Primary Meteorological Factors Affecting Concentration of Pollutants
The primary meteorological factors affecting the concentration of air pollutants is wind speed, wind direction and atmospheric stability. Wind speed or velocity is influenced by topography. Movement of air near the earth's surface is retarded by frictional effects proportional to the surface roughness. So wind speed will be greater farther from the ground surface. Stability is the tendency to resist vertical motion or to suppress existing turbulence. This tendency directly influences the ability of the atmosphere to disperse pollutants emitted into it. When the Stability is low vertical motion is not suppressed and pollutants will be able to be dispersed higher from the ground surface. When this is occurring the atmosphere is said to be Unstable. Stability is measured by the variation of ambient air temperature with respect to the height above the ground. A Stable atmosphere is one in which the ambient air temperature is greater further from the ground surface and an Unstable atmosphere is one in which the ambient air temperature is less further from the ground surface. Stability and wind speed are related in that when the air near the earth surface is pushed down because of greater stability the wind speed increases. The analogy to that of fluid flow in pipes holds here as you decrease the cross-sectional area the velocity must increase for the flow to remain the same (the law of continuity). From the figure if there is no wind (that is on a calm day) it is worse to have a greater stability condition because the pollutants when emitted will tend to stay concentrated closer to the ground.
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