Air Pollution as a Climate Forcing: A Workshop
Day 2 Presentations
Black Carbon Aerosols: A Burning Question
School of Atmospheric Sciences, Georgia Tech, and Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, MD, U.S.A.
The importance of black carbon (BC) aerosol as a climate forcing agent has recently drawn considerable attention. Due to its highly absorbing ability, BC aerosol, in contrast with other major type of aerosols, causes warming effects on global climate instead of cooling. Yet, quantifying the BC radiative forcing faces very large uncertainties, not only because it is highly sensitive to the mixing state of BC with other aerosol types, but more fundamentally, because the emissions of BC from the burning of fuels and biomass are not well determined, especially from biomass burning.
Here we present results of BC simulated from the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model. In our simulation, the seasonal and interannual variations of biomass burning emissions were estimated based on satellite observed fire counts and an absorbing aerosol index (1). Climatologically, total annual BC emission from biomass burning is estimated at 11 Tg/yr, nearly twice as much as that estimated in other studies and in the IPCC present day emission scenario or SC1 (5.63 Tg/yr). This biomass burning source exceeds the fuel combustion (6.4 Tg/yr) by a factor 1.7. Figure 1 shows the annual emission rates of BC from fuel combustions and biomass burning used in the model. It should be mentioned that biomass burning emission varies strongly with season and is different from year to year.
The modeled carbonaceous aerosol concentrations are compared with observations at the surface sites in North America from the IMPROVE network, and the modeled optical thickness are compared with the satellite observations from MODIS and the sun photometer observations from AERONET at biomass burning dominated places. The model results are consistent with the observations, especially in seasonal variations. Figure 2 plots the comparisons of modeled and measured BC concentrations at Big Bend and Yellowstone National Park, and total AOT at Cuiaba (Brazil) and Mongu (southern Africa).
The short wave direct radiative forcing by BC alone (i.e., no mixing with other aerosols) at the top of the atmosphere (TOA) is calculated at 0.54 W m-2 on global average. Of that amount, 0.16 W m-2 (30%) is due to industrial emission of BC, and 0.38 W m-2 (70%) is due to biomass burning. If the IPCC SC1 emission is used, the total BC forcing at TOA is only 0.35 W m-2, 0.19 W m-2 less than that from our standard model results. This difference can be attributed entirely to the higher BC biomass emission rates used in our standard simulation. Figure 3 shows the zonal averaged annual BC forcing at the TOA from industrial (red) and biomass burning (green) BC sources. For comparison, the total BC forcing from the IPCC SC1 emission scenario is also superimposed (dashed line).
Clearly, there is much to be learned about the BC aerosols, from their emission to their climate forcing. While the BC aerosol forcing sensitively depends on the mixing state with other aerosols, as demonstrated by several studies, the amount of BC in the atmosphere is determined by the emission. Therefore, in order to understand the present and future BC climate forcing, the first and the most straightforward step is to reduce the uncertainties in estimating BC emission. For biomass burning emission, uncertainties involve the methods used to estimate total burned biomass, emission factors in different burning stages and for different vegetation types, and the emission altitudes. A desirable global biomass burning inventory requires the synthesis of satellite fire data, field and laboratory observations, and parameters linking these data to emissions that can be used in the models.
- 1. Duncan, B.,. R. Martin, A. Staudt, R. Yevich, and J. Logan, Interannual and seasonal variability of biomass burning emissions constrained by satellite observations, J. Geophys. Res., submitted, 2002.