An aerosol can be defined as a dispersion of solid and liquid particles suspended in gas. Atmospheric aerosols, unsurprisingly, refer to solid and liquid particles suspended in air. Aerosols are produced by dozens of different processes that occur on land and water surfaces, and in the atmosphere itself. Aerosols occur in both the troposphere and the stratosphere, but there are considerable differences in the size ranges, chemical nature and sources of the aerosols that occur in these two atmospheric layers. Many research efforts are under way to measure, characterize and model aerosols. This is because aerosols have important consequences for global climate, ecosystem processes, and human health. Aerosols influence the amount of solar radiation reaching the surface of the earth, a consequence that is described as climate forcing, or radiative forcing. Radiative forcing is usually expressed in units of W/m2, and can be a positive or negative term. A reduction of solar radiation reaching the earth is considered a negative forcing. Aerosols both absorb and scatter solar radiation. Most of the light extinction caused by aerosols is due to scattering. Particles in the 0.1 - 1.0 µm size range are especially active in this regard, as their radii are comparable to the wavelengths of visible solar radiation. Scattering of light in this size interval (Mie scattering) is characterized by the Mie theory, which states that particles interact with radiation as a function of their surface. Aerosols smaller than 0.1 µm are called optically small particles. They may also scatter solar radiation via a process called Rayleigh scattering. Rayleigh scattering is inversely proportional to the fourth exponent of the wavelength of the radiation. The cooling effect that aerosols have on the surface of the Earth due to direct reflection of solar radiation is referred to as the direct effect, or direct climate forcing. Aerosol particles also influence the size, abundance, and rate of production of cloud droplets. Thus they influence cloud cover, cloud albedo, and cloud lifetime. The effects of aerosols on the radiative properties of Earth's cloud cover is referred to as the indirect effect of aerosols, or indirect climate forcing. The liquid cloud forcing consists of two parts: the 1st indirect effect (change in droplet number associated with increases in aerosols) and the 2nd indirect effect (change in precipitation efficiency associated with increases in aerosols). Aerosols are transported by prevailing winds and convection once they are in the atmosphere. For this reason, the elements contained in aerosols are seldom redeposited on the surface of the Earth in the same location that they were produced. The dry or wet deposition of aerosols can serve as a source of elements to an ecosystem distinct from other sources, such as weathering. When sulfate (SO4-2) and nitrate (NO3-) containing aerosols are incorporated into cloud droplets, they lead to acidic deposition, often hundreds of miles away from the source of the aerosols or precursor gases. Although aerosols are produced by myriad natural processes, human activities are responsible for generating much of the aerosol load in today's atmosphere. Biomass and fossil fuel burning, agricultural activities, desertification, and industrial pollution all inject aerosol particles directly into the atmosphere (examples of primary production), or produce precursor gases that condense in the troposphere or stratosphere to form aerosols (examples of secondary production).
The size of aerosol particles is usually given as the diameter of the particle
(assuming a spherical shape). Aerosols are usually assigned into one of the
following three size categories.
Aitken particles, or nucleation mode: (0.001 - 0.1 µm diameter) Large particles, or accumulation mode: (0.1 - 1 µm diameter) Giant particles, or coarse particle mode: (> 1 µm diameter)The terms nucleation mode and accumulation mode refer to the mechanical and chemical processes by which aerosol particles in those size ranges are usually produced. The smallest aerosols, in the nucleation mode, are principally produced by gas-to-particle conversion (GPC), which occurs in the atmosphere. Aerosols in the accumulation mode are generally produced by the coagulation of smaller particles and by the heterogeneous condensation of gas vapor onto existing aerosol particles. These generalities apply best to secondary aerosols (those produced by precursor gases, condensation and other atmospheric processes) rather than to primary aerosols (those injected into the atmosphere as particles from the surface of the earth). For example, biogenic aerosols are primary aerosols that occur over a wide range of particle sizes (0.3 - 50.0 µm). Pollen, spores, and plant and animal fragments are generally in the coarse particle mode. Bacteria, algae, protozoa, fungi and viruses will be smaller and will fall into the accumulation mode. Similarly, primary aerosols that are produced by combustion (for example the burning of vegetation) span all three size ranges. Most aerosol particles in the nucleation mode are comprised of sulfuric compounds, and are the result of the oxidation of sulfur containing precursor gases (like SO2, H2S, CS2, COS, CH3SCH3, and CH3SSCH3) to sulfate (SO42-), and subsequent condensation into particle form (homogenous GPC). However, these miniscule sulfate aerosol particles are highly mobile and subject to coagulation: much of the sulfate aerosol produced by GPC ultimately ends up occupying the 0.1 - 1.0 µm size range. Although the number concentration (number of particles per volume air) of aerosols in the nucleation mode is high, they contribute a negligible fraction of the overall total aerosol mass. Over the continents, nitrate (NO3-) containing aerosols are generally larger than 1 µm. This precludes their formation by homogenous GPC. These larger nitrate aerosols probably originate from the evaporation of cloud droplets. Mineral dust, volcanic ash, and fly ash from biomass burning are larger particles. Most mineral aerosol will belong to the coarse particle mode.