Physical and Optical Properties of Aged Biomass Burning Aerosol from Wildfires in Siberia and the Western US at the Mt. Bachelor Observatory
Abstract. The summer of 2015 was an extreme forest fire year in the Pacific North West. Our sample site at Mt. Bachelor Observatory (MBO, 2.7 km a.s.l.) in central Oregon observed biomass burning events more than 50 % of the time during August. In this paper we characterize the aerosol physical and optical properties of 19 aged biomass burning (BB) events during August 2015. Six of the nineteen events were influenced by Siberian fires originating near Lake Baikal. The remainder of the events resulted from wildfires in Northern California and Southwestern Oregon with transport times to MBO ranging from 4.5–35 hours. Fine particulate matter (PM1), carbon monoxide (CO), aerosol light scattering (σscat) and absorption (σabs), and aerosol number size distributions were measured throughout the campaign. We found that the Siberian events had significantly higher Δσabs/ΔCO enhancement ratio, higher mass absorption efficiency (MAE; Δσabs/ΔPM1), lower single scattering albedo (ω), and lower Absorption Ångström exponent (AAE) when compared with the regional events. We suspect the Siberian events observed represent a portion of the fire plume that has hotter flaming fire conditions that enabled strong pyro-convective lofting and long transport to MBO. These plumes would then have preferentially higher black carbon emissions and thus absorption enhancement. The lower AAE values in the Siberian events compared to regional events indicates 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 to range from 2.50–4.76 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 ranging from 138–229 nm and geometric standard deviations ranging from 1.53–1.89. We found MSE’s 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.