Factors Affecting Air Quality
The amount and kind of pollutants that are released into the air play a
major role in determining the degree of air pollution in a specific area. However, other factors are involved, mainly:
- topography (terrain), such as mountains and valleys;
- weather, such as wind, temperature, air turbulence, air pressure, rainfall and cloud cover; and
- the physical and chemical properties of pollutants.
The following information is from A Primer on Air Quality in British Columbia (Environment Canada, Ministry of Environment and other partners):
Poor air quality can result from a combination of factors. Regional air quality is affected by how air behaves as a result of the interaction of topography and weather, and by the emission
The Red-Dye Example: How Air Behaves
Let’s consider an analogy using water to help explain how air and air emissions behave. Imagine pouring red dye into the ocean. You’ll see a red pool of water
for a few seconds, but it quickly disappears as the waves mix the dye into the huge mass of water. It is a combination of the amount of water, and the vigorous mixing that makes the dye disappear.
Now imagine pouring that same amount of red dye into a bathtub. The bath water will turn very red because there is far less water than in the ocean, and there is no mixing with a source of clean water
to dilute the dye.
Just as red dye disappears in the ocean, air pollutants mix and disperse quickly in a large airshed because the air flow is not limited by topography but can travel and mix over great distances. This
results in good air quality in the airshed.
Sometimes, however, topography and weather combine to prevent pollutants from mixing and dispersing. In this case the pollutants become trapped within the area, like the red dye in the bathtub that
continually builds up with nowhere to go. This results in poor air quality in the airshed.
Weather, Wind and Geography
Once pollutants are emitted into the air, the weather largely determines how well they disperse. Turbulence mixes pollutants into the surrounding air. For example, during a hot summer day, the air near
the surface can be much warmer than the air above. Sometimes large volumes of this warm air will rise to great heights. This results in vigorous mixing.
Wind speed also contributes to how quickly pollutants
are carried away from their original source. However, strong winds don't always disperse the pollutants. They can transport pollutants to a larger area, such as the smoke from open burning or forest
Sometimes the condition of the atmosphere is very still (stable) and there is very little mixing. This occurs when the air near the surface of the earth is cooler than the air above (a temperature
This cooler air is heavier and will not want to move up to mix with the warmer air above. Any pollutants released near the surface will get trapped and build up in the cooler layer of air near the surface.
Temperature inversions are very common in B.C., especially in mountain valleys, often forming during calm clear nights with light winds. They can even persist throughout the day during the winter.
A Temperature Inversion in a Valley
The arrows symbolize the colder air being
pushed down to the surface by warmer air.
This inversion and the valley walls trap pollution.
(From A Teacher's Guide to Clean Air, Ministry of Environment)
Pollutants are released into the air from natural and human sources, from point and nonpoint sources. The number and size of emission sources in each area, along with weather conditions and topography,
will determine the level of pollutants in the air within an airshed.
Many pollutants undergo chemical reactions when they encounter other pollutants in the air. The products of these chemical reactions are called secondary pollutants, as opposed to primary pollutants
that are emitted directly into the atmosphere. Ground-level ozone is an example of a secondary pollutant that forms when nitrogen dioxide (NO2
) and volatile organic compounds (VOCs) mix in the presence