Research Programs

Determining Fire Emissions from Satellite Observations and Global Modeling
FY2006 activities

TIIMES Theme:
BGS

Compiled by Louisa Emmons - ACD - TIIMES

Reserach Team: Louisa Emmons-ACD - TIIMES, Peter Hess-ACD - TIIMES, Gabriele Pfister, Jean-François Lamarque-ACD - TIIMES, David Edwards-ACD - TIIMES

Collaborators: James Randerson (University of California, Irvine) and those in the publication list below

CO Source Inverse Modeling over South America

Figure 1. Monthly mean percentage difference for MOPITT minus MOZART CO averaged over South America, for a priori emissions (blue) and a posteriori emissions (red).

CO emitted from South American biomass burning sources has a large contribution to the Southern Hemisphere CO load during the fire season that lasts roughly from July through December. We performed an inverse modeling study for the year 2004 using the NCAR chemistry transport model MOZART and CO observations from the remote sensing instrument MOPITT. The inverse setup is similar to the one described in Pfister et al. (2005). In the inverse setup we account for CO that is transported into the region of South America by assimilating MOPITT CO mixing ratios into MOZART outside the region of interest. Biomass burning emissions from the Global Fire Emissions Database (GFED), version 2 (van der Werf et al., 2006) were used. The a priori fire emissions for South America were developed by Christine Wiedinmyer (ACD, NCAR). This emission inventory has a daily resolution and is based on MODIS fire counts (Wiedinmyer et al., 2006). The fire emissions have been injected over an altitude range 0-5 km in the model to account for fire related convection.

Biomass burning contributes up to 50% of the total CO burden over South America, peaking in August-September. Figure 1 shows a comparison of MOPITT and MOZART CO over South America simulated with a priori and a posteriori emissions. Using the a priori emissions the modeled CO fields are clearly too high throughout the year with largest differences during the biomass burning season. Employing the a posteriori sources significantly improves the agreement, reducing percentage differences to 5% and below. The largest absolute deviation remaining is seen during September with the a posteriori CO being biased high by about 10 ppb on average. The contribution of biomass burning emissions for South America in relation to the global source is calculated as 20% in our study. This is significant and demonstrates the importance of an optimal constraint on this source.

Radiative forcing from the wildfires in Alaska/Canada in summer 2004

The extreme drought conditions in the North American boreal zone in summer 2004 caused a record fire season. Burning of biomass can cause significant increases in tropospheric pollution levels, an in further consequence might also impact climate by affecting the radiative balance of the atmosphere through changes in surface albedo and through the release of greenhouse gases and pre-cursors, and aerosols. The sign and magnitude of the radiative forcing depends on the location and timing of the fires, the fire emissions, and the physical properties of the surface. Large uncertainties still exist about the sign and the magnitude of the radiative forcing of boreal forest fires. Jim Randerson (UC Irvine) has led a study that found that after the first year of a boreal forest fire ozone (O₃), carbon dioxide (CO₂), and aerosols emitted or produced from fire emissions cause a warming effect, while averaged over a 30 year period, changes in the surface albedo dominate the balance causing a cooling effect (Randerson et al., 2006).

Figure 2. Different components of the radiative forcing of the wildfires in Alaska and Canada in summer 2004: Ozone, CO2, black carbon aerosols (CB), organic carbon aerosols (OC), organic and black carbon aerosols combined (OCCB). Radiative forcing has been estimated based on IPCC estimates for relating atmospheric concentrations to radiative forcing.

In a forerunner study we have been using the MOZART-4 chemistry transport model to calculate the amounts of O₃, CO₂, and black and organic carbon aerosols produced by the fires and estimated their radiative impacts using relations between atmospheric concentrations and radiative forcing provided by the Intergovernmental Panel on Climate Change (IPPC, 2001). CO₂ is not simulated within MOZART-4, but atmospheric concentrations have been determined by scaling emissions of carbon monoxide and distributing CO₂ homogeneously within the atmosphere. The results of this study are shown in Figure 2. Aerosols can have a large impact on the radiative transfer and whether they contribute to cooling or warming strongly depends on their optical properties and the relative amounts of absorbing versus scattering aerosols, e.g. black carbon versus organic carbon aerosols. The results indicate that the largest impact is due to the combined effect of carbon aerosols causing a cooling effect, and that this cooling is somewhat mitigated by warming due to ozone and CO₂ with CO₂ being less effective then O₃ during the first year of the fire.

Figure 3. Difference in the net total radiation (W m-2) at the top of the atmosphere between the summers of 2000 and 2004. Left: Model simulations with CAM-Chem. Right: Observations with the CERES remote sensing instrument.

In a follow-up study using the Community Atmosphere Model with Chemistry (CAM-Chem) the radiative forcing within the simulation  is directly calculated, thus accounting for spatial and temporal inhomogeneities. The model was run for two different years: the year 2000 where fire activity in the Northern Hemispheric boreal zone was low, and for the record fire season in 2004. In Figure 3 the model results are compared to observations taken by the Cloud and the Earth’s Radiant Energy System (CERES) remote sensing instrument. The model shows a very good representation of the observations and both indicate a cooling effect in 2004 compared to 2000.

Keeping in mind, the results from the previous study, we can expect that the aerosols emitted from the 2004 fires play an important role in the differences observed between the years 2000 and 2004. However, surface temperatures were also significantly higher in 2004 compare to 2000, on average by about 5K over Alaska, and this also has an impact on the radiative balance. In further model simulations that are currently under work, we separate the different elements (ozone, black and organic carbon aerosols, sulfate aerosols) and determine to what extent changes in meteorology and to what extent changes in the atmospheric composition are accountable for the observed features.

References

Pfister G., P. G. Hess, L. K. Emmons, J.-F. Lamarque, C. Wiedinmyer, D. P. Edwards, G. Pétron, J. C. Gille, G. W. Sachse (2005), Quantifying CO emissions from the 2004 Alaskan wildfires using MOPITT CO data, Geophys. Res. Lett., 32, L11809, doi:10.1029/2005GL022995.

Ramaswamy, V., et al. (2001), in Climate Change 2001: The Scientific Basis. Contributions of Working Group 1 to the Third Assessment Report of the Intergovernmental Panel on Climate Change, J.T. Houghton et al., Eds. (Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2001), pp 350-416.

Randerson, J. et al. (2006), The Impact of Boreal Forest Fire on Climate Warming, Science, in press.
van der Werf, G.R., J.T. Randerson, L. Giglio, J. Collatz, P.S. Kaskbhatla, A.F. Arellano Jr. (2006), Interannual variability of global biomass burning emissions from 1997 to 2004, Atmos. Chem. Phys. Discuss., 6, 3175-3226.

Wiedinmyer, C., B. Quayle, C. Geron, A. Belote, D. McKenzie, X. Zhang, S. O’Neill, and K.K. Wynne, (2006), Estimating emissions from fires in North America for air quality modeling, Atmos. Environ.