Smog
About smog
The term ‘smog’ describes poor air quality that is often associated with reduced visibility. Smog is a mixture of air pollutants, mainly ground-level ozone and particulate matter.
Other air pollutants that contribute to smog include:
- carbon monoxide (CO)
- sulphur dioxide (SO2)
- nitrogen dioxide (NO2)
- gaseous ammonia (NH3)
- volatile organic compounds (VOCs)
Smog can be caused by a combination of:
- air pollutants that are emitted and those formed through reactions and other processes in the air. Key sources of air pollutant emissions are:
- industrial activities
- electricity generation from coal and natural gas
- residential wood burning
- wildfires
- agricultural activities
- transportation powered by fossil fuels
- seasonal conditions
Smog affects our health, and each year is linked to:
- thousands of premature deaths
- millions of asthma and acute respiratory symptom days
- increased hospitalizations and doctor visits
- hundreds of thousands of lost days at work and school
Learn more about the effects of smog on health.
Pollutants in smog also contribute to climate change and biodiversity loss, and have other environmental impacts such as:
- acidification
- water pollution
- damage to crops and vegetation
- poor visibility and haze
- damage to buildings and other structures
Learn more about the effects of smog on the environment.
Seasonal smog
Smog problems occur throughout the year and across the country. The factors that lead to smog vary by season and by region due to:
- sources of pollution
- the way smog is formed in the air
- differences in atmospheric conditions
- topography
In the summer, smog is typically associated with increased sunlight and warmer temperatures. Summer conditions encourage the production of ground-level ozone and lead to a greater frequency of air stagnation.
Summer smog is primarily composed of:
- ground-level ozone (ozone)
- nitrogen oxides (NOx)
- volatile organic compounds (VOCs)
Summer smog is affected by:
- clear skies
- light winds and stagnant air
- periods of intense sunlight
In the winter, smog occurs when pollutants build up near the ground – a metrological term called temperature inversion. Winter smog is usually caused by increased wood burning for heating and vehicle usage in the winter months.
Winter smog is primarily composed of particulate matter but also includes:
- nitrogen oxides (NOx)
- carbon monoxide (CO)
- volatile organic compounds (VOCs)
Winter conditions can trap pollutants closer to the ground. This trapping effect may last several days. It can lead to the accumulation of pollutants, especially in areas with significant industrial activity or reliance on residential wood heating systems.
Weather and smog
The severity of summer and winter smog is influenced by how much the pollutants spread out, or disperse, in the atmosphere. The less the pollutants spread out, the higher the level of pollutants in smog. How much the pollutants spread in the atmosphere mainly depends on:
- wind speed
- where they are in the atmosphere
Wind speed pushes and spreads the pollutants horizontally. No wind means the air doesn’t move, or stagnates, which allows pollutants to build up in the air. This effect is more common in the summer.
The maximum height the pollutants can reach if they spread vertically is referred to as the mixing height.
In normal situations, the mixing height is enough to spread the pollutants high into the atmosphere. The pollutants are carried up by the layer of warm rising air to the colder air higher up (diagram 1).
Long description
Pollutants spread upwards from the ground during normal situations. They are carried up from the layer of hot air to the layer of cool air and finally spreading out through the layer of cold air well above ground level. This helps prevent smog from forming at ground level.
In the case of temperature inversion, the pollutants are trapped at ground level where they may cause the most harm. An inversion occurs when cold air becomes trapped under a layer of warm air that acts as a lid. This might happen ahead of a warm front or in a broad surface ridge or in the late evening. The pollutants in the cooler layer can’t spread and they stay concentrated at ground level (diagram 2). This effect is more common in the winter.
Long description
Pollutants are trapped at ground level when cold air is trapped under a layer of warmer air. This prevents the pollutants from spreading out causing smog to form.
Topography
Topography also affects pollutant levels. The physical ‘walls’ of a valley, for example, restrict air movement in the valley. High levels of pollutants in the air can’t sufficiently spread out and smog will form. Smog also forms in places with stretches of rolling terrain where air pollutants can get trapped in the terrain's pockets.
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Topography
Topography also affects pollutant levels. The physical ‘walls’ of a valley, for example, restrict air movement in the valley. High levels of pollutants in the air can’t sufficiently spread out and smog will form. Smog also forms in places with stretches of rolling terrain where air pollutants can get trapped in the terrain's pockets.
Related links
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