Pollutant Removal Processes

Pollutants are released in various forms and from various sources (sulfur dioxide from factory stacks, carbon monoxide from automobile exhausts, etc.). These pollutants are mixed into the air and transported down wind. Many of these pollutants do not remain in the atmosphere but are removed by natural processes that occur within the planetary boundary layer. We call these processes removal mechanisms, and the duration a pollutant resides or is suspended in the atmosphere is referred to as its residence time. There are basically three removal mechanisms that act on airborne pollutants:

  1. wet deposition
  2. dry deposition
  3. chemical reactions

You should spend most of the time with this topic reading and using the pollutant dispersion calculator. This calculator presents a sample of how we might calculate the dispersion of a pollutant downwind from a point source, such as stack of some height emitting pollutants at some rate. The reading included in this calculator is longer than normal, and the calculator is more complicated than most of the calculators in this course.

Quick Quiz: What happens to the downwind concentration of a pollutant as one goes from 1 km to 10 km in downwind distance from the point source? Once you have collected some data, you might want to consider using the plotting program to visualize the behavior of your data. Use these values:

  1. the source emission rate (Q, grams per second): 300
  2. the height of the stack (zs, in meters): 200
  3. the height of the mixed layer (zi, in meters): 400
  4. the wind speed (M, in meters per second): 5
  5. the Deardorff velocity (w*, in meters per second): 1
  6. the crosswind-integrated concentration (cy, in grams per meter-squared): .05
  7. the distance downwind (x, in meters) of interest. For example, you might want to know the concentration of the pollutant at 1000 meters, 1500 m, etc.

the concentration decreases sharply, then levels out
the concentration increases sharply, then levels out
the concentration decreases exponentially
the concentration increases exponentially

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