Air Pollution as a Climate Forcing: A Workshop
Day 2 Presentations
Black Carbon Global Climate Forcing Inferred from AERONET
Makiko Sato*+, James Hansen*, Oleg Dubovik°±,
* NASA Goddard Institute for Space Studies, New York, NY, U.S.A.
+ Center for Climate System Research, Columbia University Earth Institute, New York, NY, U.S.A.
° Laboratory for Terrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, MD, U.S.A.
± University of Maryland at Baltimore County, Baltimore, MD, U.S.A.
Global climate forcing by black carbon (BC) is uncertain in magnitude. IPCC raised their estimate from 0.1 W/m2 to 0.25 W/m2 in their 2001 report, while Jacobson and Hansen have each recently argued that the BC forcing could be 0.5 W/m2 or larger. It is difficult to determine the BC forcing by using emission estimates and aerosol transport models because of uncertainties in identifying and quantifying BC sources and because of uncertainties in the simulations of aerosol removal mechanisms. The fact that much of the BC is internally mixed with other aerosol compositions further complicates estimation of the BC forcing.
We suggest an alternative empirical approach for estimating the BC forcing, based on the assumption that BC and soil dust are the primary sources of aerosol absorption at visible wavelengths. We employ the optical depths for aerosol absorption measured by AERONET photometers (Holben et al., 2001; Dubovik et al., 2002) at about 100 stations around the world. Locations of the stations that we use are shown in Figure 1. Our approach is to compare the aerosol absorption measured by AERONET with the aerosol absorption in the aerosol climatology of Koch (2001) and Tegen et al. (2000) and to find the factor by which Koch's BC amount must be multiplied to yield closest agreement with the AERONET absorption.
The various aerosol compositions are treated as if they were externally mixed in this model aerosol climatology and in the radiative forcing calculation. This does not affect our empirical estimate of BC absorption or the calculated BC climate forcing. However, it means that, if internally mixed BC is in fact more effective at absorption (Jacobson, 2001), then the actual BC mass is less than that obtained in the externally mixed approximation.
Figure 2 shows the aerosol distributions of Koch (2001) and Tegen et al. (2000). The only aerosol in this climatology, other than BC, that contributes to simulated absorption at visible wavelengths is soil dust. We found, in agreement with Dubovik et al. (2002) and Kaufman et al. (2001), that the AERONET observations reveal excessive absorption by the model aerosol climatology in regions of desert dust. Thus we increase the soil dust single scatter albedo in the Koch aerosol climatology for the purpose of reducing the error that would be introduced in the derived BC absorption. The Koch climatology has a range of dust sizes and single scatter albedos (SSAs), with a mean SSA of about 0.89, which is substantially darker than observations. We wish to obtain an estimate of the sensitivity of our derived BC absorption to the uncertainty in soil dust absorption. Thus we consider two alternative corrections to the soil dust climatology: (1) we add a maximum 0.05 to dust SSA, (2) we add a maximum 0.1 to dust SSA (the change is less for aerosol sizes for which the SSA would otherwise exceed unity). The resulting maps of aerosol SSA suggests that the actual soil dust SSA probably falls between these two cases.
If the amount of "industrial" BC, as opposed to biomass burning BC, is allowed to vary, the optimum match for global aerosol absorption occurs when the industrial BC of Koch (2001) is increased by a factor between 4.8 and 5.3. [This refers to the case with desert dust SSA increased 0.05; they are about 20% larger in the case with SSA increased 0.1]. We also tried increasing Koch's biomass BC, but we found that increased biomass BC does not yield as accurate a fit to the geographical distribution of single scatter albedo (or aerosol absorption). This may be because of an unrealistic geographical distribution or properties of biomass aerosols, rather than an indication that there is sufficient abundance of biomass aerosols in the climatology. Furthermore, an underestimate of the industrial source of BC is probably only one of the causes of the underestimate of absorption. Li (in a paper presented at this workshop) argues, for example, that there is substantial biomass burning of Northern Hemisphere forests that needs to be included in the aerosol transport models, although the Boreal biomass aerosols seem to be less absorbing.
The inferred BC forcing is 0.6-0.7 W/m2. Some of this BC absorption may be natural, e.g., a product of natural burning of forests. This natural component, in our estimation, is unlikely to be more than about one quarter of the total. It is possible that some of the absorption at visible wavelengths is due to organic material rather than elemental carbon (Krivacsy et al., 2001). If such absorption is substantial it may be appropriate to think of the derived BC absorption as being due to "carbonaceous" material. With that caveat, we conclude that the human-made direct BC forcing is of the order of 0.5 W/m2.
We note that the derived BC forcing must have some dependence on the accuracy of the aerosol climatology. We plan to investigate the dependence on aerosol climatology by repeating this study with a climatology other than that of Koch and Tegen. We also need to investigate the sensitivity of results to errors in the aerosol size distribution and phase function.
Finally, we note that Bond et al. (1998) and Derwent et al. (2001) find that some of the BC sources used for deriving aerosol climatologies are probably over-estimated. Thus our inferred BC absorption amount suggests that there must be substantial unidentified BC sources.
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