You may or may not have noticed that air pollution is worse during the winter months. Most people are surprised by this fact, as they expect air pollution to be worse during the summer months. What might explain this peculiar phenomenon? There are actually several reasons to consider when discussing air quality and cold weather.
The Relationship Between Cold Air and Air Pollutants
Air pollutants and cold air have a unique relationship. Before we explain the relationship between the two, let’s take a look at the characteristics of cold air. First and foremost, unlike warm air which rises to the sky, cold air sinks to the ground. Cold air is dense because its molecules are close together, absorb less energy and thus move slower. When the temperature drops, cold air acts as a blanket, covering the ground. Air pollutants cannot disperse into the air as they would when the weather is warm. Essentially, air pollutants are trapped beneath the layer of cold air. And, because cold air moves slowly, it does not help whisk away air pollutants. All of this is to say that air pollution during winter is worse because air pollutants have nowhere to go.
Temperature inversion, also known as thermal inversion, occurs when the atmosphere’s temperature behaves in a manner that opposes its usual pattern. In other words, a layer of cold air is covered by a layer of warm air. There are three types of temperature inversion: surface inversion, subsidence inversion and frontal inversion.
Surface inversion occurs when air closest to the earth’s surface cools faster than the air above it. This is a result of the earth’s surface losing heat quickly due to cold weather and little to no wind.
Subsidence inversion occurs when a widespread layer of air moves toward the earth’s surface and becomes colder than the air above it. Subsidence inversion is often seen near high-pressure centers.
Frontal inversion occurs when cold air and hot air meet at a weather front, and the cold air moves toward the earth’s surface, pushing the hot air above it.
What does this have to do with air quality? As previously mentioned, cold air covers the ground like a blanket when the temperature drops, trapping air pollutants. These three types of temperature inversion explain exactly how this phenomenon occurs.
Wintertime Energy Use
Energy use increases during winter in order to create enough heat to combat the cold weather. For example, you may leave your car idling while it warms up. The majority of cars, houses and businesses rely on fossil fuels for energy. This is particularly true in developing countries, some of which burn coal and garbage for energy. This increase in burned fossil fuels, garbage and coal emit copious amounts of PM2.5 and other toxic air pollutants. In combination with a temperature inversion, an increase in air pollutants contributes greatly to air pollution during winter.
Spare the Air
If possible, try to be more cognizant of your energy usage this winter. For example, wear an extra layer and allow your car to warm up on the way to your destination. The world’s air is already extremely polluted, and adding to that pollution when the weather is cold only serves to create a more toxic atmosphere.
Also keep in mind that outdoor air pollution makes it way indoors. The cold air that traps pollutants near the surface and limits air circulation, diluting air pollution in the process, means potentially worse indoor air pollution during winter too! Consider healthy home habits and source control steps, or IAQ home system upgrades to create a healthier indoor environment all winter long.