Shortwave aerosol radiative forcing

The SW aerosol radiative forcing, AF, or more precisely the 'aerosol flux change', is the effect of aerosols on the SW radiation budget at TOA, at the Earth's surface, or within the atmosphere, and it is given by

where Fi and Fciear i are the SW radiative fluxes with and without the presence of aerosols, respectively. The index i involves various aerosol forcings defined in terms of the corresponding SW fluxes. The forcings AFTOa, AFatmo, AFsurf, and AFnsurf, represent the effect of aerosols on the outgoing SW radiation at TOA, the SW radiation absorbed within the atmosphere, the downward SW radiation at the Earth's surface, and the net downward (or absorbed) SW radiation at the surface.

Aerosols cool the Earth's surface by scattering to space and through atmospheric absorption, thus reducing the solar radiation that reaches the surface. Therefore, the presence of aerosols has a significant effect on the thermal dynamics of the

Earth-atmosphere system. By cooling the surface and warming the atmosphere, aerosols act also to produce more stable atmospheric conditions by decreasing convective activity. They also reduce evaporation from the surface, and so can have a significant effect on the hydrological cycle by suppressing cloud formation and associated precipitation. The aerosol redistribution of radiative energy between the Earth's surface and the atmosphere highlights the role of increasing loads of particulate matter (amount and composition) in the atmosphere on climate by enhancing desertification processes, especially in semiarid regions such as the Mediterranean basin.

8.6.1 Aerosol forcing at TO A

In general, aerosols increase through reflection the outgoing solar radiation at TOA, by up to 6 W m~2 on a mean monthly basis, producing thus a planetary cooling (Fig. 8.22). However, aerosols can produce a planetary warming by decreasing, on a mean monthly basis, the outgoing solar radiation at TOA, by up to 4 W m~2. This is due to important absorption of atmospheric solar radiation by aerosol particles, such as those characterized by mineral components, over desert areas (e.g. Sahara, Arabia) or soot particles over continental areas (e.g. Europe, North Asia, North America). In general, the sign of the effect of aerosols on the outgoing SW radiation at TOA (aerosol forcing AFtoa) under a clear sky is determined by the values of the single scattering albedo, w, and the surface albedo, Rg. Thus, whereas planetary warming is found over Siberia in winter, there is planetary cooling in summer. The role of Rg is also shown by the contrast between the planetary cooling produced by mineral aerosols over oceanic areas of low Rg or over continental areas of Rg lower than 0.3 (sub-Sahel), and the planetary warming produced by similar mineral aerosols over highly reflecting deserts (Rg >0.3). The aerosol-induced planetary warming over highly reflecting surfaces (e.g. polar regions) is due to particle absorption caused by pollution aerosols (haze), which is enhanced through multiple reflections between ice- or snow-covered surfaces and the aerosols above. Field measurements have verified this effect, for example during the Aerosol Characterization Experiment (ACE-2). An example is the Arctic haze containing a considerable amount of anthropogenic impurities, including soot, (originating from the industrialized centres in northern Europe, North America, and Asia), and undergoing long-range transport under winter anticyclonic conditions (high-pressure regions with sinking air and low pressure gradients favouring gentle winds) that decrease substantially the albedo and act against global cooling due to anthropogenic sulphates. The Arctic aerosol type, including soot particles transported from midlatitude continental areas, can be found north of 70°N. The anticyclonic situation, favourable for this transport, persists mainly during winter through springtime, when an intense high-pressure system pushes the Arctic front to the South. Large polluted agricultural and industrialized areas of Eurasia, Japan, and North America are then within the Arctic air mass, which can move aerosols across the North

UV-visible Aerosol Forcing at TOA

flG. 8.22. Global distribution of the aerosol effect on the outgoing UV-visible radiation at top-of-atmosphere (aerosol forcing AFrOA, W m-2) for January, using GADS aerosol climatological data. (Hatzianastassiou et al. 2004a)

flG. 8.22. Global distribution of the aerosol effect on the outgoing UV-visible radiation at top-of-atmosphere (aerosol forcing AFrOA, W m-2) for January, using GADS aerosol climatological data. (Hatzianastassiou et al. 2004a)

Pole. This atmospheric activity is further amplified by the lack of clouds and precipitation. The magnitude of AFtoa is determined by the incoming solar radiation, the reflectance of the aerosol layer, directly associated with AOT, and the clear-sky fraction. The maximum summer tropical and subtropical AFtoa values are associated with relatively small cloud cover (e.g. over India in January and over the Mediterranean in July), and large incoming solar radiation values, along with relatively large AOT values. The AFtoa values in midlatitudes of the Northern Hemisphere (Europe, USA) are larger in summer than in winter, mainly because of larger incoming solar fluxes at TOA and less cloudiness during this season.

8.6.2 Aerosol forcing of atmospheric absorption

Significant aerosol-induced increase in solar atmospheric absorption can be seen in Figs. 8.23 and 8.24. Aerosols are found to cause increases, on a mean monthly basis, up to 25 W m~2 in the UV-visible and 7 W m~2 in the near-infra-red in areas (e.g. Sahara) characterized by significant amounts of absorbing aerosols (such as mineral dust) over surfaces with large surface albedo (>0.3). The solar atmospheric absorption is also found to be increased by less than 10 W m~2, due to aerosols, over Europe, North America, South, and South-East Asia, but also over South Africa, the Amazon basin and Australia. Note that model computations indicate a significant aerosol-induced solar atmospheric absorption of 5-12 W m~2 in the sub-Sahelian region (5-10°N) in January, which is the local dry season. According to the Global Fire Atlas, fires are largely concentrated in this

UV-visible Aerosol Forcing in Atmosphere

flG. 8.23. Global distribution of the aerosol effect on atmospheric absorption of UV-visible radiation (aerosol forcing Afatmo, W m~2) for January, using GADS aerosol climatological data. (Hatzianastassiou et al. 2004a)

flG. 8.23. Global distribution of the aerosol effect on atmospheric absorption of UV-visible radiation (aerosol forcing Afatmo, W m~2) for January, using GADS aerosol climatological data. (Hatzianastassiou et al. 2004a)

area in winter (January). Of course, the effect of aerosols on the SW atmospheric absorption (AFatmo) in the sub-Sahelian region can also be attributed to desert dust advected from Sahara southward toward the equator, by the Harmattan trade winds occurring in the dry season. In contrast, during Northern Hemisphere summer (July), with the dry season shifted to the Southern Hemisphere, the biomass burning is dominant to the south of the equator, with maximum concentration between 5 and 15° S. Model computations indicate an aerosol-induced increase in solar atmospheric absorption during summer, of about 5-8 W m~2 in South Africa and within the Amazon basin.

8.6.3 Aerosol forcing at the Earth's surface

Model results indicate that aerosols can decrease regionally the monthly mean downward UV-visible radiation at the Earth's surface by up to 28 W m~2, and the absorbed solar radiation by the surface by as much as 23 W m~2, thus producing surface radiative cooling. The largest decreases in surface solar radiation are found over areas with significant aerosol amounts (AOT); thus, AOT, AFsurf, and AFnsurf values are correlated. Therefore, a large effect of aerosols on the downward solar radiation at the Earth's surface is encountered over land areas (especially over deserts) while smaller forcing values are found over oceans, due to optically thin aerosol layers above. According to model computations,

Near-IR Aerosol Forcing in Atmosphere

flG. 8.24. Global distribution of the aerosol effect on atmospheric absorption of near-infra-red radiation (aerosol forcing AFatmo, W m~2) for January, using GADS aerosol data. (Hatzianastassiou et al. 2006)

flG. 8.24. Global distribution of the aerosol effect on atmospheric absorption of near-infra-red radiation (aerosol forcing AFatmo, W m~2) for January, using GADS aerosol data. (Hatzianastassiou et al. 2006)

aerosols can produce locally an effect on solar surface radiation that is up to three times larger than their effect at TOA.

The radiative effect of aerosols on the absorbed solar radiation at the Earth's surface is determined by the forcing AFsurf and the surface albedo Rg. Therefore, the features of AFnsurf are similar to those of AFsurf, except in the high-albedo polar regions. Note that in cases of small AFatmo values, the forcings AFtoa and AFnsurf are almost equal in magnitude. Under such circumstances, the aerosol forcing at the surface could be estimated from the corresponding forcing at TOA, as suggested by Ramanathan et al. (2001), for purely scattering aerosols.

8.6.4 Mean annual hemispherical aerosol forcings

In Table 8.13 are given mean annual model estimates of the three types, AFtoa, AFatmo and AFnsurf of direct aerosol forcings (combined natural and anthropogenic), under all-sky conditions, for the Northern Hemisphere (NH), Southern Hemisphere (SH), and the globe. We note that for energy conservation we have

The global cooling to space, due to all aerosols, is estimated at 1.62 W m~2, while the surface cooling is estimated to be -3.22 Wm~2, thus giving an atmospheric

Table 8.13 Model mean annual global and hemispherical (NH, SH) all-sky shortwave direct aerosol radiative forcing (W m-2), for combined natural and anthropogenic aerosols. Forcings are given in terms of: outgoing SW radiation at TOA, atmospheric absorption, and net at surface. (Hatzianastassiou et al. 2007)

AFtoa Afat mo AFnsurf

Globe L62 L60 -3.22 NH 1.72 2.63 -4.35 SH 1.51 0.58 -2.09

absorption of 1.60 W m-2. Atmospheric absorption by aerosols is greater in the Northern Hemisphere due to its heavy industrialization (more carbonaceous aerosols), whereas the Southern Hemisphere, beyond the tropical biomass burning, has natural marine sulphate aerosols that are primarily scattering. This can be seen in the AFatmo values. Similar radiative forcing values are also reported in the review by Yu et al. (2006), based on a measurement-based assessment.

The changes to global gradients in OSR will affect general circulation patterns in the atmosphere, and hence climate. Many studies of the direct aerosol SW forcing have concentrated upon the distributions and radiative effects of anthropogenic scattering sulphate aerosols that are believed to produce a substantial cooling effect. Natural aerosols are equally important to climate. On the other hand, attention has been directed to mineral-dust aerosols, especially those originating from changes in land use, because of their large contribution to atmospheric aerosol loading and absorption of solar radiation. Many global studies of direct forcing due to anthropogenic sulphate, black (elemental) carbon, carbonaceous and soot aerosols from biomass burning and fossil fuels, nitrate aerosols, soil dust, and organic matter have been carried out. We note that mineral dust (from deserts) and black carbon (industrial pollution sources) actually reduce the outgoing solar radiation and hence contribute to global warming. The global direct aerosol forcing might be comparable to the anthropogenic greenhouse forcing of longwave radiation, i.e. global warming, which is estimated to reduce the outgoing thermal infra-red radiation by about 2.4 W m~2 (IPCC 2001).

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