AIR POLLUTION METEOROLOGY Introduction Meteorology is the science/study of the atmosphere. The atmosphere is the media into which all air pollution is emitted. Atmospheric processes such as the movement of air (wind) and the exchange of heat (convection and radiation for example) dictate the fate of pollutants as they go through the stages of transport, dispersion, transformation and removal. Air pollution meteorology is the study of how these atmospheric processes affect the fate of air pollutants. Knowledge of air pollution meteorology is used to manage and control the release of pollutants into the ambient air. Managing the release of air pollutants helps ensure that ambient pollutant concentrations comply with ambient air quality standards. Knowledge of air pollution meteorology is essential in order to understand the fate and transport of air pollutants.

Composition of the Atmosphere The atmosphere surrounds the earth and rotates with the earth as it orbits the sun. As Table 11 shows, dry air consists of about 78 percent nitrogen, 21 percent oxygen and one percent argon. Trace gases such as carbon dioxide, neon and helium also exist as does water vapour. Although the water vapour content of the air is fairly small, it absorbs six times more radiation than any other atmospheric constituent and is therefore a very important component of the atmosphere.

Layers of the Atmosphere The atmosphere is divided into four distinct layers: the troposphere, stratosphere, mesosphere, and thermosphere (Figure 1-1). The lowest layer is called the troposphere which accounts for about three quarters of the mass of the atmosphere and contains nearly all of the water associatedwith the atmosphere (vapor, clouds and precipitation). The troposphere, where air masses, fronts, and storms reside, is the most unsettled layer and provides earth its weather. The depth of the troposphere varies with latitude and season. The top of the troposphere (tropopause) is about 16.5 km (54,000 ft) on average over the equator and about 8.5 km (28,000 ft) over the poles. Seasonal changes affect the thickness of the troposphere causing it tobe thicker in summer (when the air is warmer) than in winter. The depth of the troposphere changes constantly due to changes in atmospheric temperature.

Virtually all air pollution is emitted within the troposphere. Air pollution transport is governed by the speed and direction of the winds. The rate of dispersion is influenced by the thermal structure of the atmosphere as well as by mechanical agitation of the air as it moves over the different surface features of the earth. Transformation of the emitted air pollutants is impacted by exposure to solar radiation and moisture as well as other constituents in the atmosphere. The removal of pollutants depends not only on the pollutants’ characteristics but

also on weather phenomena such as rain, snow and fog. These interactive meteorological phenomena are studied as part of air pollution meteorology.

The Importance of Air Pollution Meteorology Since the atmosphere serves as the medium into which air pollutants are released, the transport and dispersion of these releases are influenced significantly by meteorological parameters. Understanding air pollution meteorology and its influence in pollutant dispersion is essential in air quality planning activities. Planners use this knowledge to help locate air pollution monitoring stations and to develop implementation plans to bring ambient air quality into compliance with standards. Meteorology is used in predicting the ambient impact of a new source of air pollution and to determine the effect on air quality from modifications to existing sources. When meteorological conditions develop that are not conducive to pollutant dispersion, governmental air pollution agencies must act fast to ensure that air pollutants don’t build up to unacceptable levels in the air we breathe. When pollutant levels become excessively high, an air pollution episode results and emissions into the atmosphere must be curtailed. Donora, Pennsylvania provides an extreme example of this situation. In 1948, Donora suffered a disastrous air pollution episode. Donora is located in the bottom of a valley surrounded by rolling hills. The townspeople were accustomed to receiving some emissions from the local steel mill, zinc smelter, and sulfuric acid plant. But, they were not prepared for the dangerously high concentrations of pollutants that built up and became trapped over the town. The meteorological conditions in Donora during this five-day period (high pressure system and strong temperature inversion) produced light winds and dense fog. The air was not able to move horizontally or vertically and just lingered over the town. The factories continued to operate, releasing their pollutants into the air. Many people became ill and 22 people died. Finally, high concentrations of pollutants subsided as the weather pattern broke, winds picked up and the valley experienced rain. ATMOSPHERIC DISPERSION The release of both natural and man made contaminants into the atmosphere can present numerous hazards to human health, animals and plants and the climate. The degree of impact is dependant on the pollutant and its concentration. Dispersion science is the study of the release, movement, dilution, loss and impact of such pollutants in the atmosphere. Atmospheric dispersion modelling is the use of computer models to simulate these processes and is the means by which we can predict the impact of pollutants over distances ranging from a few hundred metres to the entire globe.

Air quality dispersion model Air quality models are used to predict ground level concentrations down point of sources. The object of a model is to relate mathematically the effects of source emissions on ground level concentrations, and to establish that permissible levels are, or are not, being exceeded. Models have been developed to meet these objectives for a variety of pollutants and time circumstances. Models may be described according to the chemical reactions involved. So-called nonreactive models are applied to pollutants such as CO and SO2 because of the simple manner in which

their chemical reactions can be represented. Reactive models address complex multiplespecies chemical mechanism common to atmospheric photochemistry and apply to pollutants such as NO, NO2, and O3. Models can be described as simple or advanced based on the assumptions used and the degree of sophisticated with which the important variables are treated. Advanced models have been developed for such problems as photochemical pollution, dispersion in complex terrain, long-range transport, and point sources over flat terrain. The most widely used models for predicting the impact of relative unreactive gases, such as SO2, released from smoke stacks are based on Gaussian diffusion. In Gaussian models, the spread of a plume in vertical horizontal directions is assumed to occur by simple diffusion along the direction of the mean wind. The maximum ground level concentration is calculated by means of the following Equation.

Where

Cx

=

Q u H

= = =

σy

=

σz

=

y e

= =

ground level concentration at some distance x downwind (g/m3) average emission rate (g/sec) mean wind speed (m/sec) effective stack height (m) standard deviation of wind direction in the horizontal (m) standard deviation of wind direction in the vertical (m) off-centre line distance (m) natural log equal to 2.71828

The parameters σy and σz describe horizontal and vertical dispersion characteristics of a plume (form of emissions in air) at various distances downwind of a source as function of different atmospheric stability conditions. Values are determined from the graphs found n the figure. The effective stack height H is equal to the physical stack height (h) plus the height of the plume (plume rises, Δh) determined from where the plume bends over. Plume rises must be calculated from model equations before the effective stack height can be calculated.

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