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Sources of Black Carbon in the Western United States Mountain Ranges

  • Author(s): Mao, Yuhao
  • Advisor(s): Li, Qinibin
  • et al.
Abstract

This dissertation investigates the sources of black carbon (BC) in the Western United States (WUS) mountain ranges using a global 3-dimensional chemical transport model (CTM). We quantify the relative contributions from different sources and source regions to BC in the WUS mountain ranges by analyzing surface BC observations for 2006 from the Interagency Monitoring of PROtected Visual Environment (IMPROVE) network. Major discrepancies between modeled and observed surface BC concentrations are found at elevated mountainous sites during the July-October fire season when simulated BC concentrations are negatively biased by a factor of two. We attribute these low biases largely to the underestimated (by more than a factor of two) biomass burning BC emissions in the model, not only in the absolute magnitudes of fire emissions but also in the timing and location of fires.

We improve estimates of biomass burning and anthropogenic BC emissions in the WUS for 2006 by inverting surface BC concentrations from the IMPROVE network using a global CTM and its adjoint. We first use active fire counts from the Moderate Resolution Imaging Spectroradiometer (MODIS) to improve the spatiotemporal distributions of the biomass burning BC emissions from the Global Fire Emissions Database (GFEDv2). This adjustment primarily shifts emissions from late to middle and early summer (a 33% decrease for September-October and a 56% increase for June-August) and leads to appreciable increases in modeled surface BC concentrations in early and middle summer, especially at the 1-2 and 2-3 km altitude ranges.

We then conduct analytical and adjoint inversions at both 2� � 2.5� and 0.5� � 0.667� (nested over North America) horizontal resolutions. The a posteriori biomass burning and anthropogenic BC emissions in the WUS for July-September are 16.4-31.7 Gg (increased by a factor of 2.4-4.7 relative to the corresponding a priori) and 9.1-33.5 Gg (48-190% of the corresponding a priori), respectively. There are large differences in the a posteriori emissions between the analytical and adjoint inversions, mostly evident in different BC emission sectors. The anthropogenic BC emissions in the WUS increase by about a factor of two from the adjoint inversions, but decrease by ~50% from the analytical inversions. The biomass burning BC emissions increase by about factors of 2 after the adjoint inversions and 3 after analytical inversions. The differences are partially because that the inversion system has trouble effectively distinguishing collocated anthropogenic and biomass burning emissions at the grid-based resolution and tends to falsely impose larger anthropogenic emissions in the regions where biomass burning emissions are severely underestimated. Simulated surface BC concentrations with the a posteriori emissions capture the observed major fire episodes at most sites and the substantial enhancements at the 1-2 and 2-3 km altitude ranges, especially at 0.5� � 0.667�. The a posteriori emissions also lead to large bias reductions in modeled surface BC concentrations (~30% on average) and significantly better agreement with observations (increases in Taylor skill scores of ~40-200%).

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