Presentation on theme: "AIR POLLUTION AND METEOROLOGY"— Presentation transcript:
1AIR POLLUTION AND METEOROLOGY Dr.K. Subramaniam,Senior Lecturer (Environmental Health and Safety )ATTENTION/MOTIVATION: The following are two old weather proverbs:The smoke from chimneys straight ascends,Then spreading back to earth it bends.When the ditch and pond offend the nose,Then look for rain and stormy blows.Meteorology is the study and forecasting of weather changes resulting from large-scale atmospheric circulation. Meteorological principles and a knowledge of circulation patterns are major factors in effective air pollution control. Since we have no control over the weather, emission rates must be controlled so that pollution problems are less likely to develop during adverse weather periods.
2METEOROLOGY OF AIR POLLUTION Transport and dispersionRemoval mechanisms
3Important Aspects of Air Pollution Meteorology Atmospheric TurbulenceScales of Atmospheric/Turbulent MotionPlume BehaviorPlanetary Boundary Layer (PBL)Effects on DispersionApplicationsOVERVIEW: This lecture is the start for the following air pollution lectures since it’s the basis for the meteorological effects of pollutants in the atmosphere. The following lectures pertain to air quality compliance, sources & effects of air pollution, air emissions inventory, Clean Air Act monitoring requirements, and air pollution control technologies. During this lecture, we will discuss the following:Atmospheric PollutantsTurbulence/Atmospheric StabilityPlanetary Boundary LayerPlumes and Dispersion BehaviorCirculation Patterns
4Meteorological Parameters that Influence Air Pollution TurbulenceWind Speed and DirectionTemperatureStabilityMixing Height
5Atmospheric Turbulence Responsible for dispersion/transport of pollutantsRefers to the apparently chaotic nature of fluid motions (in this case, atmospheric motions)Irregular, almost random fluctuations of such parameters as:velocitytemperaturescalar concentrations (pollutants)Atmospheric turbulence is responsible for dispersion and transport of pollutants in a chaotic motion. This chaotic motion affects parameters such as velocity, temperature, and pollutant concentrations.
6Atmospheric Turbulence Sources Mechanical ForcingBuoyant or Thermal Forcing
7Atmospheric Turbulence (Sources) Mechanical Forcing:Air flowing over irregular surfaceChange in horizontal wind speed with heightFactors Influencing Mechanical Forcing:Speed of local windsRoughness of terrain over which wind is blowingTwo major sources of atmospheric turbulence are Mechanical Forcing and Buoyant Forcing. Mechanical Forcing involves air flowing over an irregular surface causing changes in horizontal wind speed with respect to height. Factors affecting this type of forcing are the speed of the local winds and the roughness of the terrain in which the wind is blowing over.
10Adiabatic Lapse RateIt is the temperature profile of what would happen to a parcel of air that is raised or lowered vertically, and allowed to cool or heat from expansion or contraction with no exchange of energy or heat.
11Atmospheric Turbulence (Sources) Buoyant Forcing (Thermal):Air rises or sinks based on temperature; heated air becomes less dense & rises on its own; cooled air becomes more dense & sinksFactors Affecting Buoyant Forcing“Stability” of the atmosphereVertical temperature profile of the atmosphereLapse Rate; specifically the Dry Adiabatic Lapse Rate which is:1oC/100m = 10oC/km = 5.4oF/1000 ftNext is Buoyant Forcing, in which air rises or sinks depending on the air temperature. Heated air becomes less dense and rises on its own. Cooled air becomes more dense and sinks. However, what happens when no heat is added or extracted to a parcel of air? When a parcel of air rises, it expands, and the temperature decreases. Likewise, when air sinks, it compresses, and the temperature increases. This phenomenon occurs without adding or withdrawing energy from the parcel. When a parcel of air, either dry or containing water vapor, rises or sinks without the addition or extraction of heat, that process is said to be a dry adiabatic process. The dry Adiabatic Lapse Rate is 1oC/100m = 10oC/km = 5.4oF/1000 ft. This means that a parcel of dry air lifted 100 meters will cool 1 degree Celsius.The stability of the atmosphere is basically determined by comparing the lapse rate of a parcel air (dry adiabatic process) to the lapse rate of the surrounding air (environment). If the lapse rate of the surrounding air is greater than the lapse rate of the parcel of air, then the stability condition is unstable. If the lapse rate of the surrounding air is equal to the lapse rate of the parcel of air, then the stability condition is neutral. Finally, if the lapse rate of the surrounding air is less than the lapse rate of the parcel of air, then the stability condition is stable. Let’s look at some examples.
12Atmospheric Turbulence (Buoyant Forcing) Cooler AirWarmer AirDRY ADIABATIC PROCESSGroundWhen a parcel of air rises, it expands, and the temperature decreases. When air sinks, it compresses, and the temperature increases. When this process occurs with out the addition or withdrawal of heat from the parcel, this process is called dry adiabatic.
13Atmospheric Turbulence (Buoyant Forcing) Cooler AirWarmer AirUnstable Conditions - Turbulence is producedGroundDisplaced warmer airwill now rise on its own(Thermals; Thunderstorm updrafts)When a parcel of air is lifted and continues to rise after the lifting force disappears, the atmosphere is unstable. In an unstable layer, the lapse rate of a rising parcel is less than the lapse rate of the environment. Though the parcel cools as it rises, its temperature remains warmer than the surrounding air during its ascent through an unstable layer. Because the parcel is warmer than the environment, the parcel has positive buoyancy and continues to rise on its own.
14Atmospheric Turbulence (Buoyant Forcing) Warmer AirCooler AirStable Conditions - Turbulence is suppressedGroundDisplaced cooler airwill sink back to starting pointIn a stable atmosphere, if you lift a parcel of air, the temperature of rising air will decrease fast enough that its temperature will be colder than the temperature of the environment. If the force pushing the parcel disappears, the parcel would sink back down to its original position. Even though a parcel of air is being forced up, it has negative buoyancy meaning it wants to sink to its original position where it was in equilibrium with the environment.
15Atmospheric Turbulence (Buoyant Forcing) Neutral Atmospheric ConditionsEnvironmentAir ParcelGroundIn a neutral atmosphere, the two lapse rates are equal. If a parcel of air is lifted through a neutral layer, the temperature and pressure of the parcel will be identical to the temperature and pressure of the surrounding air at every height and is always in equilibrium with the environment. Thus, the parcel is not buoyant.
16Planetary Boundary Layer (PBL) Top of the atmospheric boundary layer can be defined as the lowest level in the atmosphere at which the ground surface no longer influences the meteorological parameters through turbulence transfer of massDuring day this corresponds to Mixing height (up to 3 km in height)Processes include:Roughness of terrainObstructed flowHeat and energy transferPBL is that layer of air (up to 3 km in height) which is influenced by friction from the earth's surface and the processes that occur there such as: roughness of terrain, obstructed flow, and heat and energy transfer.
17The effect of boundary layer stability on plume behavior In a well-mixed turbulent boundary layer on a hot day (forced by buoyancy), the turbulent eddies may be large and intense enough to advert the whole plume down to the ground. This can result in extremely high plume concentrations in the vicinity of the source.
18The effect of boundary layer stability on plume behavior This is the kind of form assumed for a Gaussian plume, when the boundary layer is well-mixed and turbulent eddies are smaller than the plume scale. The plume forms a cone downstream.
19The effect of boundary layer stability on plume behavior In a stable boundary layer, the plume spreads out horizontally at its level of neutral buoyancy. Vertical motion is weak, so there is little upward spread, but the plume forms a `fan' when viewed from above. The plume is not well-mixed in the vertical, which implies relatively slow dilution, but there are not likely to be high plume concentrations at the ground. Unfortunately, this kind of plume may be the precursor to a `fumigation' event if the inversion is subsequently mixed to ground level.
20The effect of boundary layer stability on plume behavior At early evening, if a surface inversion is developing, vertical motion may be inhibited below the plume while remaining active above: the plume is diluted but does not reach the ground. This is a favorable situation.
21The effect of boundary layer stability on plume behavior There is a strong inversion restricting mixing above, and the plume is mixed throughout the boundary layer. This can occur quite rapidly. For example, after sunrise when the nocturnal inversion is being eroded from below by buoyant eddies, plume-level air of high concentration may be brought down to the surface over a wide area.
22Effects of PBL Height on Stack Pollutant Dispersion PBL below stack top: little or no concentration of pollutants at the surfaceHorizontal WindsPBL TopPBLIn this example, horizontal winds are prevalent in the free atmosphere. There’s little energy dissipation due to the lack of friction and turbulence only occurs in convective clouds and near jet streams. As a result, there really is no concentration of pollutants at the earth’s surface where we would be most affected.
23Effects of PBL Height on Stack Pollutant Dispersion PBL TopBuoyantTurbulencePBLIn this example, turbulence occurs due to the warmer air being more buoyant that cooler air, allowing it to rise and mix with the surrounding air. In this example, there would be an increase in concentration of pollutants below the PBL line, but not much at the surface.PBL well above stack top: decreased concentrations of pollutants at the surface
24Effects of PBL Height on Stack Pollutant Dispersion PBL just above stack top: increased concentrations of pollutants at the surfacePBL TopPBLBuoyantTurbulenceThis example clearly demonstrates the increased concentration of pollutants at the surface due to the PBL just above the stack. Turbulence increases dramatically as well as the increase in energy dissipation due to friction with the earth’s surface. You can now see the direct impact the PBL has on pollutant concentrations based on the height of the PBL.
25Temperature Profile in Atmosphere 1. INVERSIONS2. ATMOSPHERIC STABILITY
26Effects of Stability on Stack Pollutant Dispersion Unstable Conditions: leads to greater dispersion of pollutantsPBL TopPBLEnvironmental lapse rateDry adiabatic lapse rateheightTemptLet’s discuss stability again, but with respect to stack pollutant dispersion. The stability of the atmosphere often dictates the behavior of a plume in terms of the height it will rise and to what degree it will mix into the environment. For example, a plume released in an unstable atmosphere will display a looping pattern. This pattern occurs when updrafts from warming air at the surface carry a segment of the plume upward while compensating downdrafts force the adjacent section downward.
27Effects of Stability on Stack Pollutant Dispersion Stable conditions: lead to less dispersion of pollutantsPBL TopPBLA stable atmosphere, commonly marked by an inversion on clear nights, yields a fanning pattern. A plume released into a stable atmosphere will not rise or mix unless it encounters turbulence.Dry adiabaticlapse rateEnvironmental lapse rateHeightTemp
28Effects of Stability (Ground Source Pollutant Dispersion) BuoyantTurbulenceHere’s another example of a Looping pattern, but the source is at the surface.XXXUnstable Conditions: Lead to lower concentration ofpollutants at surface
29Effects of Stability (Ground Source Pollutant Dispersion) Stable Conditions: Leads to greater concentration ofpollutants at surfaceAnother example of a Fanning pattern at the surface. Due to this pattern, it will yield greater concentrations at the surface than the Looping pattern.XXX
30WIND SPEED AND DIRECTION Mesoscale circulationLarge scale circulation
31Mesoscale Circulations Affecting Dispersion Land-Sea Breeze: Daytime (Sea Breeze)Upper Level Return FlowAir Warmed over Land Expands(Becomes Less Dense)Air Cooled over Water Contracts(Becomes More Dense)Sea Breeze (arises due to density differences)Remember that Mesoscale phenomena have an approximate size between 1km and 100 km and a time span between a few minutes to a day. This scale includes weather such as sea breezes and mountain- valley breezes. Air flow in this scale is influenced by differential heating or cooling over an area and by differences in terrain which can accentuate this process. The sea and land breeze circulations are good examples of thermal circulations. The sea-breeze forms during the day, the land breeze at night. The sea breeze forms due to the differential heating of the air over a region where a large body of water and land meet. Let's look at a situation that might occur at the beach. Land surfaces generally absorb and emit radiation more efficiently than water surfaces. Therefore, the air above the land will heat and expand more rapidly than the air over the water. A relative high pressure area will form aloft over the land, and a relative low pressure area will form over the water. The pressure gradient will force air movement toward the low pressure area, and a thermally driven circulation will form. The breeze that forms along the surface blowing in from the water (called an onshore breeze) brings in cool air. This is why, during mid-day at the beach, the air is often much cooler than it is just a few miles inland. This air is also very moist, and can form clouds and even storms where it rises over the low pressure area. This separation zone between the warm inland air and the cool sea breeze air is known as a sea breeze front. This front can bring significant precipitation under the right circumstances. For example, in Florida, sea breeze fronts move in from the west and from the east, then converge over central Florida. This convergence pushes more air upward, producing strong thunderstorms. At night, however, the opposite effect takes place, called the land breeze. Because the land also cools more rapidly than the water, a similar situation occurs late at night, but in the opposite direction. Here, the cooled air over the land contracts, forming a low pressure aloft over land and a relative high pressure aloft over the water. The resulting circulation is known as a land breeze, because the wind now blows offshore.Warmer LandCooler WaterReverses at Night as Water Remains Warmer than Land to Make Land Breeze
32Mesoscale Circulations Affecting Dispersion 1. Mountain/Valley WindsDay:Night:WarmMtnCoolMtnA similar pattern occurs along mountain slopes. The valley air warms rapidly and rises along the mountain slope to form the valley breeze. At night, the surface air quickly cools and slides down the mountain slope, forming the mountain breeze. Although this is due to the terrain of the mountains, it is, technically speaking, thermally driven. These, like the sea and lake breezes, are examples of mesoscale thermal circulations.2. Urban/Heat Island (Night)PBL TopCITY
33Large Scale Circulation Transboundary air pollutionAcid depositionOzone transport
34Applications of Air Pollution Meteorology Atmospheric Dispersion ModelingStudy of Accidental Release of Hazardous Substances Including Radioactive NuclidesApplications of air quality meteorology can be used for dispersion modeling, i.e., predicting the path of the pollutant concentration and for calculations of ground sources, such as hazardous waste spills.Let’s first look at dispersion modeling.Applications of air quality meteorology can be used for dispersion modeling, ie, predicting the path of the pollutant concentration and for calculations of ground sources, such as hazardous waste spills. Let’s first look at dispersion modeling.
35Air Pollution Meteorology Meteorology very important factor in developing strategies for air pollution controlState of the lower troposphere (PBL) plays large role in dispersion of pollutants and plumes:Mechanical TurbulenceBuoyant TurbulenceCirculationConclusion/Remotivation: Okay, we have gone over the basis for air pollution by discussing some of the general concepts of air quality meteorology. We have discussed the effects of atmospheric conditions such as turbulence, planetary boundary layer, stability, and circulation and their effects on pollutant concentrations. In addition, we discussed the applications of air quality meteorology as it relates to BEEs. Again, this is the first lecture of air pollution and the rest of the lectures will involve emissions inventories, measurements, and air pollution control. If you would like further information on this subject, go to the following website:
36Wind Speed and Direction The average ground level wind speed is about 4.5 m/s.“Calm” wind is less than 0.5m/sWind speed almost always increases with height.ground friction slows lower level winds
39Wind Speed With Height Deacon’s power law: u2 / u1 = (z2 / z1)p where: u1 is the wind speed at elevation z1u2 is the wind speed at elevation z2and p is an exponent that depends on stability and ground characteristicsNote: Wind speed measured by the NWS is usually obtained at z = 10 meters (z1)
44Temperature GradientAir temperature is not uniform with altitude at a given location.Reasons:heating by the groundheating by the suncloud coverevaporative cooling over the oceansexpansion of gases due to air movement
46Stability and Lapse Rate The lapse rate determines how readily parcels of air move upward or downward.In stable atmospheres = vertical movement is opposed by the temperature gradientIn unstable atmospheres = vertical movement is enhancedIn neutral atmospheres = neither
47Stability Classes A = very unstable B = moderately unstable C = slightly unstableD = neutralE = slightly stableF = stable
48Why is stability important? Stability affects plume rise.Plume rise can be calculated using information about the stack gases and meteorology.Stability can effect the dispersion and appearance of plumes being emitted from stacks.
49InversionsAn inversion is a situation of increasing temperature with height.Pre-dawn mornings have an inversion that reached up to about 1000 ft (100m).Atmospheres within an inversion are extremely stable, with damped vertical mixing.
50Surface Temperature Inversions: Are very commonAre easy to recognizeAffect the dispersal of very small spray droplets suspended in the airDo not increase the amount of off-site movementCan increase the potential for offsite affects & the distance at which affects can be observed
51Atmospheric Stability Indicator of atmospheric turbulenceDepends on static stability, thermal and mechanical turbulenceUnstable : Lapse rate > dry adiabatic lapse rateNeutral : Lapse rate = dry adiabatic lapse rateStable : Lapse rate < dry adiabatic lapse rateTurner method: solar angle, cloud cover and wind speed
52IMPORTANCE OF METEOROLOGY DispersionTransportWind speed and directionTemperatureStabilityMixing height