Abstract
Abstract. We estimate black carbon (BC) emissions in the western United States for July–September 2006 by inverting surface BC concentrations from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network using a global chemical transport model (GEOS-Chem) and its adjoint. Our best estimate of the BC emissions is 49.9 Gg at 2° × 2.5° (a factor of 2.1 increase) and 47.3 Gg at 0.5° × 0.667° (1.9 times increase). Model results now capture the observed major fire episodes with substantial bias reductions (~ 35 % at 2° × 2.5° and ~ 15 % at 0.5° × 0.667°). The emissions are ~ 20–50 % larger than those from our earlier analytical inversions (Mao et al., 2014). The discrepancy is especially drastic in the partitioning of anthropogenic versus biomass burning emissions. The August biomass burning BC emissions are 4.6–6.5 Gg and anthropogenic BC emissions 8.6–12.8 Gg, varying with the model resolution, error specifications, and subsets of observations used. On average both anthropogenic and biomass burning emissions in the adjoint inversions increase 2-fold relative to the respective {a priori} emissions, in distinct contrast to the halving of the anthropogenic and tripling of the biomass burning emissions in the analytical inversions. We attribute these discrepancies to the inability of the adjoint inversion system, with limited spatiotemporal coverage of the IMPROVE observations, to effectively distinguish collocated anthropogenic and biomass burning emissions on model grid scales. This calls for concurrent measurements of other tracers of biomass burning and fossil fuel combustion (e.g., carbon monoxide and carbon isotopes). We find that the adjoint inversion system as is has sufficient information content to constrain the total emissions of BC on the model grid scales.
Highlights
Black carbon (BC) is directly emitted from the incomplete combustion of carbonaceous fuels (Bond et al, 2004)
Our standard adjoint inversion is at 2◦ × 2.5◦, with uncertainties of 50 % for anthropogenic emissions, 500 % for biomass burning emissions, and 30 % for the observation (Case 1, Table 1)
The a posteriori emissions are 49.9 Gg at 2◦ × 2.5◦ and 47.3 Gg at 0.5◦ × 0.667◦ for July–September, substantially higher than the a priori (24.3 Gg), because the modeled surface BC concentrations are largely biased low at most Interagency Monitoring of Protected Visual Environments (IMPROVE) sites (Mao et al, 2011, 2014)
Summary
Black carbon (BC) is directly emitted from the incomplete combustion of carbonaceous fuels (Bond et al, 2004). Mao et al.: Estimates of black carbon emissions in the western United States tion may provide an efficient near-term solution to mitigate global warming and to improve air quality and public health simultaneously (Ramanathan and Carmichael, 2008; Shindell et al, 2008, 2012; Smith et al, 2009; Ramana et al, 2010; Bond et al, 2013). Long-term records have shown an increase in fires in terms of both fire frequency and burned area in the WUS over the past 30 years because of the rising spring and summer temperatures (Westerling et al, 2006; Peterson and Marcinkowski, 2014; Jin et al, 2014) and increasing urbanization (e.g., Cannon and DeGraff, 2009) This upward trend is expected to continue as a result of the warming climate (Spracklen et al, 2009; Yue et al, 2013). A See Fig. 1 in Mao et al (2014) for the geographical definitions of the BC source regions. b Scaling factors are in parentheses. c The a priori biomass burning emissions uniformly increased by 2.5 Mg in every model grid box
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