Abstract
Abstract. Biomass burning emissions of atmospheric aerosols, including black carbon, are growing due to increased global drought, and comprise a large source of uncertainty in regional climate and air quality studies. We develop and apply new incremental four-dimensional variational (4D-Var) capabilities in WRFDA-Chem to find optimal spatially and temporally distributed biomass burning (BB) and anthropogenic black carbon (BC) aerosol emissions. The constraints are provided by aircraft BC concentrations from the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites in collaboration with the California Air Resources Board (ARCTAS-CARB) field campaign and surface BC concentrations from the Interagency Monitoring of PROtected Visual Environment (IMPROVE) network on 22, 23, and 24 June 2008. We consider three BB inventories, including Fire INventory from NCAR (FINN) v1.0 and v1.5 and Quick Fire Emissions Database (QFED) v2.4r8. On 22 June, aircraft observations are able to reduce the spread between a customized QFED inventory and FINNv1.0 from a factor of 3. 5 ( × 3. 5) to only × 2. 1. On 23 and 24 June, the spread is reduced from × 3. 4 to × 1. 4. The posterior corrections to emissions are heterogeneous in time and space, and exhibit similar spatial patterns of sign for both inventories. The posterior diurnal BB patterns indicate that multiple daily emission peaks might be warranted in specific regions of California. The US EPA's 2005 National Emissions Inventory (NEI05) is used as the anthropogenic prior. On 23 and 24 June, the coastal California posterior is reduced by × 2, where highway sources dominate, while inland sources are increased near Barstow by × 5. Relative BB emission variances are reduced from the prior by up to 35 % in grid cells close to aircraft flight paths and by up to 60 % for fires near surface measurements. Anthropogenic variance reduction is as high as 40 % and is similarly limited to sources close to observations. We find that the 22 June aircraft observations are able to constrain approximately 14 degrees of freedom of signal (DOF), while surface and aircraft observations together on 23/24 June constrain 23 DOF. Improving hourly- to daily-scale concentration predictions of BC and other aerosols during BB events will require more comprehensive and/or targeted measurements and a more complete accounting of sources of error besides the emissions.
Highlights
Black carbon (BC) makes significant contributions to shortterm climate (Bond et al, 2013) and human health (Janssen et al, 2012) as a component of aerosolized fine particulate matter (PM2.5) in the atmosphere
In order to match the regional climatological aerosol optical depth (AOD) scaling factors for the western US, we scale all Quick Fire Emissions Database (QFED) BC sources by ×1/3. This scaling is already taken into account in the prior emissions shown in Sect. 3.3, and without it FINNv1.0 and QFED would differ by ×10 during the ARCTAS-CARB campaign
The positive definite nature of atmospheric chemical emissions combined with uncertainties that are potentially greater than 100 % sets them apart from most control variable vector (CV) sought in meteorological data assimilation
Summary
Black carbon (BC) makes significant contributions to shortterm climate (Bond et al, 2013) and human health (Janssen et al, 2012) as a component of aerosolized fine particulate matter (PM2.5) in the atmosphere. Adjoint-based four-dimensional variational data assimilation (4D-Var) is able to account for nonlinear behavior between the emission sources and observation receptors by calculating exact gradients across physical processes Such an approach does not have the limitations imposed by mass balance, LPDM, or analytical inversions, but does require development of an adjoint. Sahu et al (2012) used BC total mass measurements from a singleparticle soot photometer (SP2) and other simultaneous gasphase measurements to identify and characterize anthropogenic and BB plumes in California By using these observations and surface measurements from every third day from the Interagency Monitoring of PROtected Visual Environment (IMPROVE) network (Malm et al, 1994), we provide top–down estimates of BC surface fluxes using 4D-Var. The mixture of anthropogenic and BB sources distributed across complex terrain and biomes is a difficult system to characterize. We conclude with a summary and recommendations for future measurements and emission inversion research
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