Central Campus Air Quality Model (CCAQM) Instructions

Mixing Height and Inversions

The mixing height is the height of vertical mixing of air and suspended particles above the ground. This height is determined by the observation of the atmospheric temperature profile.  A parcel of air rising from the surface of the Earth will rise at a given rate (called the dry-adiabatic lapse rate). As long as the parcel of air is warmer than the ambient temperature, it will continue to rise. However, once it becomes colder than the temperature of the environment, it will slow down and eventually stop. It is at this junction where the temperature of the parcel crosses the curve denoting the vertical environmental temperature profile determines the mixing height. 

Inversions are a result of the vertical temperature profile of air.  Temperature normally decreases as altitude increases in the troposphere (at an average rate of 1�C per 100 meters).  However, an increase of temperature as altitude increases can occur and is called an inversion.  Thus, the colder air layer is below the warmer air, resulting in a stable temperature profile that restricts vertical mixing.   Because of the restricted mixing volumes of air due to the inversion, pollution becomes stagnant and does not dissipate.  

Los Angeles is a prime example of a location that is prone to inversions. Thus, in this location, pollution does not dissipate, causing large amounts of smog and air pollution.    

Inversions affect plume rise.  On the left there is no inversion present, while the inversion is above the stack in the figure on the right