Abstract
Reliable assessment of the impact of aerosols emitted from boreal forest fires on the Arctic climate necessitates improved understanding of emissions and the microphysical properties of carbonaceous (black carbon (BC) and organic aerosols (OA)) and inorganic aerosols. The size distributions of BC were measured by an SP2 based on the laser-induced incandescence technique on board the DC-8 aircraft during the NASA ARCTAS campaign. Aircraft sampling was made in fresh plumes strongly impacted by wildfires in North America (Canada and California) in summer 2008 and in those transported from Asia (Siberia in Russia and Kazakhstan) in spring 2008. We extracted biomass burning plumes using particle and tracer (CO, CH3CN, and CH2Cl2) data. OA constituted the dominant fraction of aerosols mass in the submicron range. The large majority of the emitted particles did not contain BC. We related the combustion phase of the fire as represented by the modified combustion efficiency (MCE) to the emission ratios between BC and other species. In particular, we derived the average emission ratios of BC/CO = 2.3 +/- 2.2 and 8.5 +/- 5.4 ng/cu m/ppbv for BB in North America and Asia, respectively. The difference in the BC/CO emission ratios is likely due to the difference in MCE. The count median diameters and geometric standard deviations of the lognormal size distribution of BC in the BB plumes were 136-141 nm and 1.32-1.36, respectively, and depended little on MCE. These BC particles were thickly coated, with shell/core ratios of 1.3-1.6. These parameters can be used directly for improving model estimates of the impact of BB in the Arctic.
Highlights
CIT GIT Asian biomass burning (BB) Sulfate AMS0.49 tation of this relationship is not straightforward because DVSC and DCO are both smoldering products
This trend may be sensitive to the relative emissions of light‐scattering particles (LSP) and carbon monoxide (CO) to those of CO2 because both the DVSC/DCO2 and DCO/DCO2 ratios decreased with the increase in modified combustion efficiency (MCE)
The present analysis demonstrates the necessity of taking into account the dependence of the emission ratios of aerosols on the burning phase (e.g., MCE) in improving the uncertainty in estimating aerosol emissions from BB
Summary
[2] Boreal forest fires are one of the most important sources of aerosols transported to the Arctic [e.g., Stohl et al, 2006, 2007; Treffeisen et al, 2007; Eck et al, 2009]. Predominantly in the form of carbonaceous aerosols, namely black carbon (BC) and organic aerosols (OA), are emitted from biomass burning (BB) These particles strongly absorb and scatter solar visible radiation (downwelling and upwelling), influencing the radiation budget in the Arctic. [9] The most important parameters and processes presented by this study are the evolution of the size distributions and mixing state (coating thickness) of BC, the emission ratios of BC versus CO and CO2, and their dependence on the conditions of combustion. We have elucidated the difference of these parameters between the BB plumes from Asia and North America, which are the two major regions of boreal forest fires These parameters can be used directly for improving model estimates of the impact of BB on climate, especially in the Arctic
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