A top-down approach for apportioning black carbon aerosols to emission sources in regions with multiple competing contributors

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Black carbon (BC), generated from incomplete combustion of carbonaceous fuels, is one of the two major airborne pollutants that drive climate change and degrade regional air quality. With a warming potential second to CO2, BC contributes the most uncertainty to climate modeling due to its short retention time in the atmosphere and poorly understood mixing, deposition, and contribution from various emission sources. BC from fossil fuel and biomass burning possesses distinct light absorbing properties and aging processes, and hence differing warming potentials. Therefore, improved source characterization is crucial for determining targets, formulating mitigation strategies, and increasing the accuracy of input anthropogenic aerosols in climate models. However, conventional bottom-up emission inventory (EI) methods have intrinsic limitations such as inaccurate emission factors, incomplete coverage of sources/sinks that cause large uncertainties. This research aims to develop an innovative, dual-isotope (14C-13C) based top-down approach to apportion BC to major sources as this approach eliminates the need of measuring emission factors or comprehensive surveying of sources. The near objective is to establish the method by improving the experimental procedures to minimize or quantify the level of interferences through a pilot study in Alberta where 1/3 of Canada BC is emitted from multiple competing sources. Molecular tracer studies combined with remote sensing data and air mass trajectory analysis will serve to validate the results of this method during the wildfire season. A mechanistic study is designed to investigate the removal mechanism of the two types of BC, so as to explain the discrepancy between the bottom-up and top-down approaches. The mid- and long term objectives are to apply this approach for investigating important BC emission regions in Canada to construct a multi-region database containing source-resolved BC emissions, based on which the impact of source-differentiated BC on the regional climate will be assessed. The outcome of this research is anticipated to bring immediate benefit to the BC community in that this method, with increased adaptivity, can possibly be adopted by other researches for sourcing BC at other important emission regions in the world. In the mid-long run, the datasets to be collected in this research will enable a better simulation of the Canadian climate system by providing more accurate source parameterization. The simulation results will in turn impact the regulatory sector by providing specific constraints for bottom-up emission inventories, which will assist policymakers to develop regionally tailored mitigation strategies for black carbon by selecting the right sector to target.