Where does Pollution Go?

Where Does Pollution Go?

Weather is an important part of air pollution picture. It affects the movement and build-up of pollutants in our atmosphere. Wind can carry emissions large distances. Thermal inversions trap pollution near the ground. Sunlight drives photochemical reactions, generating smog. Rain and snow wash pollution out of the air. Weather is a complex system affecting the quality of the air we breathe.

There are four factors affecting the pollution levels in a given area. The sources, air buoyancy, wind and dispersion all influence pollutant concentrations.


Both mobile and station sources emit primary pollutants directly into the atmosphere. These include: oxides of nitrogen (NOx), volatile organic compounds (VOCs), sulphur dioxide and particulate matter. Secondary pollutants, such as ozone and sulphates are generated when the primary pollutants react in the atmosphere.

See: A.C. Stern et al.; Fundamentals of Air Pollution, 2nd Edition.
Academic Press, Orlando, FL, 1984.

Air Buoyancy

Smoke rises. When pollutants are emitted from smokestacks, tailpipes etc., they are warmer than the surrounding air and tend to rise. The hotter the emissions, the faster they will float up into the atmosphere. This follows the same principle as a hot air balloon., Air buoyancy is an important aspect of the dispersion of pollutants through the air.


Wind carries many pollutants long distances from their sources., For example, the precursors for ground-level ozone, NOx and VOCs, are stable in the atmosphere and can generate ozone in rural areas far from where they originated. Depending on wind patterns, pollution can accumulate in remote areas.


Air pollution reaches the ground by mixing, or by wet or dry deposition. Dry deposition is the settling out of dust particles from the atmosphere due to the force of gravity. Larger particles settle out faster, while the smaller, more dangerous particles remain suspended longer. Wet deposition occurs when pollution dissolves in rain or snow. Precipitation cleans most of the pollution from the air.


Thermal inversions occur when the air in the upper atmosphere is warmer than the air below it. This limits the upward movement of pollution because the warmer air acts as a barrier. If there is no dispersing wind, pollution can accumulate to dangerous levels.

Thermal Inverstions Lead to High Air Pollution Levels, Weather Patterns Affect Pollutant Transport

Did you know?

Acid rain is the wet deposition of sulphates and nitrates, more than half of which come from the US.

Prevailing winds from the south carry NOx and VOCs from the US, which produce half of the ozone found in Ontario, and probably half of the sulphate of PM10 in southern Ontario comes from the US as well.

When the Air Quality Index reaches or is expected to exceed 50, a Smog Advisory is issued.

What can you do?

•Pollution levels may be high during calm weather conditions.

•Listen for Smog Advisories.

•Limit outdoor activities if an advisory is issued.

•Contact your local MP and ask about the progress of the Canada-US Agreement.

Get Involved

•Start or join a citizen’s group.

•Write letters to the Minister of the Environment and to your local MPP.

•Register complaints about air quality.