Air Pollution as a Climate Forcing: A Workshop
Day 2 Presentations
Direct Aerosol Forcing
John H. Seinfeld
California Institute of Technology, Pasadaena, CA, U.S.A.
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Of all the chemical species present as atmospheric aerosol, sulfate is considered to be the largest contributor to anthropogenic direct climate forcing. Direct radiative forcing by sulfate aerosols has received considerable attention over the past decade; estimates of global mean forcing range between -0.29 and -0.95 W/m2. Disagreement over the atmospheric sulfate burden and the fraction of that burden that is caused by human activities accounts for some, but not all, of this uncertainty. Figure 1 shows various anthropogenic sulfate direct forcing estimates as a function of the corresponding anthropogenic sulfate burden. This plot makes it clear that even researchers assuming the same amount of anthropogenic sulfate may reach very different conclusions about the magnitude of the resulting forcing. The sulfate forcing efficiency, defined as the amount of forcing per mass of sulfate, implicit in these numbers, varies from -130 to -370 W/g SO42-.
In addition to sulfate, accumulation mode aerosols contain significant amounts of nitrate in polluted areas. Both nitrate and sulfate are generally neutralized to a substantial degree by ammonia, which exists in the aerosol phase as the ammonium cation. Most importantly, inorganic aerosols are hygroscopic and contain water under nearly all atmospheric conditions. The amount of aerosol nitrate, ammonium, and water influence, with sulfate, the optical properties of the aerosol.
Of all these components, water plays perhaps the greatest role in determining aerosol optical behavior, simply because it constitutes most of the aerosol mass. Moreover, water uptake is highly nonlinear. Ammonium sulfate particles, for example, triple in volume as relative humidity increases from 85% to 95%, but grow by less than 20% (using the metastable hysteresis curve) from 50% to 60% RH. Water uptake also depends on the degree to which sulfate is neutralized by ammonia, with sulfuric acid being more hygroscopic than ammonium bisulfate or ammonium sulfate except near 100% RH. As a result, water uptake by inorganic aerosol is highly variable in time and space as relative humidity and aerosol composition change.
Adams et al. (2001) estimate the present-day anthropogenic direct aerosol forcing to be -0.95 W/m2 for sulfate and -0.19 W/m2 for nitrate. Moreover, based on the SRES A2 IPCC emissions scenario with especially strong increases in NOx emissions, they predict that the nitrate forcing will increase to -1.28 W/m2 by the end of this century, even while sulfate forcing declines to -0.85 W/m2. This result shows that future estimates of aerosol forcing based solely on predicted sulfate concentrations may be misleading and that the potential for significant amounts of ammonium nitrate aerosol needs to be considered in estimates of future climate change.
A key finding is that the calculated direct forcing is extremely sensitive to how the effect of water uptake on aerosol scattering behavior is taken into account. In particular, Adams et al. (2001) find that the large amount of water taken up by the aerosol above 95% relative humidity increases the total forcing by about 60%. This is important because a method commonly used in previous global models for parameterizing the effect of water uptake on optical properties has been to assume a low RH scattering coefficient of 5 m2/g SO42- and to correct for that at higher humidities with an empirical f(RH) factor. In principle, there is nothing wrong with this approach, but in practice, lack of data about f(RH) at high relative humidity has caused investigators to conservatively limit it to values that are unrealistically low.
Carbonaceous particles consist of a complex mixture of chemical compounds. Such particles are usually divided into two fractions, black (or elemental) carbon (BC) and organic carbon (OC). Organic carbon can be emitted directly into the atmosphere as products of fossil fuel combustion or biomass burning. This is called primary organic aerosol (POA). By contrast, secondary organic aerosol (SOA) is formed in the atmosphere as the oxidation products of certain volatile organic compounds (VOCs) condense on pre-existing aerosols. Both anthropogenic and biogenic VOCs can lead to SOA; on a global scale, biogenic hydrocarbons are estimated to be the predominant source.
The global distribution of carbonaceous aerosols is simulated online in the GISS GCM II-prime (Chung and Seinfeld, 2002). We include black carbon (BC) and primary organic aerosols (POA) from fossil fuel and biomass burning, as well as secondary organic aerosols (SOA) from the oxidation of biogenic hydrocarbons.
Anthropogenic black carbon is predicted to contribute to a globally and annually averaged net radiative forcing of +0.5 W m-2 when considered to be externally mixed and +0.78 W m-2 when occurring in an internal mixture of BC, OC, and sulfate. Externally mixed OC has a radiative forcing of -0.08 to -0.17 W m-2, depending on the amount of water uptake. Globally averaged and taken together, anthropogenic BC, OC and sulfate are predicted to exert a radiative forcing of -0.4 to -0.76 W m-2, depending on the exact assumptions of aerosol mixing and water uptake by OC. Even though the net radiative effect is one of cooling, warming of up to 3 W m-2 is predicted to occur in regions of large BC concentrations and high surface albedo. The annual combined cycle of BC, OC, and sulfate radiative forcing is nearly constant in the SH but varies strongly with season in the NH, with maximum cooling occurring during summer. Regional climate perturbations are expected to lead to climate feedbacks that warrant further study. Figure 2 shows the range of global mean forcing estimates for BC as a function of global BC burden. We note the strong dependence of the estimated forcing on the aerosol mixing assumptions. The global BC burden is also a key variable, which is affected by assumptions of wet removal as well as the global BC emissions inventory itself.
-- FIGURE 2 -- Figure 2. Estimates of black carbon (BC) direct forcing versus global BC burden (Tg).
The large contribution to the forcing made by aerosols at high relative humidity emphasizes the need to understand potential correlations between aerosols, humidity, and clouds. Subgrid variability in these factors and correlations between them could be important for improving our estimate of direct aerosol forcing. These are effects that, by their nature, are not well represented in GCMs. Little work has been done on this topic, but what has been done indicates that estimates of forcing based on GCM grid cell average humidity, such as the ones presented here, may be underestimates.
Second, further efforts should be made to evaluate GCM predictions of relative humidity. This will be difficult to achieve because there is a lack of data on the large spatial scales required to validate a global model with coarse resolution. Moreover, as long as one is interested primarily in cloud cover and precipitation rates, the details of the GCM-predicted relative humidity can be overlooked as long as they do not result in substantial errors in simulated clouds and rain. It is understandable, then, that GCM relative humidities have not been thoroughly and systematically evaluated. On the other hand, if one uses a GCM to estimate direct forcing of a hygroscopic aerosol, the details of the spatial and temporal distribution of grid-cell average relative humidity become important and require further attention.
- Adams, P. J., Seinfeld, J. H., Koch, D., Mickley, L., and D. Jacob, General circulation model assessment of direct radiative forcing by the sulfate-nitrate-ammonium-water inorganic aerosol system, J. Geophys. Res., 106, 1097-1111, 2001.
- Chung, S. H., and J. H. Seinfeld, Global distribution and climate forcing of carbonaceous aerosols, J. Geophys. Res., (in press).