Uncertainties in Global Aerosols and Climate Effects due to Biofuel Emissions
May 14, 2015
Jack Kodros
Committee: Jeff Pierce (advisor), Sonia Kreidenweis, John Volckens (Mechanical Engineering, Environmental and Radiological Health Sciences)
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Abstract
Prevalent combustion of biofuel as a source of energy for domestic tasks emits large quantities of black carbon and organic aerosol, causing potentially large impacts to air quality and climate. While reducing aerosol emissions through improving technologies is a necessary step for improving health and lifestyle in developing regions, the net effect on climate is largely unconstrained due to large uncertainties surrounding black carbon’s radiative and cloud effects. We use the chemical transport model, GEOS-Chem, with TOMAS microphysics to explore the sensitivity of the direct and cloud-albedo indirect effects due to uncertainties from black and organic carbon emission factors, optical mixing state, and modeled nucleation and background secondary organic aerosol. We find the direct radiative effect is strongly dependent on assumed mixing state, absorptivity of organic aerosol (brown carbon), and the emissions BC to OA ratio, while the cloud-albedo indirect effect is strongly dependent on emissions mass, aerosol size distribution, and choice of model nucleation and secondary organic aerosols. We find the global-mean direct radiative effect of biofuel emissions ranges from -0.02 to +0.06 W m-2 across all simulation/mixing state combinations with regional effects in source regions ranging from -0.2 to +1.2 W m-2. The global-mean cloud-albedo aerosol indirect effect ranges from +0.01 to -0.02 W m-2 with regional effects in source regions ranging from -1.0 to -0.05 W m-2. We find the sign of the net climate effect from biofuel combustion emissions uncertain, thus substantially limiting our ability to introduce mitigation strategies aimed at reducing black carbon to counter warming effects from greenhouse gases. To better understand the climate impact of particle emissions from biofuel combustion, we recommend field/laboratory measurements to narrow constraints on: 1) emissions mass, 2) emission size distribution, 3) mixing state, and 4) ratio of black carbon to organic aerosol.
