UNIT II - meteorology, occurrence, process

UNIT II - meteorology, occurrence, process

1. Unit II - Meteorology

2. Meteorology Effects of meteorology on Air Pollution - Fundamentals , Atmospheric stability, Inversion, Wind profiles and stack plume patterns- Atmospheric Diffusion Theories – Dispersion models, Plume rise .

3. What is Meteorology Meteorology is a branch of science which includes atmospheric chemistry and atmospheric physics with a major focus on weather forecasting. Meteorology is the scientific study of the atmosphere that focuses on weather processes and forecasting.

4. Basic Meteorology Pollutants circulate the same way the air in the troposphere circulates. Air movement is caused by solar radiation and the irregular shape of the earth and its surface, which causes unequal absorption of heat by the earth's surface and atmosphere. This differential heating and unequal absorption creates a dynamic system. The dynamic thermal system of the earth's atmosphere also yields differences in barometric pressure, associated with low-pressure systems with both hot and cold weather fronts. Air movement around low-pressure fronts in the Northern Hemisphere is counterclockwise and vertical winds are upward, where condensation and precipitation take place.

5. Basic Meteorology High-pressure systems bring sunny and calm weather – stable atmospheric conditions - with winds (in the Northern Hemisphere) spiraling clockwise and downward. Low- and high-pressure systems, commonly called cyclones and anticyclones Anticyclones are weather patterns of high stability, in which dispersion of pollutants is poor The high-pressure area indicates a region of stable air, where pollutants build up and do not disperse.

6. Basic Meteorology

7. Meteorological data Identifies the sources of pollutants Predicts air pollution events Simulates air quality using computer models

8. Meteorological Parameters Primary Parameters Wind direction and speed Temperature Atmospheric stability Mixing height Secondary Parameters Humidity Precipitation Solar Radiation Visibility

9. Meteorological Parameters Wind direction and speed Dispersion of pollutant is based on the prevailing wind speed and wind direction. Higher speed – carries the pollutants to longer distances Low speed – dilution of pollutant is less Concentration of pollutant is inversely proportional to the wind speed

10. Temperature Temperature is a physical property of atmosphere that quantitatively expresses hot and cold . It is the manifestation of thermal energy. Temperature is measured using mercury Thermometer which works on principles of thermal expansion M-Thermometers are useful up to 350° C

11. Mixing Height “Mixing Height” or “Mixing Depth” signifies the height above the surface throughout which a pollutant such as smoke can be dispersed. During times of surface temperature inversions (typically nighttime with clear skies), the mixing height goes to zero and smoke dispersion is minimal.

12. Stability The lapse rate is the rate at which an atmospheric variable, normally temperature falls with altitude. The Environmental lapse rate is the rate of change of the temperature of the environment (atmosphere) with changing altitude or The lapse rate of non raising air

13. Stability The Adiabatic lapse rate is the rate at which the temperature of an air parcel changes in response to the compression or expansion associated with elevation change The actual stability of an air parcel is determined by the orientation of the environmental lapse rate in comparison with either the dry or moist adiabatic lapse rates.

14. Stability Stability is associated with how air parcels behave once they are displaced vertically from their initial positions . Three types 1. Stable or Positively stable 2. Neutral or Neutrally stable 3. Unstable or Negatively stable

15. Stable or Positively stable Stable or Positive stability implies that a displaced air parcel will return to its initial position; associated with high pollution Environmental lapse rate < adiabatic lapse rate

16. Neutral or Neutrally stable Neutral stability implies that a displaced air parcel will remain at its new position; associated with moderate pollution Environmental lapse rate = adiabatic lapse rate

17. Unstable or Negatively stable Negative stability, or instability, means that a displaced air parcel will continue to accelerate away from its rest position; associated with low pollution Environmental lapse rate > adiabatic lapse rate

18. Stability Paper Mill plume after Sunrise and at dawn

19. Temperature Inversion Temperature inversion is the deviation from the normal change of an atmospheric property with altitude. Temperature inversion , also called thermal inversion , a reversal of the normal behaviour of temperature in the troposphere, in which a layer of cool air at the surface is overlain by a layer of warmer air.

20. Atmospheric stability The atmosphere is neutral when air only moves based on adiabatic heating or cooling When conditions are extremely stable, cooler air near the surface is trapped by a layer of warmer air above it = inversion – no vertical air motion

21. Inversions, Stability, and Mixing

22. Temperature Inversions Inversions are important because they suppress vertical dispersion of pollution and often trap pollution near the surface where we live.

23. Temperature Inversions

24. Temperature Inversions Subsidence –Created by sinking air associated with ridges – Can limit daytime mixing depth and plays important role in daytime pollutant concentrations Nocturnal or radiation – Created by cooling ground at night –Strongest with clear skies, light winds, and long nights – Can trap emissions, released during the overnight hours, close to the ground (e.g., wood smoke) Advection – Created when warm air aloft moves over cooler air below – Can occur ahead of an approaching cold front – Can cause poor air quality, despite the lack of an aloft ridge

25. Dispersion = Advection (Transport) + Dilution (Diffusion) Fick’s law of diffusion J= - D * D C/Dx Where, J= Mass Flux; D = Diffusivity coefficient,; D C/Dx = Concentration gradient Diffusion of pollutants occur due to turbulence, which further depends upon many factors: Ambient temperature Temperature of emissions Roughness factors Wind velocity Wind direction Humidity Stability Source Receptor Transport Re-entrainment

26. Air Pollutant Cycle

27. Dispersion General mean air motion Turbulent velocity fluctuations Diffusion due to concentration gradients – from plumes Aerodynamic characteristics of pollution Particles Size Shape Weight

28. Turbulence Not always completely understood Two types: Atmospheric heating Causes natural convection currents --- discussed Thermal eddies Mechanical turbulence Results from shear wind effects Result from air movement over the earth’s surface, influenced by location of buildings and relative roughness of terrain.

29. Lapse Rate Important characteristic of atmosphere is ability to resist vertical motion: stability Affects ability to disperse pollutants When small volume of air is displaced upward Encounters lower pressure Expands to lower temperature Assume no heat transfers to surrounding atmosphere Called adiabatic expansion

30. Adiabatic Expansion To determine the change in temp. w/ elevation due to adiabatic expansion Atmosphere considered a stationary column of air in a gravitational field Gas is a dry ideal gas Ignoring friction and inertial effects ( dT/dz)adiabatic perfect gas = - (g M/ C p ) T = temperature z = vertical distance g = acceleration due to gravity M = molecular weight of air Cp = heat capacity of the gas at constant pressure

31. Adiabatic Expansion ( dT/dz)adiabatic perfect gas = -0.0098°C/m or ( dT/dz)adiabatic perfect gas = -5.4°F/ft Change in Temp. with change in height

32. Lapse rate Lapse rate is the negative of temperature gradient Dry adiabatic lapse rate = Metric: Γ = - 1°C/100m or SI: Γ = - 5.4°F/1000ft

33. Conti…. Important is ability to resist vertical motion: stability Comparison of Γ to actual environment lapse rate indicates stability of atmosphere Degree of stability is a measure of the ability of the atmosphere to disperse pollutants

34. Atmospheric Stability Affects dispersion of pollutants Temperature/elevation relationship principal determinant of atmospheric stability Stable Little vertical mixing Pollutants emitted near surface tend to stay there Environmental lapse rate is same as the dry adiabatic lapse rate 4 common scenarios

35. Stability Classes Developed for use in dispersion models Stability classified into 6 classes (A – F) A: strongly unstable B: moderately unstable C: slightly unstable D: neutral E: slightly stable F: moderately stable

36. Vertical Temperature Profiles Environmental lapse rate (ELR) Dry adiabatic lapse rate (DALR) If, ELR > DALR =sub adiabatic condition, atmosphere is stable . ELR >> DALR = Inversion conditions. Very stable atmosphere. ELR= DALR = atmosphere is neutral . ELR< DALR = super adiabatic condition, atmosphere is unstable . Shapes of plumes depends upon atmospheric stability conditions.

37. Mixing Height of atmosphere The height of the base of the inversion layer from ground surface .

38. General Characteristics of Stack Plumes Dispersion of pollutants Wind – carries pollution downstream from source Atmospheric turbulence -- causes pollutants to fluctuate from mainstream in vertical and crosswind directions Mechanical & atmospheric heating both present at same time but in varying ratios Affect plume dispersion differently

39. Plume Types Plume types are important because they help us understand under what conditions there will be higher concentrations of contaminants at ground level.

40. Looping Plume High degree of convective turbulence Superadiabatic lapse rate -- strong instabilities Associated with clear daytime conditions accompanied by strong solar heating & light winds High probability of high concentrations sporadically at ground level close to stack. Occurs in unstable atmospheric conditions.

41. Coning Plume Stable with small-scale turbulence Associated with overcast moderate to strong winds Roughly 10° cone Pollutants travel fairly long distances before reaching ground level in significant amounts Occurs in neutral atmospheric conditions

42. Fanning Plume Occurs under large negative lapse rate Strong inversion at a considerable distance above the stack Extremely stable atmosphere Little turbulence If plume density is similar to air, travels downwind at approximately same elevation

43. Lofting Plume Favorable in the sense that fewer impacts at ground level. Pollutants go up into environment. They are created when atmospheric conditions are unstable above the plume and stable below.

44. Fumigation Most dangerous plume: contaminants are all coming down to ground level. They are created when atmospheric conditions are stable above the plume and unstable below. This happens most often after the daylight sun has warmed the atmosphere, which turns a night time fanning plume into fumigation for about a half an hour.