In the past century, the mean surface temperature of the Earth has increased by 0.6˚ according to the International Panel of Climate Change [IPCC, 2001]. Figure 1.1 shows the mean Earth’s surface temperature since 1860. The red bars are the mean annual temperature deviation from the 1961 – 1990 average and the red line is a 10 year average. The black line in Figure 1.1 shows a distinct trend upward. Since the industrial revolution, large-scale industrious human activities have accounted for an increase in greenhouse gas concentrations in the atmosphere. Greenhouse gases like carbon dioxide (CO2) and methane (CH4) absorb infrared radiation, thereby influencing the radiation balance and warming the Earth. Furthermore, human activities have accounted for a large increase in small solid or liquid atmospheric particles, called aerosols.
The effect of aerosols on the radiation budget is complicated. Figure 1.2 [IPCC, 2001] shows the level of understanding of the radiative effects, called radiative forcing, resulting from an increase in aerosol and greenhouse gas concentrations [IPCC, 2001]. The radiative forcing caused by aerosols is still very poorly understood, but might even be as large (but negative) as the radiative forcing caused by greenhouse gasses. The first mechanism by which aerosols influence our climate system is by reflecting and absorbing Solar radiation and infrared thermal radiation from the Earth’s surface. This is called the aerosol direct effect. Some aerosol species, like for example black carbon, absorb radiation at long wavelength, thus warming the Earth’s climate system. Other species, like for example desert dust, effectively scatter Solar radiation, thus increasing the planetary albedo and cooling the Earth’s climate system. Aerosol direct forcing is currently considered to give a cooling effect [Kaufman et al, 2001], although some aerosol types, like black carbon, may give a warming effect. The second mechanism by which aerosols influence our climate system is by using as condensation nuclei for water vapor, thereby enhancing the process of cloud forming, resulting in more clouds. Furthermore, these clouds consist of more cloud droplets, which are smaller in size. This increases both the reflectivity and lifetime of clouds. This is called aerosol indirect effect. Clouds reflect Solar radiation, giving a cooling effect, and they reflect infrared radiation from the Earth’s surface, giving a warming effect. The net effect of the aerosol indirect effect is currently considered to be cooling the Earth’s surface [Kaufman et al., 2002]. The effects of aerosols in our climate system described above illustrate the need to monitor aerosols. Besides the effect on climate, other reasons for the need to monitor aerosols are that high aerosol concentrations in urban regions can cause smog, which may lead to human health problems, and that aerosols affect the chemical composition of the atmosphere by the alteration of photolysis rates and by direct chemical interaction with gasses [Kaufman et al, 2002].
Aerosols can be grouped into five categories: dustlike soil, soot, sulfate, sea salt and organic aerosols [Liou, 2002]. Dustlike soil and sea salt aerosol particles have a typical diameter larger than 1 m while soot, sulfate and organic aerosol have a typical diameter smaller than 1 m. Aerosol concentrations are highly variable in space and time, caused by the relatively short lifetime of an aerosol particle, and the differences in aerosol sources. Examples of aerosols sources are forest fires, human industrial activities and sand storms. Furthermore, it is difficult to distinguish anthropogenic aerosols from natural aerosols. Except for marine aerosol, all types of aerosols can have both natural and anthropogenic sources. However, it is possible to estimate the anthropogenic contribution to the total aerosol load using satellite data, aerosol models and information on fire practices and agricultural and industrial activities.
Joris de Vroom
. The Contribution of Dutch GLOBE Schools to Validation of Aerosol Measurements from Space
KNMI number: TR-267, Year: 2003, Pages: 47