AIR POLLUTION AND METEOROLOGY

1. AIR POLLUTION AND METEOROLOGY Dr.K. Subramaniam, Senior Lecturer (Environmental Health and Safety )

2. METEOROLOGY OF AIR POLLUTION • Transport and dispersion • Removal mechanisms

3. Important Aspects of Air Pollution Meteorology • Atmospheric Turbulence • Scales of Atmospheric/Turbulent Motion • Plume Behavior • Planetary Boundary Layer (PBL) • Effects on Dispersion • Applications

4. Meteorological Parameters that Influence Air Pollution • Turbulence • Wind Speed and Direction • Temperature • Stability • Mixing Height

5. 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: • velocity • temperature • scalar concentrations (pollutants)

6. Atmospheric Turbulence Sources • Mechanical Forcing • Buoyant or Thermal Forcing

7. Atmospheric Turbulence (Sources) • Mechanical Forcing: • Air flowing over irregular surface • Change in horizontal wind speed with height • Factors Influencing Mechanical Forcing: • Speed of local winds • Roughness of terrain over which wind is blowing

10. Adiabatic Lapse Rate • It is the temperatureprofile 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.

11. 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: 1oC/100m = 10oC/km = 5.4oF/1000 ft

12. DRY ADIABATIC PROCESS Cooler Air Cooler Air Warmer Air Ground Atmospheric Turbulence (Buoyant Forcing)

13. Unstable Conditions - Turbulence is produced Cooler Air Warmer Air Cooler Air Displaced warmer air will now rise on its own (Thermals; Thunderstorm updrafts) Ground Atmospheric Turbulence (Buoyant Forcing)

14. Stable Conditions - Turbulence is suppressed Warmer Air Warmer Air Cooler Air Displaced cooler air will sink back to starting point Ground Atmospheric Turbulence (Buoyant Forcing)

15. Neutral Atmospheric Conditions Environment Environment Air Parcel Ground Atmospheric Turbulence (Buoyant Forcing)

16. 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: • Roughness of terrain • Obstructed flow • Heat and energy transfer

17. 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.

18. 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.

19. 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.

20. 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.

21. 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.

22. Effects of PBL Heighton Stack Pollutant Dispersion PBL below stack top: little or no concentration of pollutants at the surface Horizontal Winds PBL Top PBL

23. Effects of PBL Height on Stack Pollutant Dispersion PBL Top Buoyant Turbulence PBL PBL well above stack top: decreased concentrations of pollutants at the surface

24. 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

25. Temperature Profile in Atmosphere 1. INVERSIONS 2. ATMOSPHERIC STABILITY

26. Effects of Stability on Stack Pollutant Dispersion Unstable Conditions: leads to greater dispersion of pollutants PBL Top PBL

27. Effects of Stability on Stack Pollutant Dispersion Stable conditions: lead to less dispersion of pollutants PBL Top PBL

28. Effects of Stability(Ground Source Pollutant Dispersion) Buoyant Turbulence XXX Unstable Conditions: Lead to lower concentration of pollutants at surface

29. Effects of Stability(Ground Source Pollutant Dispersion) Stable Conditions: Leads to greater concentration of pollutants at surface XXX

30. WIND SPEED AND DIRECTION • Mesoscale circulation • Large scale circulation

31. Mesoscale Circulations Affecting Dispersion Land-Sea Breeze: Daytime (Sea Breeze) Upper Level Return Flow Air Warmed over Land Expands (Becomes Less Dense) Air Cooled over Water Contracts (Becomes More Dense) Sea Breeze (arises due to density differences) Cooler Water Warmer Land Reverses at Night as Water Remains Warmer than Land to Make Land Breeze

32. Mesoscale Circulations Affecting Dispersion 1. Mountain/Valley Winds Day: Night: Warm Mtn Cool Mtn 2. Urban/Heat Island (Night) PBL Top CITY

33. Large Scale Circulation • Transboundary air pollution • Acid deposition • Ozone transport

34. 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. • Let’s first look at dispersion modeling.

35. 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

36. 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

37. A Wind Rose

38. A Wind Rose

39. Wind Speed With Height • Deacon’s power law: u2 / u1 = (z2 / z1)p where: u1 is the wind speed at elevation z1 u2 is the wind speed at elevation z2 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 (z1)

41. Impact of Fixed Geographic Features • TERRAIN EFFECTS • Sea breeze • Valley wind • Drainage wind • Flow patterns due to topographical features

44. TemperatureGradient • Air temperature is not uniform with altitude at a given location. • Reasons: • heating by the ground • heating by the sun • cloud cover • evaporative cooling over the oceans • expansion of gases due to air movement

46. 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

47. Stability Classes A = very unstable B = moderately unstable C = slightly unstable D = neutral E = slightly stable F = stable

48. 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.

49. 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.

50. Surface Temperature Inversions: • Are very common • Are easy to recognize • Affect the dispersal of very small spray droplets suspended in the air • Do not increase the amount of off-site movement • Can increase the potential for offsite affects & the distance at which affects can be observed