Aerosols are solid or liquid particles that are small enough to remain suspended in the atmosphere for hours to days. They are created by both natural and human-caused processes, and are either directly emitted from sources or are formed in the atmosphere from the condensation of gases. Aerosols can perturb climate either directly, by the scattering or absorption of radiation, or indirectly, through the modification of clouds and precipitation. The effects of aerosols on climate represent the single largest source of uncertainty in the understanding of global warming. While the magnitude of the effect is unclear, most observations and models agree that the impact of aerosols is to cool the earth.
Typically, particles that comprise aerosols range in size from the length of a large molecule (one nanometer) to one-tenth the width of a human hair .0004 in. (10 microns). The number of these particles in a cubic centimeter of air at the Earth’s surface is highly variable in time and space, and ranges from 100 to 100,000. "Primary aerosols" are formed directly at the source, examples of which include mineral dust, sea salt, and combustion products such as soot and black carbon. "Secondary aerosols" are those formed in the atmosphere from reacted gases, and are often complex mixtures of oxygen, hydrogen, sulfur, nitrogen, and carbon. Most aerosols contain a mix of primary and secondary constituents. Aerosols are the major part of smog, and, in addition, contribute to many environmental and human problems such as acid rain and human respiratory ailments.
The impact of aerosols on climate has been classified in two ways. The direct radiative effect describes the scattering and absorption of sunlight and the earth’s radiated heat energy by aerosols. Indirect radiative effects include the suite of possible impacts of aerosols on climate through the modification of cloud properties.
Scientists quantify the impact of an external agent such as aerosols on climate by its radiative forcing, expressed in units of Watts per square meter (W m-2). A positive radiative forcing tends, on average, to warm the Earth’s climate, whereas a negative forcing tends to cool the Earth.
The direct radiative effect of aerosols is closely related to "global dimming," which is the observed reduction of sunlight received at the surface of the Earth in areas where pollution aerosols play a role. Recent assessments of the direct effect have benefited greatly from improvements in satellite and surface-based observations, which provide near-global coverage of aerosol optical properties. These observations have been matched by improvements in, and are often integrated into, global atmospheric computer models that can provide estimates of direct radiative forcing of aerosols. The most recent estimate places the value of direct effect radiative forcing between minus 0.9 and minus 0.1 W m2.
The indirect effect highlights a key role that aerosols play in the atmosphere as the seeds upon which water vapor condenses to form clouds. Variations in the number of these seed particles, known as cloud condensation nuclei (CCN), result in changes in the radiative properties, amounts, and lifetime of clouds. The effectiveness of an aerosol to act as a CCN is a function of its size, composition, shape, and surrounding environment. These details are known best about liquid water clouds, whereas knowledge of ice clouds is limited.
Several types of indirect effects have been identified. The first is the albedo effect, also known as the Twomey or 1st indirect effect. Albedo is a measure of the reflectivity of a cloud, and the albedo effect refers to an increase in the number of CCN, which leads to the formation of more numerous, but smaller, cloud droplets. This leads to an increase in cloud reflectivity, resulting in a negative radiative forcing estimated to be between minus 0.3 and minus 1.8 W m2.
The cloud lifetime effect or 2nd indirect effect, describes how increased CCN can modify the hydro-logical cycle and cause feedbacks that ultimately affect the earth’s climate. Examples include drizzle suppression, increased cloud height, and increased cloud lifetime. Drizzle suppression, for example, can result in increases in wind-blown dust, which would lead to increased aerosol concentrations that would directly and indirectly impact climate. A third classification, the semi-direct effect, refers to certain types of aerosols, such as black carbon, that absorb sunlight and heat the atmosphere, which suppresses cloud formation. There are no estimates available for the impact of these other indirect effects.
The overall impact of aerosols on climate includes direct, semi-direct, and indirect effects. All model simulations agree that the total effect is greater over the Northern Hemisphere than the Southern Hemisphere. Direct and semi-direct effects in models are generally smaller compared to indirect effects. The average overall forcing ranges from minus 0.2 to minus 2.3 W m2, and the resulting effect on precipitation ranges from 0 to minus .05 in. (1.3 mm.) day-1. No climate simulation accounts for all aerosol-cloud interactions, therefore the net aerosol effect on clouds deduced by models is inconclusive.
