Info

Free Power Secrets

Making Your Own Fuel

Get Instant Access

particles, the effects of such natural aerosols often act in a synergistic manner with anthropogenic aerosol particles.

In short, the direct effects of aerosol particles in terms of backscattering solar radiation out to space and hence leading to cooling are reasonably well understood qualitatively and provided the aerosol composition, concentrations, and size distribution are known, their contribution can be treated quantitatively as well. However, major uncertainties exist in our knowledge of the physical and chemical properties, as well as the geographical and temporal variations, of aerosol particles and it is these uncertainties that primarily limit the ability to accurately quantify the direct effects at present.

b. Absorption of Solar Radiation

Depending on their chemical composition, aerosol particles can not only scatter incoming solar radiation but, in some cases, absorb it as well. This absorbed energy is converted to heat, which can contribute to warming of the troposphere. In addition, since energy absorbed by such particles does not reach the surface but heats the atmosphere directly, changes in the lapse rate may result as well, and this can contribute to global change by altering atmospheric circulation patterns (e.g., see Penner et al., 1994; and Tegen et al., 1991).

Although sulfate aerosols do not appreciably absorb incoming solar radiation, elemental or black (graphitic) carbon particles do. In addition, mineral dust particles absorb in the visible, primarily due to the presence of iron compounds such as hematite (Fe203) (Patterson, 1981; Sokolik et al., 1993). As discussed in Chapter 9.A.4, some recent studies suggest there may be a previously unrecognized, but substantial, contribution of complex organics to light absorption (Malm et al., 1996). However, since relatively little is known about the nature and concentrations of these complex organics, and the dust contribution to solar radiation absorption is generally assumed to be small compared to that of elemental carbon (which may not be justified in some regions; e.g., see Patterson, 1981), we focus here on the contribution of elemental carbon.

As described in Chapter 9 (see also Penner and Novakov, 1996), tropospheric particles containing carbon are often referred to as "carbonaceous aerosols." The form of the carbon may be organic or elemental, the latter often being referred to as graphitic or black carbon due to its strong absorption of visible light. The expression given in Eq. (U) for direct radiative forcing by aerosol particles can be modified to include contri butions due to absorption (Chylek and Wong, 1995): A Fr= ~Ac)T2

where rscal = aRII/RII5S()2- is the effective aerosol optical scattering depth and rabs is the optical depth due to absorption. For no absorption, rahs = 0 and Eq. (W) reduces to Eq. (U) as expected. Note that for absorption when scattering is small, A FR becomes positive, i.e., warming results. In short, carbonaceous aerosols can both scatter solar radiation, causing negative radiative forcing, and absorb light, leading to positive forcing. The net effect depends on the composition of the aerosols as well as their particle size and vertical distribution.

Several groups have carried out detailed global 3-D model studies of carbonaceous aerosol particles and their effects on radiation balance (e.g., Liousse et al., 1996; Haywood and Ramaswamy, 1998; Penner et al., 1998). For example, Figure 14.33 shows the model estimates by Penner et al. (1998) for direct radiative forcing due to aerosol particles from fossil fuel combustion. These were assumed to include both black carbon, which absorbs solar radiation, and organic carbon, which scatters it. The black carbon over Europe, Asia, and, to a lesser extent, the eastern United States leads to positive radiative forcing in these areas due to direct absorption of solar radiation. Similar results are predicted by other models, although the absolute values of the radiative forcing may differ somewhat, depending on the details of the particular model used and the emission inventory chosen for the black carbon (e.g., see Schult et al., 1997; Haywood and Ramaswamy, 1998; and Penner et al., 1998).

Figure 14.34 shows one model prediction for the net radiative forcing due to a combination of sulfate, biomass, and fossil fuel particles containing both black and organic carbonaceous compounds (Penner et al., 1998). The net result is predicted negative radiative forcing over the industrialized areas of the Northern Hemisphere due to aerosol particles (remember this does not include the positive forcing due to greenhouse gases). Positive radiative forcing is predicted over the highly reflecting ice-covered surfaces at high latitudes and over a small portion of Asia. The predicted average values of radiative forcing were —0.65 to —1.07 W m~2 for the Northern Hemisphere, —0.33 to —0.51 W m~2 for the Southern Hemisphere, and —0.51 to —0.88 W m"2 for the global average.

Jayaraman et al. (1998) measured the aerosol optical depth, aerosol size distribution, and the solar flux close to the coast of India, over the Arabian Sea, and then

C. AEROSOL PARTICLES, ATMOSPHERIC RADIATION, AND CLIMATE CHANGE 180° 120°W 60°W 0° 60°E 120°E 180°

C. AEROSOL PARTICLES, ATMOSPHERIC RADIATION, AND CLIMATE CHANGE 180° 120°W 60°W 0° 60°E 120°E 180°

Nyada Dilsiz Harita

180° 120°W 60°W 0° 60° E 120°E 180° FIGURE 14.33 Calculated direct radiative forcing for the combination of black carbon and organic carbon from fossil fuel combustion. The numbers shown are the lower limits of the ranges included in each area. The boundaries of the regions are +10, +5, ±2, ±1, +0.5, ±0.2, +0.1, and 0. Thus -5 represents the -5 to — 10 W m~2 region, —2 represents the —2 to — 5 W m~2 region, etc. (adapted from Penner et al., 1998).

180° 120°W 60°W 0° 60° E 120°E 180° FIGURE 14.33 Calculated direct radiative forcing for the combination of black carbon and organic carbon from fossil fuel combustion. The numbers shown are the lower limits of the ranges included in each area. The boundaries of the regions are +10, +5, ±2, ±1, +0.5, ±0.2, +0.1, and 0. Thus -5 represents the -5 to — 10 W m~2 region, —2 represents the —2 to — 5 W m~2 region, etc. (adapted from Penner et al., 1998).

over the more remote Indian Ocean. The aerosol optical depth increased from 0.1 or less in the remote region to 0.2-0.4 over the Arabian Sea to values up to 0.5 close to the Indian coast. This paralleled the trend in aerosol mass concentrations, which varied from a few fx g m"3 over the Indian Ocean to ~80 yu,g m"3 near the coast. The data indicated that the contribution of light absorption by aerosols near the coast was larger than that over the remote ocean, suggesting a contribution from black carbon and perhaps other or-ganics associated with anthropogenic activities.

The presence of clouds can also affect the net light absorption by black carbon, indeed even more than for sulfate (Haywood and Shine, 1997; Haywood and Ramaswamy, 1998; Liao and Seinfeld, 1998). For example, Liao and Seinfeld (1998) calculate that the net heating

FIGURE 14.34 Model-predicted net direct radiative forcing due to sulfate and carbonaceous particles. The numbers shown are the lower limits of the ranges included in each area. The boundaries of the regions are + 10, ±5, ±2, +1, +0.5, +0.2, ±0.1, and 0. Thus, -5 represents the -5 to -10 W itT2 region, -2 represents the -2 to -5 W m~2 region, etc. (adapted from Penner et al., 1998).

FIGURE 14.34 Model-predicted net direct radiative forcing due to sulfate and carbonaceous particles. The numbers shown are the lower limits of the ranges included in each area. The boundaries of the regions are + 10, ±5, ±2, +1, +0.5, +0.2, ±0.1, and 0. Thus, -5 represents the -5 to -10 W itT2 region, -2 represents the -2 to -5 W m~2 region, etc. (adapted from Penner et al., 1998).

effect can be enhanced by as much as a factor of three in the presence of a low, thick stratus cloud below the particles, due to the enhanced scattering of solar radiation back to the absorbing particles (see Chapter 3.C.2g). On the other hand, a thick cirrus cloud above the black carbon reduces its heating effect for an overhead sun because of reduced transmission of the direct solar radiation. As a result of this sensitivity to the location of clouds relative to the carbon particles, knowing the vertical distribution of black carbon is important (e.g., Haywood and Shine, 1997).

There is another interaction of absorbing aerosol particles with clouds that has been proposed. Hansen et al. (1997b, 1997d) suggest there is a "semi-direct" effect in that warming caused by light absorption may reduce cloud cover and result in net warming. Their calculations suggest that at values of the single-scatter albedo (defined as the fraction of extinction that appears as scattered radiation) below about 0.85, this semi-direct effect predominates and heating, rather than cooling, results.

In summary, aerosol particles containing compounds that can absorb solar radiation, such as elemental (or black) carbon and possibly some organic compounds as well, can also contribute to direct radiative forcing. This absorption of solar radiation generally results in positive radiative forcing. This effect occurs simultaneously with scattering, which results in negative radiative forcing. It should be noted that such particles also affect the amount of solar radiation reaching the low troposphere that is available for photochemistry (e.g., see Haywood and Shine, 1997). See Chapter 3.C.2f for a more detailed discussion of this issue.

c. Absorption of Long-Wavelength Infrared

Species such as sulfate and black carbon can absorb the long-wavelength thermal infrared emitted by the earth's surface, leading to positive radiative forcing, in principle, the same is true of other infrared-absorbing particle components such as nitrate, ammonium, formate, acetate, and oxalate for the bands that are not in the region of the spectrum that is already saturated (Marley et al., 1993). It has been proposed that if these species and/or sulfate are present at sufficiently high concentrations in particles, for example in or downwind of urban areas, they can contribute to radiative forcing by direct absorption of infrared on local to regional scales (Marley et al., 1993; Gaffney and Marley, 1998). On a global basis, Haywood and Shine (1997) and Haywood et al. (1997a) estimate that the contribution of sulfate and black carbon to long-wavelength direct forcing is at least an order of magnitude less than that due to the scattering, and in the case of black carbon, absorption of solar radiation.

However, this is not the case for airborne particles composed of crustal materials formed by erosion processes. As discussed in Chapter 9.C, mineral dust consists primarily of such crustal materials. Despite the fact that soil dust particles tend to be quite large, of the order of a micron and larger, they can be carried large distances. These particles not only scatter and absorb solar radiation but also absorb long-wavelength infrared emitted by the earth's surface.

Figure 14.35, for example, shows the real (n) and imaginary (k) parts of the index of refraction (17 = n -ki) of three samples of dust collected in the Barbados, but thought to be transported from the Sahara Desert (Volz, 1973), from Afghanistan (Sokolik et al., 1993), and from Whitehill, Texas, in the southwestern United States (Patterson, 1981), respectively (Sokolik et al., 1998). Regions of absorption in the infrared due to some common dust components are also shown: the asymmetric C-O stretch of carbonate in calcite near 7 /jum seen in the Texas dust, the asymmetric Si-O-Si

Was this article helpful?

0 0
Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment