Training on atmospheric dispersion in urban environments
3. Basics of atmospheric transport and dispersion
After being released into the atmosphere, an air pollutant is transported, or advected, away from the source, and is dispersed or spread out by the process of turbulent diffusion. The pollutant may be removed from the atmosphere, by scavenging within clouds (rainout) or below clouds (washout), as well as through wet and dry deposition on the ground. The most relevant mechanisms involved in atmospheric transport and dispersion are illustrated in figure 3.1. Chemical or radiological transformations can also occur.
Figure 3.1: The most important processes affecting the transport and dispersion of airborne material released into the atmosphere [1].
Dispersion is primarily governed by turbulence, which mixes ambient air with the plume, whose average concentration drops with increasing distance from the source. The three kinds of turbulence that act to disperse a plume are mechanical, shear and buoyancy. Mechanical turbulence is caused by air flowing over surface features, such as buildings, trees, and hills. Not only does mechanical turbulence disperse a plume, but if the surface features are large enough, it can change the direction the plume is traveling. Wind shear turbulence results from abrupt changes in wind speed or direction. Buoyancy turbulence is caused by bubbles of warm air rising, either due to explosion, fire, or vertical instability, e.g. daytime heating of air near the ground.
The concentration of an air pollutant at a given place depends on several factors, including the amount of the pollutant released at the source (the emission rate and duration), the distance from the source, and the atmospheric conditions. The most important atmospheric conditions are wind speed, wind direction, and the vertical temperature characteristics. If the air temperature decreases strongly with height, this results in an unstable atmosphere that tends to mix pollutants vertically in a deeper layer , resulting in low to moderate pollutant concentrations at ground level. If the temperature increases or is constant with height, then the atmosphere is stable: vertical mixing is limited, pollutants accumulate in low levels, and concentrations near the ground will be high.
The concentration of pollutants is often expressed in terms of the total mass of the pollutant in a standard volume of air. The most frequently used measure in metric units is micrograms of pollutant in one cubic meter of air (μg/m3). This measure can be used either for particles or for gases. Concentrations of gases can also be expressed as parts per million (ppm), where 1 ppm represents 1 cubic meter of the pollutant dispersed into 1 million cubic meters of air. Using the density of each gaseous pollutant, its concentration can be converted from ppm to μg/m3, or vice versa. As example for sulfur dioxide, at reference conditions, 1 ppm = 2.620 μg/m3.
Models that take into account these processes (transport, diffusion, scavenging, deposition, transformation) can be used to predict airborne concentrations and surface deposition at locations downwind of the release point.
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