Plume characteristics
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  • Why plume expands: concentration and temperature gradients The history of plume depends on: Physical/chemical nature of pollutants (e.g. particle size, reaction rate in the atmosphere (NO2)); Meteorological factors (rainy vs clear day, windy vs calm day); Location of source relative to potential obstructions (i.e. dispersion and dilution); Topography (flat plain vs valley or mountain or buildings)
  • Higher Effective Stack Height: taller stack, lower wind speed, higher stack gas temperature and exit velocity
  • Unstable. Calm wind. During daytime. Good for dispersing pollutants.
  • Stability class: Neutral to isothermal lapse rates Good for dispersing pollutants. Cloudy or windy days or at night Little vertical mixing
  • Stability class: Formed under ground- based inversion conditions (stable) Top view: Expands horizontally but not vertically Low ground level concentration.
  • Inversion layer above During the day after sunrise Ground level concentration is zero for some distance, then peaks and then decreases. Good because that’s when ground level concentration is higher.
  • Ground level inversion layer. Late afternoon or early evening under clear skies (if the stack is tall). Transition to fanning. Ground level concentration: 0.
  • Wake is smaller for round than for square. Turbulent mixing at unstable condition can destroy the wake.
  • Conclusion: The higher the stack height, the better the dispersion. Stack downwind better than upwind of building. Generally, the stack should be twice high of the building.
  • Stack downwash: stack exit velocity is low compared to the wind velocity.
  • In a valley, the streamline remains straight, but lateral diffusion is suppressed in the vicinity of the valley walls. In the beginning at point (a), the plume follows the regular pattern. When the plume reaches the wall,. It becomes confined. So, the concentration along the walls increases very fast, and the available valley volume is filled with stack effluent.
  • Acid deposition Elevated rural O3 levels Arctic Haze Movement of long-lived pollutants Why only long-lived? Ans: short-lived are consumed/reacted.

Transcript

  • 1. Dispersion from Point Sources • Pollutants emitted in plume form Why does plume expand downwind? What are the factors that influence the history of plume? www.epa.gov/.../muncpl/landfill/s w_combst.htm Impact on air quality depends on dispersion, which depends on the height of plume03/30/13 Aerosol & Particulate Research Lab 1
  • 2. • Plume rise affects transport – Effects maximum ground level concentrations (MGLCs) – Effects distance of MGLCs www.atmos.ucla.edu/.../ch imneyplumes/Note03.html Under what conditions can we have a higher Effective Stack Height?03/30/13 Aerosol & Particulate Research Lab 2
  • 3. Stack Plume: Looping Strong turbulence http://www.uwm.edu/~kahl/CoVis/Plume/stable.gif Q: Is it at stable or unstable condition? High or low wind speed? Does it happen during the day or night? Is it good for dispersing pollutants?03/30/13 Aerosol & Particulate Research Lab http://www.med.usf.edu/~npoor/3 3
  • 4. 03/30/13 Aerosol & Particulate Research Lab 4
  • 5. Stack Plume: Coning Strong wind, no turbulence What is the stability class? Good vertical mixing? On sunny or cloudy days? Good for dispersing pollutants? What is the ground level concentration as a function of distance from the stack?03/30/13 Aerosol & Particulate Research Lab 5
  • 6. 03/30/13 Aerosol & Particulate Research Lab 6
  • 7. Stack Plume: Fanning http://www.med.usf.edu/~npoor/4 Q: What is the stability class? What is the top view of the plume? What is the ground level concentration as a function of distance from the stack?03/30/13 Aerosol & Particulate Research Lab 7
  • 8. 03/30/13 Aerosol & Particulate Research Lab 8
  • 9. Stack Plume: Fumigation Q: Why can’t the pollutants be dispersed upward? Does it happen during the day or night? What’s your opinion about requiring power plants to reduce their power output from 3 am to 3 hours after sunrise? What is the ground level concentration as a function of distance from the stack?03/30/13 Aerosol & Particulate Research Lab 9
  • 10. 03/30/13 Aerosol & Particulate Research Lab 10
  • 11. Stack Plume: Lofting Q: Why can’t the pollutants be dispersed downward? When does it happen? Lofting is a transition stage. To which type of plume? What is the ground level concentration as a function of distance from the stack?03/30/13 Aerosol & Particulate Research Lab 11
  • 12. 03/30/13 Aerosol & Particulate Research Lab 12
  • 13. Stack Plume: Trapping Q: What is the stability class? What is the ground level concentration as a function of distance from the stack? http://www.uwm.edu/~kahl/CoVis/Plume/03/30/13 Aerosol & Particulate Research Lab 13
  • 14. Effect of Surface Discontinuity (a) (b) Warm land Cold water (c) Warm land Cold water Cold land Warm water Q: What is the ground level concentration as a function of distance from the stack?03/30/13 Aerosol & Particulate Research Lab 14
  • 15. 03/30/13 Aerosol & Particulate Research Lab 15
  • 16. Effects of Terrain on the Plume Pattern Q: Does the shape of the building matter? Is the wake bigger at stable or unstable condition?03/30/13 Aerosol & Particulate Research Lab 16
  • 17. Impact of Building and Stack Location Backwash Downwash Q: Is stack height an important parameter?03/30/13 Aerosol & Particulate Research Lab 17
  • 18. Impact of Stack Height: Stack Upwind of Building03/30/13 Aerosol & Particulate Research Lab 18
  • 19. Impact of Stack Height: Building Supported Stack03/30/13 Aerosol & Particulate Research Lab 19
  • 20. Impact of Stack Height: Stack Downwind of Building Conclusions?03/30/13 Aerosol & Particulate Research Lab 20
  • 21. Cases of Downwash03/30/13 Aerosol & Particulate Research Lab 21
  • 22. Plume Affected by Natural Terrain Irregularity03/30/13 Aerosol & Particulate Research Lab 22
  • 23. Plume Near Very Large Obstacle Unstable Stable03/30/13 Aerosol & Particulate Research Lab 23
  • 24. Plume in a Valley03/30/13 Aerosol & Particulate Research Lab 24
  • 25. Heat Island Effect Q: When does the effect break?03/30/13 Aerosol & Particulate Research Lab 25
  • 26. Plume Affected by Heat Island Effect Toward a city Within a city03/30/13 Aerosol & Particulate Research Lab 26
  • 27. Long-Range Transport • Transport of pollutants hundreds/thousands of miles; resulting in air quality problems far away from the source Examples? Planetary Transport • Stable air above PBL retards vertical mixing • Transport out of PBL to free troposphere takes few hours to few days • Transport to top of troposphere with uniform mixing takes about a week (for long-lived pollutants, e.g. CO2, CH4, CFC) Why only long-lived?03/30/13 Aerosol & Particulate Research Lab 27
  • 28. Summary Take 2 minutes to summarize here what you have learned from this section03/30/13 Aerosol & Particulate Research Lab 28