The PBL is where nearly all of our weather is produced. Temperature and pressure gradients caused by differential heating force the winds that drive air masses together producing warm, cold and occluded fronts. The lifting mechanisms, reviewed in Session 6, produce the upward motion which causes the cooling necessary for cloud development to occur and precipitation to form. Though, each of these processes are important in the role they play in the production of various weather events, these processes in the PBL are also important in the role they play in the transport, dispersion, and removal of pollution.
Pollutants are released in various forms and from various sources (sulfur dioxide from factory stacks, carbon monoxide from automobile exhausts, etc. (see Session 1's discussion on Air and Air Quality). These pollutants are mixed into the air and transported down wind. Many of these pollutants do not remain in the atmosphere but are removed by natural processes that occur within the planetary boundary layer. We call these processes removal mechanisms, and the duration a pollutant resides or is suspended in the atmosphere is referred to as its residence time. There are basically three removal mechanisms that act on airborne pollutants: 1) wet deposition, 2) dry deposition, and 3) chemical reactions.
Deposition, in general, refers to the transfer of airborne pollutants to the earth's surface where they either react with or adhere to some surface and thus, are removed from the atmosphere. Wet deposition involves the absorption of pollutants, both particles and gases, into liquid droplets or ice crystals. These pollutants are transferred, in most cases, to the surface in the form of precipitation. Wet deposition is also commonly referred to as precipitation scavenging, wet removal, rainout, and washout. However, wet deposition is not limited to precipitation. It also includes the deposition that occurs when low lying fog or haze droplets come into contact with a surface such as plant life or natural and man-made structures. Acid rain and acid fog are two examples of wet deposition that can adversely affect the surfaces onto which they are deposited. Session 8: Physical Meteorology and Visibility will discuss, in detail, the role of aerosols and other pollutants in cloud droplet and ice formation and their significance to cloud physics and the production of fog, haze, and smog.
Dry deposition, as the name suggests, refers to the removal of pollutants that are not absorbed into liquid or ice, but rather, are removed as either dry particles or gases. Dry deposition involves three stages known as the aerodynamic, surface, and transfer stages. The aerodynamic stage involves the transport of pollutants through the surface layer of the atmosphere down to within a few millimeters of the surface onto which the pollutant is deposited. The turbulence of the atmosphere and the aerodynamic resistance of the pollutant control the success of this stage of dry deposition.
The surface stage refers to the diffusion of the pollutant through this remaining thin layer to make contact with the surface. Any resistance through this layer occurs on the molecular level rather than due to turbulent diffusion, as in the aerodynamic stage.
The transfer stage, refers to the actual removal of the pollutant from the atmosphere. A pollutant is not considered to be removed by dry deposition until it is taken up or absorbed by the surface it contacts. Removal occurs, for example, when a pollutant is dissolved into a body of water or surface moisture, incorporated into soil, or taken up by the respiration of plants. The transfer stage is controlled by the reactivity between the pollutant and the surface. Some inert gases, such as helium, are rarely, if ever, removed by dry deposition because they are non-reactive and are never absorbed into a surface.
The transformation of substances through chemical reactions is also considered, by some professionals, a removal mechanism. If two substances react with one another, they are, in a sense, removed because the original substances no longer exist. In the case of acid rain, sulfur dioxide (SO2) combines with moisture to produce sulfuric acid (H2SO4). When this reaction occurs, the sulfur and oxygen are not removed, rather, they exist as sulfuric acid. The sulfur dioxide, however, no longer exists. To learn more about atmospheric chemistry, refer to our companion course, Computational Atmospheric Science.
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