Poor Air Quality Can be Caused by Many Things

Air quality where you live can vary depending on how much air pollution is emitted in your community, how much pollution is carried into your community on the wind, and by weather conditions.

Ground level ozone forms when two key pollutants, nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight. These pollutants are precursors to ozone formation, meaning they must be present in the air for ozone to form. Small particles can be directly emitted (like smoke from a woodstove), but a lot of particles form when gases react in the air. NOx and sulfur dioxides (SO2) contribute to particle formation.

These ozone and particle-forming pollutants come from a wide variety of sources, including mobile sources like vehicles, area sources such as woodstoves or burn piles and point or industrial sources. Natural sources contribute, too: wildfires and volcanoes contribute to particle pollution, while trees and other vegetation can contribute both to particle and ozone pollution.

Weather plays a big role in the levels of ozone and particle pollution in your community. Sunlight and heat, for example, promote ozone formation. Light winds and temperature inversions can keep pollution from dispersing. Depending on its direction, the wind can bring in more pollution – sometimes from hundreds of kilometres away. Geography can affect pollution levels too; mountain ranges can trap air masses and prevent pollution from dispersing.

Weather affects your air quality

High Pressure – Responsible for Pretty Days – but Sometimes Polluted Ones, Too

• High pressure: affects a number of weather conditions, often contributing to an increase in the concentration of air pollutants. Pollution concentrations tend to increase on the tail end of high pressure systems, when the centre of the high has passed by. Temperatures and humidity increase, so the air tends to be more polluted.

• Temperature: In general, higher temperatures promote chemical reactions. This applies to particle pollution as well as ozone. On hot humid summer days, poor visibility is the result of small particles in the atmosphere.

• Wind speed: As pressure builds over an area, winds become lighter. Light winds- or absence of wind- allow pollutants that create ozone and particle pollution to build up, and provide a more favorable environment for the chemical reactions necessary to create ozone and particle pollution to take place.

• Relative humidity: This term refers to the amount of moisture in the air. Moisture helps clouds form by causing air to rise and cool. When air is dry, it does not move as much, and pollutants build up. For example, on days when ozone is high, the relative humidity is often very low. Humidity adds water to the atmosphere, and this moisture is absorbed by particles, causing them to swell and impair visibility even more. Therefore, poor visibility on humid days is the result of particle pollution and moisture interactions.

• Sinking air: Air sinks in a high pressure system, which prevents it from cooling and forming clouds. This sinking creates days with abundant sunshine — a key ingredient needed to start the chemical reactions that form ozone.

• Inversions: Sometimes a layer of cooler air is trapped near the ground by a layer of warmer air above. This is called an inversion, and can last all day, or even for several days. When the air cannot rise, pollution at the surface also is trapped and can accumulate, leading to higher concentrations of ozone and particle pollution. There are different conditions that cause inversions to form. The most common is a nighttime inversion, when clear skies allow air at the surface to cool faster than the air above

Wind Direction – Responsible for Transport and Travel of Pollutants. Large weather systems dictate the predominant wind direction, which can have a considerable effect on the quality of air in a specific city or region.

• Upwind Sources: Air quality can worsen in your community if the wind is blowing from a region that contains numerous sources of pollution. If the winds are coming from areas with little or no pollution, they can make your air quality better. Very light winds or no wind, such as those in a strong high pressure system, can be a problem for urban areas, because all the pollution that a city creates stays in one place.

• Recirculation: The clockwise rotation of winds in a high pressure system can create a phenomenon called recirculation. When this occurs, a dirty parcel of air in one location may circle back around to where it started — after accumulating even more pollution. Sea breezes circulations can also cause pollutants to recirculate over a particular area. In coastal areas, the wind often blows from land toward the sea during the nighttime hours, pushing pollutants offshore. In the daytime, the wind shifts and begins blowing pollutants back onshore. Recirculation often plays a role in the worst air quality episodes.
Source: Air Quality Awareness 101

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