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

摘要

The summer of 2015 was an extreme forest fire year in the PacificNorthwest. Our sample site at the Mt. Bachelor Observatory (MBO,2.7 km a.s.l.) in central Oregon observed biomass burning (BB) events morethan 50 % of the time during August. In this paper we characterize theaerosol physical and optical properties of 19 aged BB events during August2015. Six of the 19 events were influenced by Siberian firesoriginating near Lake Baikal that were transported to MBO over 4–10 days.The remainder of the events resulted from wildfires in Northern Californiaand Southwestern Oregon with transport times to MBO ranging from 3 to 35 h.Fine particulate matter (PM), carbon monoxide (CO), aerosol lightscattering coefficients (), aerosol light absorptioncoefficients (), and aerosol number size distributionswere measured throughout the campaign. We found that the Siberian events hada significantly higher Δ∕ΔCO enhancementratio, higher mass absorption efficiency (MAE; Δ∕ΔPM), lower single scattering albedo (), and lower absorption Ångström exponent (AAE) when compared withthe regional events. We suggest that the observed Siberian events representthat portion of the plume that has hotter flaming fire conditions and thusenabled strong pyroconvective lofting and long-range transport to MBO. TheSiberian events observed at MBO therefore represent a selected portion of theoriginal plume that would then have preferentially higher black carbonemissions and thus an enhancement in absorption. The lower AAE values in theSiberian events compared to regional events indicate a lack of brown carbon(BrC) production by the Siberian fires or a loss of BrC during transport. Wefound that mass scattering efficiencies (MSE) for the BB events ranged from2.50 to 4.76 m g. We measured aerosol size distributions with ascanning mobility particle sizer (SMPS). Number size distributions rangedfrom unimodal to bimodal and had geometric mean diameters ()ranging from 138 to 229 nm and geometric standard deviations() ranging from 1.53 to 1.89. We found MSEs for BB events tobe positively correlated with the geometric mean of the aerosol sizedistributions ( = 0.73), which agrees with Mie theory. We did not find any dependence on event size distribution to transport time or fire sourcelocation.