The Interaction of Light with Particles
All solids and liquids—including atmospheric particles—have some ability to reflect light. Atmospheric particles can reflect incoming sunlight, with the consequence that some of it is directed back into space and so is unavailable later for absorption at the surface (see Figure 6-18). The particles can also reflect outgoing infrared light, with the consequence that some of it is redirected back toward the Earth's surface rather than escaping from the atmosphere. The redirection of light by a particle is sometimes called scattering; reflection backward is hackscattering.
Certain types of suspended particulates in air reflect some of the sunlight that shines on them back into space and so have a significant albedo value; this reflection of sunlight by the aerosol cools the air mass and the surface below it, since none of the reflected light is subsequently absorbed and then converted to heat.
Some types of aerosol particles can absorb certain wavelengths of light (Figure 6-18a). Once absorbed, the energy that was associated with the light is rapidly converted into heat, which then is shared with the surrounding air molecules as a result of their collisions with the heated particle. Thus absorption of light by a particle leads to warming of the air immediately surrounding it. The absorption of sunlight, with consequent warming, is significant only for dark-colored particles such as those composed primarily of soot, often called carbon black, and of ash particles from volcanoes. The contribution of carbon black to global warming has only recently been fully appreciated. The emission into the atmosphere of carbon black is greatest in developing countries, where incomplete combustion of coal and biomass is widespread. Its effect globally is to increase air temperatures by its absorption of sunlight, with the subsequent export of this tropospheric air to other areas. However, its local effect may be cooling, since it blocks sunlight from reaching the surface. Carbon black's effects on local climate may be substantial, increasing drought in some areas and flooding in others.
Recall from Chapter 3 that the sulfur dioxide gas predominantly released as a pollutant from the burning of fossil fuels—especially coal—and from the smelting of nonferrous metals creates a sulfate aerosol. Pure sulfate aerosols do not absorb sunlight since none of their constituents—water, nitric and sulfuric acids, and the ammonium salts thereof—absorb light in the visible or the UV-A regions. The sulfate aerosols are not particularly effective in trapping outgoing thermal IR emissions. Only if tropospheric sulfate aerosols incorporate some soot will absorption of sunlight by these particles be significant.
Overall, however, anthropogenic sulfate-rich aerosols produced in abundance—especially in the Northern Hemisphere—reflect sunlight back into space much more effectively than they absorb it, so they significantly increase Earth's average albedo. As a result, less sunlight is available to be absorbed and converted to heat in the lower troposphere and at the surface. Thus the net effect of the sulfate aerosols is to cool the air near ground level and thereby to offset some of the global warming induced by greenhouse gases.
In addition to the direct effect of sulfate aerosols in reflecting sunlight, there are indirect effects that arise because the sulfate particles act as nuclei for the formation of small water droplets.
• Such small droplets are more effective in backscattering light than are an equal mass of larger ones (see Figure 6-18b).
• Small droplets are also less likely to coalesce into raindrops, so their clouds are longer-lived than otherwise expected and can thus reflect sunlight for longer periods.
Both these indirect effects result in more sunlight being reflected back into space, thereby cooling the Earth's surface. In addition, the "Asian brown cloud" (Chapter 4) formed by aerosol particles reduces the strength of the essential monsoon rains over India and Asia.
Some scientists have proposed that sulfate particles be injected artificially into the stratosphere, where they would reflect sunlight and thereby offset some of the effects of global warming. The lifetime of the particles in the stratosphere is several years, depending on altitude, so the sulfate would have to be replenished regularly. Although the injection of sulfate particles is considered to be a short-term solution, until controls for carbon dioxide emissions have been put in place, a few scientists have proposed injecting solid reflecting objects of macroscopic size high above the atmosphere, where they would reflect sunlight and counter global warming on a long-term basis. Such geoengineering of the Earth's climate is considered controversial by many scientists and policymakers because of the uncertainties involved in its potential side effects.
A short-term, dramatic example of the effects of atmospheric aerosols on climate occurred as a consequence of the massive eruption of substances into the troposphere and stratosphere by the Mount Pinatubo volcano in the Philippines in 1991. Initially, the lower stratosphere was warmed by the dominant effect of the large volcanic ash particles, which absorbed some of the incoming sunlight and subsequently converted it to heat, and by their interception of outgoing infrared from the surface. Due to their relatively large size, the ash particles were not long-lived in the stratosphere. The longer-term effect of the Pinatubo eruption was to significantly decrease air temperatures at ground level. The stratospheric aerosol that remained suspended after a few months was formed by the oxidation of the 30 million tonnes of SC)2 that the volcano had blasted directly into the lower parts of this region. The sulfate aerosol remained there for several years, during which time it efficiently reflected sunlight back into space. Many regions, including North America, experienced several cool summers in the early 1990s as a result. Due to the gradual sedimentation of the aerosol, we returned to 1990-1991 temperatures by 1995.
Continue reading here: Aerosols and Global Warming
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