol light scattering coefficients (σ_scat), aerosol light absorption coefficients (σ_abs), and aerosol number size distributions were measured throughout the campaign. We found that the Siberian events had a significantly higher Δσ_abs∕ΔCO enhancement ratio, higher mass absorption efficiency (MAE; Δσ_abs∕ΔPM₁), lower single scattering albedo (ω), and lower absorption Ångström exponent (AAE) when compared with the regional events. We suggest that the observed Siberian events represent that portion of the plume that has hotter flaming fire conditions and thus enabled strong pyroconvective lofting and long-range transport to MBO. The Siberian events observed at MBO therefore represent a selected portion of the original plume that would then have preferentially higher black carbon emissions and thus an enhancement in absorption. The lower AAE values in the Siberian events compared to regional events indicate a lack of brown carbon (BrC) production by the Siberian fires or a loss of BrC during transport. We found that mass scattering efficiencies (MSE) for the BB events ranged from 2.50 to 4.76 m² g⁻¹. We measured aerosol size distributions with a scanning mobility particle sizer (SMPS). Number size distributions ranged from unimodal to bimodal and had geometric mean diameters (D_pm) ranging from 138 to 229 nm and geometric standard deviations (σ_g) ranging from 1.53 to 1.89. We found MSEs for BB events to be positively correlated with the geometric mean of the aerosol size distributions (R² = 0.73), which agrees with Mie theory. We did not find any dependence on event size distribution to transport time or fire source location.