Basic Air Pollution Meteorology - Class 2

What is a occluded front?

When cold and warm fronts merge (the cold front overtaking the warm front) occluded fronts form. Occluded fronts can be called cold front or warm front occlusions, but, in either case, a colder air mass takes over an air mass that is not as cold.

What are some factors regarding plume rise?

A combination of the gases' momentum and buoyancy causes the gases to rise. This is referred to as plume rise and allows air pollutants emitted in this gas stream to be lofted higher in the atmosphere. Since the plume is higher in the atmosphere and at a further distance from the ground, the plume will disperse more before it reaches ground level.

Describe the dry adiabatic lapse rate:

A dry air parcel rising in the atmosphere cools at the dry adiabatic rate of 9.8°C/1000m and has a lapse rate of −9.8°C/1000m. Likewise, a dry air parcel sinking in the atmosphere heats up at the dry adiabatic rate of 9.8°C/1000m and has a lapse rate of 9.8°C/1000m. Air is considered dry, in this context, as long as any water in it remains in a gaseous state. The dry adiabatic lapse rate is a fixed rate, entirely independent of ambient air temperature. A parcel of dry air moving upward in the atmosphere, then, will always cool at the rate of 9.8°C/1000 m, regardless of its initial temperature or the temperature of the surrounding air. You will see later that the dry adiabatic lapse rate is central to the definition of atmospheric stability.

Describe the wet adiabatic lapse rate:

A rising parcel of dry air containing water vapor will continue to cool at the dry adiabatic lapse rate until it reaches its condensation temperature, or dew point. At this point the pressure of the water vapor equals the saturation vapor pressure of the air, and some of the water vapor begins to condense. Condensation releases latent heat in the parcel, and thus the cooling rate of the parcel slows. This new rate, called the wet adiabatic lapse rate, is shown in Figure 4-2. Unlike the dry adiabatic lapse rate, the wet adiabatic lapse rate is not constant but depends on temperature and pressure. In the middle troposphere, however, it is assumed to be approximately −6 to −7o C/1000 m.

What is an advection inversion?

Advection inversions are associated with the horizontal flow of warm air. When warm air moves over a cold surface, conduction and convection cools the air closest to the surface, causing a surface-based inversion. This inversion is most likely to occur in winter when warm air passes over snow cover or extremely cold land. Another type of advection inversion develops when warm air is forced over the top of a cooler air layer. This kind of inversion is common on the eastern slopes of mountain ranges, where warm air from the west overrides cooler air on the eastern side of the mountains. Denver often experiences such inversions. Both kinds of advection inversions are vertically stable but may have strong winds under the inversion layer.

Why does air movement occur?

Air moves in an attempt to equalize imbalances in pressure that result from differential heating of the earth's surface. While moving from areas of high pressure to low pressure, wind is heavily influenced by the presence or absence of friction. Thus, surface winds behave differently than winds aloft due to frictional forces acting near the earth's surface. The rotation of the earth modifies atmospheric motion but does not cause it, since the atmosphere essentially rotates with the earth. The movement of air helps keep concentrations of pollutants that are released into the air from reaching dangerous levels.

What is air pollution transport governed by?

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.

list and describe the 4 different air quality dispersion models:

Air quality dispersion models consist of a set of mathematical equations that interpret and predict pollutant concentrations due to plume dispersal and impaction. These models incorporate the above mentioned dispersion estimates and various meteorological conditions including temperatures, wind speeds, stabilities, and topography. There are four generic types of models: Gaussian, numerical, statistical, and physical. The Gaussian models use the Gaussian distribution equation (see discussion on Gaussian Distribution below) and are widely used to estimate the impact of nonreactive pollutants. Numerical models are more appropriate than Gaussian models for area sources in urban locations that involve reactive pollutants, but numerical models require extremely detailed source and pollutant information and are not widely used. Statistical models are used when scientific information about the chemical and physical processes of a source are incomplete or vague and therefore make the use of either Gaussian or numerical models impractical. Lastly, physical models require fluid modeling studies or wind tunneling. This type of modeling is very complex and requires expert technical support. However, for areas with complex terrain, stack downwash, complex flow conditions, or large buildings, this type of modeling may be the best choice.

Explain important factors for inversions regarding lapse rates:

An inversion occurs when air temperature increases with altitude. This situation occurs frequently but is generally confined to a relatively shallow layer. Plumes emitted into air layers that are experiencing an inversion (inverted layer) do not disperse very much as they are transported with the wind. Plumes that are emitted above or below an inverted layer do not penetrate that layer, rather these plumes are trapped either above or below that inverted layer. High concentrations of air pollutants are often associated with inversions since they inhibit plume dispersion. The four major types of inversions are caused by different atmospheric interactions and can persist for different amounts of time.

Define "calm" conditions:

Any average wind speed below the starting threshold of the wind speed or direction sensor, whichever is greater.

What is the environmental lapse rate?

As mentioned previously, the actual temperature profile of the ambient air shows the environmental lapse rate. Sometimes called the prevailing or atmospheric lapse rate, it is the result of complex interactions of meteorological factors, and is usually considered to be a decrease in temperature with height. It is particularly important to vertical motion since surrounding air temperature determines the extent to which a parcel of air rises or falls. As Figure 4-3 shows, the temperature profile can vary considerably with altitude, sometimes changing at a rate greater than the dry adiabatic lapse rate and some times changing less. The condition when temperature actually increases with altitude is referred to as a temperature inversion. In Figure 4-4, the temperature inversion occurs at elevations of from 200 to 350 m. This situation is particularly important in air pollution, because it limits vertical air motion.

What is a stationary front?

As the name implies, the air masses around this front are not in motion. It will resemble a warm front, and will manifest similar weather conditions. The abbreviations cP and mT stand for continental Polar and maritime Tropical air masses. A stationary front can cause bad weather conditions that persist for several days

What are some buoyancy factors for air parcels?

Atmospheric temperature and pressure influence the buoyancy of air parcels. Holding other conditions constant, the temperature of air (a fluid) increases as atmospheric pressure increases, and conversely decreases as pressure decreases. With respect to the atmosphere, where air pressure decreases with rising altitude, the normal temperature profile of the troposphere is one where temperature decreases with height. An air parcel that becomes warmer than the surrounding air (for example, by heat radiating from the earth's surface), begins to expand and cool. As long as the parcel's temperature is greater that the surrounding air, the parcel is less dense than the cooler surrounding air. Therefore, it rises, or is buoyant. As the parcel rises, it expands thereby decreasing its pressure and, therefore, its temperature decreases as well. The initial cooling of an air parcel has the opposite effect. In short, warm air rises and cools, while cool air descends and warms. The extent to which an air parcel rises or falls depends on the relationship of its temperature to that of the surrounding air. As long as the parcel's temperature is greater, it will rise; as long as the parcel's temperature is cooler, it will descend. When the temperatures of the parcel and the surrounding air are the same, the parcel will neither rise nor descend unless influenced by wind flow.

What is conditional instability?

Conditional instability occurs when the environmental lapse rate is greater than the wet adiabatic lapse rate but less than the dry rate. Stable conditions occur up to the condensation level and unstable conditions occur above it.

Define the term conduction heating:

Conduction is the process by which heat is transferred through matter without the transfer of matter itself. For example, the handle of an iron skillet becomes hot due to the conduction of heat from the stove burner. Heat is conducted from a warmer object to a cooler one.

Explain source effects on plume rise:

Due to the configuration of the stack or adjacent buildings, the plume may not rise freely into the atmosphere. Some aerodynamic effects due to the way the wind moves around adjacent buildings and the stack can force the plume toward the ground instead of allowing it to rise in the atmosphere. Stack tip downwash can occur where the ratio of the stack exit velocity to wind speed is small. In this case, low pressure in the wake of the stack may cause the plume to be drawn downward behind the stack. Pollutant dispersion is reduced when this occurs and can lead to elevated pollutant concentrations immediately downwind of the source. As air moves over and around buildings and other structures, turbulent wakes are formed. Depending upon the release height of a plume (stack height) it may be possible for the plume to be pulled down into this wake area. This is referred to as aerodynamic or building downwash of the plume and can lead to elevated pollutant concentrations immediately downwind of the source.

Describe important characteristics of air pressure:

Even though you can't see it, air has weight. In any gas such as air, molecules are moving around in all directions at very high speeds. The speed actually depends on the temperature of the gas. Air pressure is caused by air molecules (e.g. oxygen, nitrogen) bumping into each other and other things and bouncing off. Air pressure is a function of the number of air molecules in a given volume and the speed at which they are moving. When air is confined within a certain boundary, heating the air increases its pressure and cooling the air decreases its pressure. Forcing air into a smaller space increases air pressure while allowing it to expand into a larger space reduces air pressure. Air pressure at any location whether it is on the earth's surface or up in the atmosphere depends on the weight of the air above. Imagine a column of air. At sea level, a column of air extending hundreds of kilometers above sea level exerts a pressure of 1013 millibars (mb) (or 1.013 kP). But, if you travel up the column to an altitude of 5.5 km (18,000 feet), the air pressure would be roughly half, or approximately 506 mb (0.506 kP).

What is the representativeness of meteorological data dependent on?

For a dispersion model to provide useful and valid results, the meteorological data used in the model must be representative of the transport and dispersion characteristics in the vicinity of the source that the model is trying to simulate. The representativeness of the meteorological data is dependent on the following: • The proximity of the meteorological monitoring site to the area under consideration • The complexity of the terrain in the area • The exposure of the meteorological monitoring site • The period of time during which the data are collected In addition, the representativeness of the data can be adversely affected by large distances between the source and the receptor of interest. Similarly, valley/mountain, land/water, and urban/rural characteristics affect the accuracy of the meteorological data for the source under consideration. For control strategy evaluations and New Source Review, the minimum meteorological data required to describe transport and dispersion of air pollutants in the atmosphere are wind direction, wind speed, mixing height and atmospheric stability (or related indicators of atmospheric turbulence and mixing). Due to the question of representativeness of meteorological data, site-specific data are preferable to data collected off-site. Typically one year of on-site data is required. If an off-site database is used (from a nearby airport for example), five years of data are normally required. With five years of data, the model can incorporate most of the possible variations in the meteorological conditions at the site.

Describe the behavior of the adiabatic process:

For the most part, a parcel of air does not exchange heat across its boundaries. Therefore, an air parcel that is warmer than the surrounding air does not transfer heat to the atmosphere. Any temperature changes that occur within the parcel are caused by increases or decreases of molecular activity within the parcel. Such changes, occur adiabatically, and are due only to the change in atmospheric pressure as a parcel moves vertically. An adiabatic process is one in which there is no transfer of heat or mass across the boundaries of the air parcel. In an adiabatic process, compression results in heating and expansion results in cooling.

Describe friction and how it affects atmospheric processes:

Friction, the third major force affecting the wind, comes into play near the earth's surface and continues to be a factor up to altitudes of about 500 to 1000 m. This section of the atmosphere is referred to as the Planetary or Atmospheric Boundary Layer. Above this layer, friction no longer influences the wind. The Coriolis force and the pressure gradient force are in balance above the Planetary Boundary Layer. As shown in Figure 3-5, the balanced forces that occur above the layer where friction influences the wind, create a wind that will blow parallel to the isobars. This is called the High Low PGF Steep Upper air wind flow Surface wind flow geostrophic wind. In the Northern Hemisphere low pressures will be to the left of the wind. The reverse is true in the Southern Hemisphere. Within the friction layer, the Coriolis force, pressure gradient force, and friction all exert an influence on the wind. The effect of friction on the wind increases as the wind approaches the earth's surface. Also, the rougher the surface of the earth is, the greater the frictional influence will be. For example, air flow over an urban area encounters more friction than air flowing over a large body of water. Friction not only slows wind speed but also influences wind direction. Friction's effect on wind direction is due to the relationship between wind speed and the Coriolis force. Remember, the Coriolis force is proportional to wind speed. Consequently, as winds encounter more friction at progressively lower altitudes within the friction layer, wind speeds decrease and so does the Coriolis force. With friction, the Coriolis force lessens in relation to the pressure gradient force; the pressure gradient force no longer exactly balances the Coriolis force as it does with the geostrophic wind above the Planetary Boundary. Instead, the pressure gradient force predominates, turning the wind toward low pressure (see Figure 3-6). The wind direction turns toward low pressure until the resultant vector of the frictional force and the Coriolis force exactly balances the pressure gradient force. As frictional forces become greater, wind directions turn more sharply toward low pressure. This change in wind direction at different altitudes within the friction layer is depicted in Figure 3-7 and is referred to as the Ekman Spiral. The turning of the wind's direction lessens with height until friction no longer influences wind flow as in the case of the geostrophic wind. The effect that friction has on wind has a profound influence on the transport of air pollutants. As a plume of air pollutants rises from a stack it will likely rise through the boundary layer of the atmosphere where friction changes the direction of the wind with height. This will spread the plume horizontally in slightly different directions. Also, pollutants released at different heights in the atmosphere may move in slightly different directions.

Describe frontal trapping/inversions:

Frontal systems are accompanied by inversions. Inversions occur whenever warm air rises over cold air and "traps" the cold air beneath. Within these inversions there is relatively little air motion, and the air becomes relatively stagnant. This frontal trapping may occur with either warm fronts or cold fronts. Since a warm front is usually slower moving than a cold front, and since its frontal surface slopes more gradually, trapping will generally be more important with a warm front. In addition, the low level and surface wind speeds ahead of a warm front (within the trapped sector) will usually be lower than the wind speeds behind a cold front. Most warm frontal trapping will occur to the west through north from a given pollutant source, and cold frontal trapping will occur to the east through south of the source.

What is "entrainment?"

Gases that are emitted from stacks are often pushed out by fans. As the turbulent exhaust gases exit the stack they mix with ambient air. This mixing of ambient air into the plume is called entrainment. As the plume entrains air into it, the plume diameter grows as it travels downwind. These gases have momentum as they enter the atmosphere. Often these gases are heated and are warmer than the outdoor air. In these cases the emitted gases are less dense than the outside air and are therefore buoyant.

Define the term convection heating:

Heat transfer by convection occurs when matter is in motion. Air that is warmed by a heated land surface (by conduction) will rise because it is lighter than the surrounding air. This heated air rises, transferring heat vertically. Likewise, cooler air aloft will sink because it is heavier than the surrounding air. This goes hand in hand with rising air and is part of heat transfer by convection. Meteorologists also use the term advection to denote heat transfer that occurs mainly by horizontal motion rather than by vertical movement of air (convection).

Describe the Coriolis force:

If the earth did not rotate, air would move directly from high pressure toward low pressure. However, since the earth does rotate, to an observer standing on the surface of the rotating earth there is an apparent deflection of air. The Coriolis force causes this deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis force is an apparent force due to the earth rotating under the moving air. Observed from space, this movement of air (or any freely moving object for that matter) would appear to follow a straight line. But to an observer on earth this movement appears to be deflected. Imagine a spinning turntable rotating around its center axis like the earth. If you were to hold a ruler still and draw a straight line across the spinning turntable you would see a straight line from your vantage point. If the turntable were the earth, your vantage point would be space. However, the line you would draw on the turntable would actually be curved. So from the turntable's point of view, the line was deflected. This is the same thing that happens when the wind blows. This apparent force on the wind: • Increases as wind speed increases • Remains at right angles to wind direction • Increases with an increase in latitude (i.e., force is greatest at the poles and zero at the equator) The effect of this deflecting force is to make the wind seem to change direction on earth. Actually, the earth is moving with respect to the wind. Winds appear to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

What conditions form fumigating plumes?

If the plume is released just under an inversion layer, a serious air pollution situation could develop. As the ground warms in the morning, air below an inversion layer becomes unstable. When the instability reaches the level of the plume that is still trapped below the inversion layer, the pollutants can be rapidly transported down toward the ground. This is known as fumigation. Ground-level pollutant concentrations can be very high when fumigation occurs. Sufficiently tall stacks can prevent fumigation in most cases.

Describe the weather flow from 30 and 60 degree latitudes:

Instead of moving toward the equator, some surface air at the 30o latitudes moves toward the poles. The Coriolis force deflects these winds to the east in both hemispheres. These surface winds blow from west to east and are called the prevailing westerlies or westerlies in both hemispheres. Between 30 to 60o latitudes, traveling pressure systems and associated air masses (discussed later) help transport energy. Moist air from southerly regions are transported northerly. This moisture condenses, liberating energy that helps heat the air in the northerly latitudes. In the areas between the 60o latitudes and the poles, the polar easterlies prevail. This easterly wind forms a zone of cold air that blows to the southwest (Northern Hemisphere) and to the northwest (Southern Hemisphere) until it encounters the warmer westerly winds. The interface between the polar easterlies and the westerlies is the polar front that moves as these two air masses push back and forth against each other. The polar front travels from west to east helping to move cold air southward and warm, moist air northward (N. Hemisphere), thereby bringing heat energy toward the polar regions. As warm, moist air characteristics of the westerlies push up and over the cold, drier easterlies, stormy weather develops. Therefore, clouds and precipitation typically accompany the polar front.

What is a frontal inversion?

Lesson 3 mentions frontal trapping, the inversion that is usually associated with both cold and warm fronts. At the leading edge of either front, the warm air overrides the cold, so that little vertical motion occurs in the cold air layer closest to the surface. The strength of the inversion depends on the temperature difference between the two air masses. Because fronts are moving horizontally, the effects of the inversion are usually short-lived, and the lack of vertical motion is often compensated by the winds associated with the frontal passage. However, when fronts become stationary, inversion conditions may be prolonged

What is the review order for new source review?

New major stationary sources or major modifications to existing sources of air pollution are required by the Clean Air Act to obtain an air quality permit before construction is started. This process is called New Source Review (NSR), and it is required for any new major stationary source or major modification to an existing source regardless of whether or not the National Ambient Air Quality Standards (NAAQS) are exceeded. Sources located in areas which exceed the NAAQS (nonattainment areas) would undergo nonattainment New Source Review. New Source Review for major sources in areas where the NAAQS are not violated (attainment areas) would involve the preparation of a Prevention of Significant Deterioration (PSD) permit. Some sources will have the potential to emit pollutants for which their area is in attainment (or unclassifiable) as well as the potential to emit pollutants for which their area is nonattainment. When this is the case, the source's permit will contain terms and conditions to meet both the PSD and nonattainment area major NSR requirements because these requirements are pollutant specific. In most cases, any new source must obtain a nonattainment NSR permit if it will emit, or has the potential to emit, 100 tons per year or more of any regulated NSR pollutant for which that area is in nonattainment. However, the Clean Air Act has established five categories of nonattainment, from "marginal" to "extreme." In areas where air quality problems are more severe, EPA has established lower thresholds for three criteria pollutants: ozone (VOCs),1 particulate matter (PM10), and carbon monoxide. The "significance levels" are lower for modifications to existing sources. In general, a new source located in an attainment or unclassifiable area must get a PSD permit if it will emit, or has the potential to emit, 250 tons per year (tpy) or more of any criteria or NSR regulated pollutant. If the source is on EPA's list of 28 PSD source categories, a PSD permit is required if it will or may emit 100 tpy or more of any NSR regulated pollutant. The "significance levels" are lower for modifications to existing sources. In addition, PSD review would be triggered, with respect to a particular pollutant, if a new source or major modification is constructed within 10 kilometers of a Class I area (see below) and would have an impact on such area equal to or greater than 1mg/m3 , (24 hour average) for the pollutant, even though the emissions of such pollutant would not otherwise be considered "significant".

What are the two major components of the atmosphere?

Nitrogen and Oxygen, with 1% argon.

Describe differential heating:

Not only do different amounts of solar radiation reach the earth's surface, but different earth surfaces absorb heat energy at different rates. For example, land masses absorb and store heat differently than water masses. Also, different types of land surfaces vary in their ability to absorb and store heat. The color, shape, surface texture, vegetation and presence of buildings can all influence the heating and cooling of the ground. Generally, dry surfaces heat and cool faster than moist surfaces. Plowed fields, sandy beaches, and paved roads become hotter than surrounding meadows and wooded areas. During the day, the air over a plowed field is warmer than over a forest or swamp; during the night, the situation is reversed. The property of different surfaces which causes them to heat and cool at different rates is referred to as differential heating. Absorption of heat energy from the sun is confined to a shallow layer of land surface. Consequently, land surfaces heat rapidly during the day and cool quickly at night. Water surfaces, on the other hand, heat and cool more slowly than land surfaces for the following reasons: • Water movement distributes heat • The sun's rays are able to penetrate the water surface • More heat is required to change the temperature of water due to its higher specific heat. (It takes more energy to raise the temperature of water than it does to change the temperature of the same amount of soil.) • Evaporation of water occurs which is a cooling process

What conditions form lofting plumes?

Obviously a major problem for pollutant dispersion is an inversion layer, which acts as a barrier to vertical mixing. The height of a stack in relation to the height of the inversion layer may often influence ground-level pollutant concentrations during an inversion. When conditions are unstable above an inversion the release of a plume above the inversion results in effective dispersion without noticeable effects on groundlevel concentrations around the source. This condition is known as lofting.

What increased protection(s) does PSD Class I areas have?

PSD Class I areas have the most stringent PSD increments, and therefore, must be protected not only from high pollutant concentrations, but also from the additional problems pollutants in the atmosphere can cause. Under the Clean Air Act, PSD Class I areas must be evaluated for visibility impairment. This may involve a visibility impairment analysis. According to EPA regulations, visibility impairment is defined as any humanly perceptible change in visibility (visual range, contrast, or coloration) from natural conditions. Therefore, any location is susceptible to a visibility impairment due to air pollution sources. Since PSD Class I areas (national parks and wilderness areas) are known for their aesthetic quality, any change or alteration in the visibility of the area must be analyzed.

Describe the factors and types of pollution deposition:

Pollutant deposition is the process of pollutants being removed from the atmosphere and deposited onto the surface of the earth. Stack plumes contain gases and a small amount of particles that are not removed from the gas stream. When the plume emerges from the stack, these particles are carried with it. Once airborne, the particles begin to settle out and become deposited on the ground and on surface objects. There are basically two ways the particles can be deposited: dry deposition (gravitational settling) or wet deposition (precipitation scavenging). Depending on the meteorological conditions during the time of pollutant emission, these particles may: 1. Settle out quickly due to their weight and the effect of gravity; 2. Be transported further downwind of the source due to buoyancy and wind conditions; or 3. Be washed out of the atmosphere by precipitation or clouds (wet deposition). In any case, the deposition of these pollution particles is important to understand and quantify since pollutants deposited upon the ground can impact human health, vegetation, and wildlife. Pollutant deposition concentrations must be predicted in order to minimize the risk to human health. In order to quantify the amount of pollutant deposition which occurs from stack emissions, air quality models can be used. These models determine pollutant deposition based on the chemical reactivity and solubility of various gases and by using detailed data on precipitation for the areas in question.

Describe emergency planning and response for emmission sources regarding air pollution:

Regulatory agencies require sources that have the potential to release hazardous materials into the atmosphere to implement emergency planning and response procedures. These procedures are designed to enable a facility owner to take emergency action for public protection. In addition, emergency planning can enable the facility owner to provide assessments of the emergency situation based on meteorological parameters and the airborne release of the hazardous chemicals. The emergency plans and procedures include specific meteorological measurements that must be evaluated in order to anticipate the transport and dispersion of any hazardous materials that could be emitted during an emergency situation at a site. Therefore, not only is it important to know which hazardous pollutants a facility is capable of releasing, it is just as important to know the meteorological conditions that are prevalent at the site in order to predict how hazardous substances would be handled if accidentally released into the atmosphere. Continuous on-site meteorological data is an important factor in assessing the transport and dispersion of accidental releases. Information on wind speed and direction, atmospheric stability and mixing height is crucial in determining the area potentially impacted by a sudden release and initiating emergency response actions such as evacuations.

What are some important factors involving unstable conditions?

Remember that an air parcel that begins to rise will cool at the dry adiabatic lapse rate until it reaches the dew point at which point it will cool at the wet adiabatic lapse rate. This assumes that the surrounding atmosphere has a lapse rate greater than the adiabatic lapse rate (cooling at more than 9.8o C/1000 m), so that the rising parcel will continue to be warmer than the surrounding air. This is a superadiabatic lapse rate. The temperature difference between the actual environmental lapse rate and the dry adiabatic lapse rate actually increases with height, and buoyancy is enhanced. As the air rises, cooler air moves underneath. It, in turn, may be heated by the earth's surface and begin to rise. Under such conditions, vertical motion in both directions is enhanced, and considerable vertical mixing occurs. The degree of instability depends on the degree of difference between the environmental and dry adiabatic lapse rates. Unstable conditions most commonly develop on sunny days with low wind speeds where strong insolation is present. The earth rapidly absorbs heat and transfers some of it to the surface air layer. There may be one buoyant air mass if the thermal properties of the surface are uniform, or there may be numerous parcels if the thermal properties vary. The air warms, becomes less dense than the surrounding air and rises. Another condition that may lead to instability is the cyclone (low pressure system), which is characterized by rising air, clouds, and precipitation.

What is screening level modeling?

Screening level modeling is conducted before refined modeling in order to get a first glance at the type of pollutant concentrations that will occur due to a particular source. Screening level modeling consists of simple models that use relatively simple estimation techniques and assumptions. Therefore, the results are conservative indicating that if refined modeling is conducted, pollutant concentration estimates should not be higher than the concentrations computed by the screening level model. Screening level modeling is typically conducted first in order to eliminate any sources that will not contribute to or cause a potential air quality problem. Sources that do not cause any air quality problems do not need to be considered in the refined modeling analysis.

What are some factors surrounding the selection of an air quality model?

Selection of an air quality model for a particular air quality analysis is dependent on the type of pollutants being emitted, the complexity of the source, and the type of topography surrounding the facility. Some pollutants are formed by the combination of precursor pollutants. For example ground-level ozone is formed when volatile organic compounds (VOCs) and nitrogen oxides (NOx) react in the presence of sunlight. Models to predict ground-level ozone concentrations would use the emission rate of VOCs and NOx as inputs. Also, some pollutants readily react once emitted into the atmosphere. These reactions deplete the concentrations of these pollutants and may need to be accounted for in the model. Source complexity also plays a role in model selection. Some pollutants may be emitted from short stacks that are subject to aerodynamic downwash. If this is the case, a model must be used that is capable of accounting for this phenomenon. Topography plays a major role in the dispersal of plumes and their air pollutants and must be considered in the selection of an air quality model. Elevated plumes may impact areas of high terrain. Elevated terrain heights may experience higher pollutant concentrations since they are closer to the plume centerline. A model which considers terrain heights should be used when elevated terrain exists.

What is the radiational balance?

Since energy from the sun is always entering the atmosphere, the earth would overheat if all this energy were stored in the earth-atmosphere system. So, energy must eventually be released back into space. On the whole, this is what happens. Incoming radiation eventually goes back out as terrestrial radiation, and a heat balance, called the radiational balance results. For every 100 units of energy that enters the atmosphere, 51 units are absorbed by the earth, 19 units are absorbed in the atmosphere and 30 units are reflected back to space. The 70 units that are absorbed by the earth-atmosphere system (51 units +19 units) are eventually reradiated to space as long wave radiation.

The four factors that govern the amount of insolation received by the earth are:

Solar constant Atmosphere's transparency Daily sunlight duration Angle at which the sun's rays strike the earth

Describe air quality modeling analysis:

Some new sources or modifications to sources that are in attainment areas may be required to perform an air quality modeling analysis. This air quality impact analysis should determine if the source will cause a violation of the NAAQS or cause air quality deterioration that is greater than the available PSD increments. PSD requirements provide an area classification system based on land use for areas within the United States. These three areas are Class I, Class II, and Class III, and each class has an established set of increments that cannot be exceeded. Class I areas consist of national parks and wilderness areas that are only allowed a small amount of air quality deterioration. Due to the pristine nature of these areas, the most stringent limits on air pollution are enforced in the Class I areas. Class II areas consist of normal, well-managed industrial development. Moderate levels of air quality deterioration are permitted in these regions. Class III areas allow the largest amount of air quality deterioration to occur. When a PSD analysis is performed, the PSD increments set forth a maximum allowable increase in pollutant concentrations, which limit the allowable amount of air quality deterioration in an area. This in turn limits the amount of pollution that enters the atmosphere for a given region. In order to determine if a source of SO2 , for example, will cause an air quality violation, the air quality analysis uses the highest estimated concentration for annual averaging periods, and the highest, second highest estimated concentration for averaging periods of 24 hours or less. The new NAAQS for PM and Ozone contain specific procedures for determining modeled air quality violations. For reviews of new or modified sources, the air quality impact analysis should generally be limited to the area where the source's impact is "significant", as defined by regulations. In addition, due to the uncertainties in making concentration estimates for large downwind distances, the air quality impact analysis should generally be limited to a downwind distance of 50 km, unless adverse impacts in a Class 1 area may occur at greater distances.

Describe the term insolation:

The amount of incoming solar radiation received at a particular time and location in the earth-atmosphere system is called insolation. Insolation is governed by four factors: • Solar constant • Transparency of the atmosphere • Daily sunlight duration • Angle at which the sun's rays strike the earth

How does the angle of the sun's rays affect atmospheric transparency?

The angle at which the sun's rays strike the earth varies considerably as the sun "shifts" back and forth across the equator. A relatively flat surface perpendicular to an incoming vertical sun ray receives the largest amount of insolation. Therefore, areas at which the sun's rays are oblique receive less insolation because the oblique rays must pass through a thicker layer of reflecting and absorbing atmosphere and are spread over a greater surface area (Figure 2-6). This same principle also applies to the daily shift of the sun's rays. At solar noon, the intensity of insolation is greatest. In the morning and evening hours, when the sun is at a low angle, the amount of insolation is small.

How does the earth attain equilibrium with heat distribution?

The atmosphere drives warm air poleward and brings cold air toward the equator. Heat transfer from the tropics poleward takes place throughout the year, but at a much slower rate in summer than in winter. The temperature difference between low and high latitudes is considerably smaller in summer than in winter (only about half as large in the Northern Hemisphere). As would be expected, the winter hemisphere has a net energy loss and the summer hemisphere a net gain. Most of the summertime gain is stored in the surface layers of land and ocean, mainly in the ocean. The oceans also play a role in heat exchange. Warm water flows poleward along the western side of an ocean basin and cold water flows toward the equator on the eastern side. At higher latitudes, warm water moves poleward in the eastern side of the ocean basin and cold water flows toward the equator on the western side. The oceanic currents are responsible for about 40 percent of the transport of energy from the equator to the poles. The remaining 60 percent is attributed to the movement of air.

What are the four layers of our 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 associated with the atmosphere (vapor, clouds and precipitation).

What are some characteristics of the atmosphere?

The atmosphere surrounds the earth and rotates with the earth as it orbits the sun. 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 vapor. Although the water vapor 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.

What is a cold front?

The cold front is a transition zone between warm and cold air where the cold air is moving in over the area previously occupied by warm air. Cold fronts generally have slopes from 1:50 to 1:150, meaning that for every kilometer of vertical distance covered by the front, there will be 50 to 150 km of horizontal distance covered. The rise of warm air over an advancing cold front and the subsequent expansive cooling of this air lead to cloud cover and precipitation following the position of the surface front. (The surface front is the location where the advancing front touches the ground.)

How does momentum and buoyancy affect plume rise?

The condition of the atmosphere, including the winds and temperature profile along the path of the plume, will largely determine the plume's rise. Two plume characteristics influence plume rise: momentum and buoyancy. The exit velocity of the exhaust gases leaving the stack contributes to the rise of the plume in the atmosphere. This momentum carries the effluent out of the stack to a point where atmospheric conditions begin to affect the plume. Once emitted, the initial velocity of the plume is quickly reduced by entrainment as the plume acquires horizontal momentum from the wind. This causes the plume to bend over. The greater the wind speed is, the more horizontal momentum the plume acquires. Wind speed usually increases with distance above the earth's surface. As the plume continues upward the stronger winds tilt the plume even further. This process continues until the plume may appear to be horizontal to the ground. The point where the plume looks level may be a considerable distance downwind from the stack. Wind speed is important in blowing the plume over. The stronger the wind, the faster the plume will tilt over. Plume rise due to its buoyancy is a function of the temperature difference between the plume and the surrounding atmosphere. In an atmosphere that is unstable, the buoyancy of the plume increases as it rises, increasing the ultimate plume height. In an atmosphere that is stable, the buoyancy of the plume decreases as it rises. Finally, in a neutral atmosphere, the buoyancy of the plume remains constant. Buoyancy is taken out of the plume by the same mechanism that tilts the plume over⎯the wind. As shown in Figure 6-2, mixing within the plume pulls atmospheric air into the plume interior. The faster the wind speed is, the faster this mixing with outside air takes place. Entrainment of ambient air into the plume by the wind "robs" the plume of its buoyancy very quickly so that on windy days the plume does not climb very high above the stack.

What conditions form coning plumes?

The coning plume is characteristic of neutral conditions or slightly stable conditions. It is likely to occur on cloudy days or on sunny days between the breakup of a radiation inversion and the development of unstable daytime conditions.

Describe important factors regarding stability and plume behavior:

The degree of atmospheric stability and the resulting mixing height have a large effect on pollutant concentrations in the ambient air. Although the discussion of vertical mixing did not include a discussion of horizontal air movement, or wind, you should be aware that horizontal motion does occur under inversion conditions. Pollutants that cannot be dispersed upward may be dispersed horizontally by surface winds. The combination of vertical air movement and horizontal air flow influences the behavior of plumes from point sources (stacks). Lesson 6 will discuss plume dispersion in greater detail. However, this lesson will describe several kinds of plumes that are characteristic of different stability conditions.

Explain some important factors about atmospheric stability:

The degree of stability of the atmosphere is determined by the temperature difference between an air parcel and the air surrounding it. This difference can cause the parcel to move vertically (i.e., it may rise or fall). This movement is characterized by four basic conditions that describe the general stability of the atmosphere. In stable conditions, this vertical movement is discouraged, whereas in unstable conditions the air parcel tends to move upward or downward and to continue that movement. When conditions neither encourage nor discourage air movement beyond the rate of adiabatic heating or cooling, they are considered neutral. When conditions are extremely stable, cooler air near the surface is trapped by a layer of warmer air above it. This condition, called an inversion, allows virtually no vertical air motion. These conditions are directly related to pollutant concentrations in the ambient air.

How does daylight duration affect atmospheric transparency?

The duration of daylight also affects the amount of insolation received: the longer the period of sunlight, the greater the total possible insolation. Daylight duration varies with latitude and the seasons. At the equator, day and night are always equal. In the polar regions, the daylight period reaches a maximum of twenty-four hours in summer and a minimum of zero hours in winter. Figure 2-5 shows how the amount of daylight changes with the seasons at a particular location

Describe terrestrial radiation:

The earth absorbs short-wave solar radiation and emits longer wavelength terrestrial radiation. In the atmosphere, clouds, water vapor, and to a lesser extent carbon dioxide absorb terrestrial radiation, which causes the atmosphere to warm. The atmosphere absorbs much more terrestrial radiation than solar radiation. The atmosphere also radiates energy to outer space and back to the earth's surface. The earth-atmosphere system emits terrestrial radiation continuously, both day and night. The atmospheric absorption of terrestrial radiation benefits the earth-atmosphere by absorbing radiation that would otherwise be lost to space. This phenomenon explains the reason air temperatures are usually warmer on nights with cloud cover than on clear nights.

How is the earth's heat distribution important to atmospheric processes?

The earth, as a whole, experiences great contrasts in heat and cold at any particular time. Warm, tropical breezes blow at the equator while ice caps are forming in the polar regions. In fact, due to the extreme temperature differences at the equator and the poles, the earth-atmosphere system resembles a giant "heat engine." Heat engines depend on hot-cold contrasts to generate power. As you will see, this global "heat engine" influences the major atmospheric circulation patterns as warm air is transferred to cooler areas. Different parts of the earth receiving different amounts of insolation account for much of this heat imbalance. As discussed earlier, latitude, the seasons, and daylight duration cause different locations to receive varying amounts of insolation.

Describe the driving force (energy) for virtually all atmospheric processes:

The energy expended in virtually all atmospheric processes is originally derived from the sun. This energy is transferred by radiation of heat in the form of electromagnetic waves. The radiation from the sun has its peak energy transmission in the visible wavelength range [0.38 to 0.78 micrometers (µm)] of the electromagnetic spectrum (Figure 2-1). However, the sun also releases considerable energy in the ultraviolet and infrared regions. Ninety-nine percent of the sun's energy is emitted in wavelengths between 0.15 to 40 µm. Furthermore, wavelengths longer than 2.5 µm are strongly absorbed by water vapor and carbon dioxide in the atmosphere. Radiation at wavelengths less than 0.29 µm is absorbed high in the atmosphere by nitrogen and oxygen. Therefore, solar radiation striking the earth generally has a wavelength between 0.29 and 2.5 µm.

What conditions form fanning plumes?

The fanning plume occurs in stable conditions. The inversion lapse rate discourages vertical motion without prohibiting horizontal motion, and the plume may extend downwind from the source for a long distance. Fanning plumes often occur in the early morning during a radiation inversion.

What is the effective stack height?

The final height of the plume, referred to as the effective stack height (H), is the sum of the physical stack height (hs) and the plume rise (∆h). Plume rise is actually calculated as the distance to the imaginary centerline of the plume rather than to the upper or lower edge of the plume (Figure 6-1). Plume rise depends on the stack's physical characteristics and on the effluent's (stack gas) characteristics. The difference in temperature between the stack gas (Ts) and ambient air (Ta) determines the plume density which affects plume rise. Also, the velocity of the stack gases which is a function of the stack diameter and the volumetric flow rate of the exhaust gases determines the plume's momentum.

Define the term albedo:

The general reflectivity of the various surfaces of the earth is referred to as the albedo. Albedo is defined as the fraction (or percentage) of incoming solar energy that is reflected back to space. Different surfaces (water, snow, sand, etc.) have different albedo values. For the earth and atmosphere as a whole, the average albedo is 30% for average conditions of cloudiness over the earth. This reflectivity is greatest in the visible range of wavelengths.

Define the greenhouse effect:

The greenhouse effect is the descriptive name given to the result of the energy exchange process that causes the earth's surface to be warmer than it would be if the atmosphere did not radiate energy back to the earth. Gases such as carbon dioxide and methane also increase the ability of the atmosphere to absorb radiation. Some scientists believe that increased manmade emissions of these naturally occurring compounds (and other similarly behaving gases often called greenhouse gases) are heating up the earth and atmosphere more rapidly than would occur naturally. This phenomenon is often referred to as global warming. Table 2-3 lists the predominant greenhouse gases. Furthermore, some scientists predict that gradual changes in climatic conditions could occur if this purported warming trend continues. Currently, studies are being conducted to determine if manmade emissions are significantly contributing to global warming.

Describe the general behavior of pressure systems:

The horizontal movement of air is directed by many forces. Surface winds are directed in a counterclockwise fashion around low pressure systems (cyclones) in the Northern Hemisphere. This same balance of forces directs air in a clockwise fashion around high pressure systems (anticyclones) in the Northern Hemisphere. At upper levels of the atmosphere where frictional forces are removed, the air moves parallel to isobars.

Define the term lapse rate:

The lapse rate is defined as the rate at which air temperature changes with height. The actual lapse rate in the atmosphere is approximately −6 to −7o C per km (in the troposphere) but it varies widely depending on location and time of day. We define a temperature decrease with height as a negative lapse rate and a temperature increase with height as a positive lapse rate. How the atmosphere behaves when air is displaced vertically is a function of atmospheric stability. A stable atmosphere resists vertical motion; air that is displaced vertically in a stable atmosphere tends to return to its original position. This atmospheric characteristic determines the ability of the atmosphere to disperse pollutants emitted into it. To understand atmospheric stability and the role it plays in pollution dispersion, it is important to understand the mechanics of the atmosphere as they relate to vertical atmospheric motion.

What conditions form looping plumes?

The looping plume occurs in highly unstable conditions and results from turbulence caused by the rapid overturning of air. While unstable conditions are generally favorable for pollutant dispersion, momentarily high ground-level concentrations can occur if the plume loops downward to the surface.

Explain the effect of topographical features to wind/atmosphere:

The physical characteristics of the earth's surface are referred to as terrain features or topography. Topographical features not only influence the way the earth and its surrounding air heat up, but they also affect the way air flows. Terrain features, as you would expect, predominantly affect air flow relatively close to the earth's surface. These features can be grouped into four categories: flat, mountain/valley, land/water, and urban. Topographical features affect the atmosphere in two ways: thermally (through heating) and geometrically (also known as mechanically). The thermal turbulence is caused by differential heating. Different objects give off heat at different rates. For example, a grassy area will not absorb and subsequently release as much heat as an asphalt parking lot. Mechanical turbulence is caused by the wind flowing over different sizes and shapes of objects. For example, a building affects the wind flowing around it differently than a cornfield would affect it

What is a radiation inversion?

The radiation inversion is the most common form of surface inversion and occurs when the earth's surface cools rapidly. As the earth cools, so does the layer of air close to the surface. If this air cools to a temperature below that of the air above, it becomes very stable, and the layer of warmer air impedes any vertical motion. Radiation inversions usually occur in the late evening through the early morning under clear skies with calm winds, when the cooling effect is greatest. The same conditions that are conducive to nocturnal radiation inversions are also conducive to instability during the day. Diurnal cycles of daytime instability and nighttime inversions are relatively common. Therefore, the effects of radiation inversions are often short-lived. Pollutants trapped by the inversions are dispersed by vigorous vertical mixing after the inversion breaks down shortly after sunrise. In some cases, however, the daily warming that follows a nocturnal radiation inversion may not be strong enough to erode the inversion layer. For example, thick fog may accompany the inversion and reduce the effect of sunlight the next day. Under the right conditions, several days of radiation inversion, with increasing pollutant concentrations, may result. This situation is most likely to occur in an enclosed valley, where nocturnal, cool, downslope air movement can reinforce a radiation inversion and encourage fog formation. In locations where radiation inversions are common and tend to be relatively close to the surface, tall stacks that emit pollutants above the inversion layer can help reduce surface-level pollutant concentrations.

What are isobars?

The roughly concentric circles around the areas of highest and lowest pressure are called isobars, which are lines of equal pressure. Isobars may follow straight lines or form rings as they do around areas of high and low pressure. The pressure readings in the diagram (not listed here) range from 1008 to 1024 millibars (mb).

What is the definition of Meteorology?

The science 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.

What is refined level modeling?

The second level in a modeling analysis is refined modeling. This level consists of more analytical and complex calculations. More detailed information about the source, meteorological conditions, and terrain is needed as inputs into the refined models. While screening level models consider worst-case meteorological conditions and simplified terrain assumptions, the refined model incorporates more complete terrain and source information and uses actual meteorological data taken from the National Weather Service or other reliable sources that record weather information. As a result of inputting more detailed information into the model, more accurate and descriptive pollutant concentration estimates can be calculated for areas all around the source. v

Define the term solar constant:

The solar constant is the average amount of radiation received at a point, perpendicular to the sun's rays, that is located outside the earth's atmosphere at the earth's mean distance from the sun. The actual amount of solar radiation received at the outer edge of the atmosphere would vary slightly depending on the energy output of the sun and the distance of the earth relative to the sun. Due to the eccentricity of the earth's orbit around the sun, the earth is closer to the sun in January than in July. Also, the radiation emitted from the sun varies slightly, probably less than a few percent. These slight variations that affect the solar constant are trivial considering the atmospheric properties that deplete the overall amount of solar radiation reaching the earth's surface. Transparency of the atmosphere, duration of daylight, and the angle at which the sun's rays strike the earth are much more important in influencing the amount of insolation actually received, which in turn influences the weather.

What is a subsidence inversion?

The subsidence inversion is almost always associated with anticyclones (high pressure systems). Recall that air in an anticyclone descends and flows outward in a clockwise rotation. As the air descends, the higher pressure at lower altitudes compresses and warms it at the dry adiabatic lapse rate. Often this warming occurs at a rate faster than the environmental lapse rate. The inversion layer thus formed is often elevated several hundred meters above the surface during the day. At night, because of the surface air cooling, the base of a subsidence inversion often descends, perhaps to the ground. In fact, the clear, cloudless days characteristic of anticyclones encourage radiation inversions, so that there may be a surface inversion at night and an elevated inversion during the day. Although the mixing layer below the inversion may vary diurnally, it will never become very deep. Subsidence inversions, unlike radiation inversions, last a relatively long time. This is because they are associated with both the semipermanent anticyclones centered on each ocean and the slow-moving migratory anticyclones moving generally west to east in the United States. When an anticyclone stagnates, pollutants emitted into a mixing layer cannot be diluted. As a result, over a period of days, pollutant concentrations may rise. The most severe air pollution episodes in the United States have occurred either under a stagnant migratory anticyclone (for example, New York in November, 1966 and Pennsylvania in October, 1948) or under the eastern edge of the Pacific semipermanent anticyclone (Los Angeles).

What is some important information on the troposphere?

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 to be 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.

What are the two types of pollution that impair visibility?

There are generally two types of air pollution which impair visibility. The first type consists of smoke, dust, or gaseous plumes which obscure the sky or horizon and are emitted from a single source or small group of sources. The second type is a widespread area of haze that impairs visibility in every direction over a large area and originates from a multitude of sources. Regardless of the type of air pollution that impairs the visibility at a particular location, any change in the meteorology or source emissions that would increase the pollutant concentration in the atmosphere will result in increased visibility impairment

What is the definition of Air pollution meteorology?

This 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.

What is a parcel of air?

This theoretically infinitesimal parcel is a relatively well-defined body of air (a constant number of molecules) that acts as a whole. Self-contained, it does not readily mix with the surrounding air. The exchange of heat between the parcel and its surroundings is minimal, and the temperature within the parcel is generally uniform. The air inside a balloon is an analogy for an air parcel.

Describe the importance of latitude in atmospheric transparency:

Transparency is a function not only of cloudiness, but also of latitude. The sun's rays must pass through a thicker layer of reflecting-scattering atmosphere at middle and high latitudes than at tropical latitudes (Figure 2-3). This effect varies with the seasons, being greatest in winter (in the northern hemisphere) when the earth's axis is tilted away from the sun causing the sun's rays to be low on the horizon

Describe the term transparency:

Transparency of the atmosphere does have an important bearing upon the amount of insolation that reaches the earth's surface. The emitted radiation is depleted as its passes through the atmosphere. Different atmospheric constituents absorb or reflect energy in different ways and in varying amounts. Transparency of the atmosphere refers to how much radiation penetrates the atmosphere and reaches the earth's surface without being depleted. Some of the radiation received by the atmosphere is reflected from the tops of clouds and from the earth's surface and some is absorbed by molecules and clouds.

Describe vapor plume induced icing:

Vapor plumes are emitted from cooling towers and stacks and consist mainly of water vapor. Although pollutant concentrations are not a major concern with vapor plumes, other problems arise when vapor plume sources are located close to frequently travelled roads and populated areas. Vapor emitted from a stack is warm and moist. When meteorological conditions are favorable, the moisture in the vapor plume condenses out and settles on cooler objects (e.g. road surfaces). This phenomena is similar to the moisture that collects on the sides of a glass of water on a warm day. If temperatures are at or below freezing when the moisture condenses, road surfaces can freeze rapidly creating hazardous driving conditions. In addition, light winds can cause the plume to remain stagnant creating a form of ground fog that can cause low visibilities as well. Water vapor plumes that lower visibility can create hazards for aircraft, especially during critical phases of flight including landings and takeoffs.

What is a warm front?

Warm fronts, on the other hand, separate advancing warm air from retreating cold air and have slopes on the order of 1:100 to 1:300 due to the effects of friction on the trailing edge of the front. Precipitation is commonly found in advance of a warm front.

What atmospheric constituent absorbs more radiation than any other?

Water Vapor.

Why is water vapor important in regards to radiation?

Water vapor, although comprising only about 3% of the atmosphere, on the average absorbs about six times as much solar radiation as all other gases combined. The amount of radiation received at the earth's surface is therefore considerably less than that received outside the atmosphere as represented by the solar constant.

What are some important factors involving stable conditions?

When the environmental lapse rate is less than the adiabatic lapse rate (cools at less than 9.8o C/1000 m), the air is stable and resists vertical motion. This is a subadiabatic lapse rate. Air that is lifted vertically will remain cooler, and therefore more dense than the surrounding air. Once the lifting force is removed, the air that has been lifted will return to its original position. Stable conditions occur at night when there is little or no wind.

What are some important factors involving neutral conditions?

When the environmental lapse rate is the same as the dry adiabatic lapse rate, the atmosphere is in a state of neutral stability. Vertical air movement is neither encouraged nor hindered. The neutral condition is important as the dividing line between stable and unstable conditions. Neutral stability occurs on windy days or when there is cloud cover such that strong heating or cooling of the earth's surface is not occurring.

Describe pressure gradient forces:

Wind is caused by nature's attempt to correct differences in air pressure. Wind will flow from areas of high pressure to low pressure. The pressure equalizing force that attempts to move air from high pressure to low pressure is called the pressure gradient force. The pressure gradient is the rate and direction of pressure change. It is represented by a line drawn at right angles to the isobars as shown in Figure 3-4. Gradients are steep where isobars are closely spaced. The wind will move faster across steep gradients. Winds are weaker where the isobars are farther apart because the slope between them is not as steep; therefore, wind does not build up as much force. wind moves from areas of high to low pressure, but because of the Coriolis force (effect of the earth's rotation), wind does not flow parallel to the pressure gradient. Also, notice that wind direction at the surface (solid lines) differs from wind direction high above the earth (dotted lines) despite the same pressure gradient forces operating. This is due to frictional forces as explained in the next section.

Describe important aspects of wind:

Wind is the basic element in the general circulation of the atmosphere. Wind movements from small gusts to large air masses all contribute to transport of heat and other conditions of the atmosphere around the earth. Winds are always named by the direction from which they blow. Thus a "north wind" is a wind blowing from the north toward the south and a "westerly wind" blows from west to east. When wind blows more frequently from one direction than from any other, the direction is termed the prevailing wind. Wind speed increases rapidly with height above the ground level, as frictional drag decreases. Wind is commonly not a steady current but is made up of a succession of gusts, slightly variable in direction, separated by lulls. Close to the earth, wind gustiness is caused by irregularities of the surface, which create eddies. Eddies are variations from the main current of wind flow. Larger irregularities are caused by convection⎯or vertical transport of heat. These and other forms of turbulence contribute to the movement of heat, moisture, and dust into the air aloft.

What document promotes consistency among modelers so that all air quality modeling activities would be based on the same procedures and recommendations?

air quality modeling is necessary to ensure that a source is in compliance with the SIP and New Source Review requirements. When air quality modeling is required, the selection of a model is dependent on the source characteristics, pollutants emitted, terrain, and meteorological parameters. The EPA has compiled the Guideline on Air Quality Modeling, (40 CFR 51 Appendix W) which summarizes the available models, techniques, and guidance in conducting air quality modeling analyses used in regulatory programs. This document was written to promote consistency among modelers so that all air quality modeling activities would be based on the same procedures and recommendations.

What is used to determine averaging times for ambient pollution concentrations?

emission limits should be based on ambient pollutant concentration estimates for the averaging time that results in the most stringent control requirements. In all cases these concentration estimates are assumed to be the sum of the pollutant concentrations contributed by the source and an appropriate background concentration. An air quality model is used to determine which averaging time (e.g. annual, 24-hour, 8-hour, 3-hour, 1-hour) results in the highest ambient impact. For example, if the annual average air quality standard is approached by a greater degree (percentage) than standards for other averaging times, the annual average is considered the restrictive standard. In this case, the sum of the highest estimated annual average concentration and the annual average background concentration provides the concentration which should be used to specify emission limits. However, if a short-term standard is approached by a greater degree and is thus identified as the restrictive standard, other considerations are required because the frequency of occurrence must also be taken into account.

What is the definition of "mixing height:"

the point at which the air parcel cooling at the dry adiabatic lapse rate intersects the ambient temperature profile "line" is known as the mixing height. This is the air parcel's maximum level of ascendance. In cases where no intersection occurs (when the environmental lapse rate is consistently greater than the adiabatic lapse rate), the mixing height may extend to great heights in the atmosphere. The air below the mixing height is the mixing layer. The deeper the mixing layer, the greater the volume of air into which pollutants can be dispersed.