Presentation on theme: "AIR POLLUTION AND METEOROLOGY Dr.K. Subramaniam, Senior Lecturer (Environmental Health and Safety )"— Presentation transcript:
AIR POLLUTION AND METEOROLOGY Dr.K. Subramaniam, Senior Lecturer (Environmental Health and Safety )
METEOROLOGY OF AIR POLLUTION Transport and dispersion Removal mechanisms
Important Aspects of Air Pollution Meteorology Atmospheric Turbulence Scales of Atmospheric/Turbulent Motion Plume Behavior Planetary Boundary Layer (PBL) Effects on Dispersion Applications
Meteorological Parameters that Influence Air Pollution Turbulence Wind Speed and Direction Temperature Stability Mixing Height
Atmospheric Turbulence Responsible for dispersion/transport of pollutants Refers to the apparently chaotic nature of fluid motions (in this case, atmospheric motions) Irregular, almost random fluctuations of such parameters as: i.velocity ii.temperature iii.scalar concentrations (pollutants)
Atmospheric Turbulence Sources Mechanical Forcing Buoyant or Thermal Forcing
Atmospheric Turbulence (Sources) Mechanical Forcing: i.Air flowing over irregular surface ii.Change in horizontal wind speed with height Factors Influencing Mechanical Forcing: a)Speed of local winds b)Roughness of terrain over which wind is blowing
Adiabatic Lapse Rate It 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.
Atmospheric 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 & sinks Factors Affecting Buoyant Forcing –Stability of the atmosphere –Vertical temperature profile of the atmosphere –Lapse Rate; specifically the Dry Adiabatic Lapse Rate which is: 1 o C/100m = 10 o C/km = 5.4 o F/1000 ft
Cooler Air Warmer Air DRY ADIABATIC PROCESS Ground Atmospheric Turbulence (Buoyant Forcing)
Cooler Air Warmer Air Unstable Conditions - Turbulence is produced Ground Displaced warmer air will now rise on its own (Thermals; Thunderstorm updrafts) Atmospheric Turbulence (Buoyant Forcing)
Warmer Air Cooler Air Stable Conditions - Turbulence is suppressed Ground Displaced cooler air will sink back to starting point
Planetary 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 mass During day this corresponds to Mixing height (up to 3 km in height) Processes include: i.Roughness of terrain ii.Obstructed flow iii.Heat and energy transfer
The 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.
The 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.
The 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.
The 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.
The 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.
PBL below stack top: little or no concentration of pollutants at the surface PBL Top Horizontal Winds PBL Effects of PBL Height on Stack Pollutant Dispersion
PBL well above stack top: decreased concentrations of pollutants at the surface PBL Top Buoyant Turbulence PBL Effects of PBL Height on Stack Pollutant Dispersion
PBL just above stack top: increased concentrations of pollutants at the surface PBL Top Buoyant Turbulence PBL Effects of PBL Height on Stack Pollutant Dispersion
Temperature Profile in Atmosphere 1.INVERSIONS 2.ATMOSPHERIC STABILITY
Unstable Conditions: leads to greater dispersion of pollutants PBL Top PBL Effects of Stability on Stack Pollutant Dispersion
Stable conditions: lead to less dispersion of pollutants PBL Top PBL Effects of Stability on Stack Pollutant Dispersion
Unstable Conditions: Lead to lower concentration of pollutants at surface XXX Buoyant Turbulence Effects of Stability (Ground Source Pollutant Dispersion)
Stable Conditions: Leads to greater concentration of pollutants at surface XXX Effects of Stability (Ground Source Pollutant Dispersion)
WIND SPEED AND DIRECTION Mesoscale circulation Large scale circulation
Air Cooled over Water Contracts (Becomes More Dense) Air Warmed over Land Expands (Becomes Less Dense) Land-Sea Breeze: Daytime (Sea Breeze) Cooler Water Warmer Land Reverses at Night as Water Remains Warmer than Land to Make Land Breeze Sea Breeze (arises due to density differences) Upper Level Return Flow Mesoscale Circulations Affecting Dispersion
1. Mountain/Valley Winds Day:Night: Warm Mtn Cool Mtn 2. Urban/Heat Island (Night) CITY PBL Top Mesoscale Circulations Affecting Dispersion
Large Scale Circulation Transboundary air pollution Acid deposition Ozone transport
Applications of Air Pollution Meteorology Atmospheric Dispersion Modeling Study of Accidental Release of Hazardous Substances Including Radioactive Nuclides Applications 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. Lets first look at dispersion modeling.
Air Pollution Meteorology Meteorology very important factor in developing strategies for air pollution control State of the lower troposphere (PBL) plays large role in dispersion of pollutants and plumes: –Mechanical Turbulence –Buoyant Turbulence –Circulation
Wind Speed and Direction The average ground level wind speed is about 4.5 m/s. –Calm wind is less than 0.5m/s Wind speed almost always increases with height. –ground friction slows lower level winds
A Wind Rose
Wind Speed With Height Deacons power law: u 2 / u 1 = (z 2 / z 1 ) p where: u 1 is the wind speed at elevation z 1 u 2 is the wind speed at elevation z 2 and p is an exponent that depends on stability and ground characteristics Note: Wind speed measured by the NWS is usually obtained at z = 10 meters (z 1 )
Impact of Fixed Geographic Features TERRAIN EFFECTS Sea breeze Valley wind Drainage wind Flow patterns due to topographical features
Temperature Gradient Air temperature is not uniform with altitude at a given location. Reasons: a)heating by the ground b)heating by the sun c)cloud cover d)evaporative cooling over the oceans e)expansion of gases due to air movement
Stability 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 gradient In unstable atmospheres = vertical movement is enhanced In neutral atmospheres = neither
Stability Classes A = very unstable B = moderately unstable C = slightly unstable D = neutral E = slightly stable F = stable
Why 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.
Inversions An 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.
Surface Temperature Inversions: a)Are very common b)Are easy to recognize c)Affect the dispersal of very small spray droplets suspended in the air d)Do not increase the amount of off-site movement e)Can increase the potential for offsite affects & the distance at which affects can be observed
Atmospheric Stability i.Indicator of atmospheric turbulence ii.Depends on static stability, thermal and mechanical turbulence iii.Unstable : Lapse rate > dry adiabatic lapse rate iv.Neutral : Lapse rate = dry adiabatic lapse rate v.Stable : Lapse rate < dry adiabatic lapse rate vi.Turner method: solar angle, cloud cover and wind speed
IMPORTANCE OF METEOROLOGY Dispersion Transport Wind speed and direction Temperature Stability Mixing height