Air Pollution - Maximum Mixing Depth and Ventilation Coefficient

IBD/Lecture Notes

Maximum Mixing Depth The amount of air available to dilute pollutants is related to the wind speed and to the extent to which emissions can rise into the atmosphere. The maximum mixing depth (sometimes called the mixing height) is obtained by projecting the dry adiabatic lapse rate line to the point of intersection with the atmospheric temperature profile.

Neutral

Height

Stable Maximum mixing depth Unstable

Temperatur e

Tmax

The product of the maximum mixing depth and the average wind speed within the mixing depth is sometimes used as an indicator of the atmosphere’s dispersive capability. This product is known as the ventilation coefficient (m2/s). Values of ventilation coefficient less than about 6000 m2/s are considered indicative of high air pollution potential (Portelli and Lewis, 1987).

Sample Problems

EX1. Suppose the following atmospheric altitude versus temperature data have been collected. Altitude,

0

100

200

300

400

500

600

IBD/Lecture Notes

m Temp. OC

20

18

16

15

16

17

18

a) What would be the mixing depth? b) How high would you expect a plume to rise if it is emitted at 21 OC

from a 100-m stack if it rises at the dry adiabatic lapse rate? Would you expect the plume to be looping, coning, fanning, or fumigating?

EX2. For the temperature profile given in the previous problem, if the maximum daytime surface temperature is 22 OC, and a weather station anemometer at 10 m height shows winds averaging 4 m/s, what would be the ventilation coefficient? Assume stability class C and use the wind at the height halfway to the maximum mixing depth.

IBD/Lecture Notes

eq .