Temperature and wind are often included on air quality maps that display real-time pollutant information. But why are these environmental factors listed, and how do they impact air quality?

Here is a review of how wind and temperature, interact with pollutants to impact air quality.

How does wind affect air pollution?

Wind can be helpful in dispersing pollutants. When pollutants linger over a region, wind may disperse pollutants out of the region and reduce concentrations of more intense pollutants in any one region.[1]This can also blow pollutants far away from their source - as happened when western U.S. wildfire smoke sent particle pollutants as far away as western Europe.

However, geography can occasionally pose a challenge for wind in dispersing pollutants. Pollutants may not be pushed out of valleys if the prevailing winds are unable to rise over mountain ranges. In those cases, pollutants can gather in higher concentrations at the base of the mountain – or worse, create a “mountain valley chimney” effect.

A 2009 study in the Journal of Geophysical Research: Atmospheres found that mountain valley breezes played a role in creating a second layer of pollution in Beijing, China.[2]

Mountain valley breezes blowing predominantly southward gathered surface pollutants from the city, flowed along the face of mountains, and created a second elevated pollution layer flowing in a northward direction over the city. Pollutants from that layer could return to the surface and affect residents a second time.

Rain dilution and coagulation

Rain can help to dilute high concentrations of airborne pollutants. Because coarse particulate matter (PM10) like dust, dirt, and pollen is larger and heavier than other particles, rain can help PM10 settle to the ground more quickly than smaller fine particulate matter (PM2.5).[3]

Rain is less effective in diluting PM2.5. Researchers in Lanzhou, China measured how much rain impacted concentrations of PM10, PM2.5, and PM1 in the air from 2005 to 2007.[4] Extremely heavy rain could reduce larger particle pollutants by a small amount, but had almost no effect on particles smaller than 2.5 microns.

As raindrops fall, they can also attract aerosolized particle pollutants in a process called coagulation. In a paper published in Atmospheric Chemistry and Physics in 2015, researchers found that the smaller a droplet, the easier it was for the water to attract aerosols.[5] Low relative humidity was also helpful in the process.

If the source of air pollution happens to be a wildfire, a heavy rain can also be helpful in dousing fires and ending smoke emissions.

Hot, sunny days can create ozone haze

Summer heat can lead to haze – a fog-like appearance most often found in urban areas. But instead of being composed of tiny water droplets like fog, summer haze is actually ground-level ozone, or smog.

When nitrogen oxides and volatile organic compounds (VOCs) emitted from combustion – often vehicles – interact with sunlight, the reaction creates ozone.

Ozone is less common when there is high humidity, rain, wind, or cooler temperatures.[6]

As warm weather is an important factor in creating ozone, famously sunny cities like Los Angeles can struggle with heavy smog days. Consequently, some cities like Bogotá, Colombia; Paris, France; and Mexico City, Mexico have limited car access to cities in an effort to reducing smog.[7]

How heatwaves lead to smoke

Sunny weather and high temperatures can have an additional negative impact on air quality. When high temperatures exceed norms and last for an extended period of time, resulting heatwaves can lay the ground for dangerous air quality conditions.

Heatwaves can fuel wildfires. In British Columbia, Canada during June and July 2021, intense heat reaching 121.2 degrees created extremely dried ground vegetation. When a severe thunderstorm rolled through the region, 29,000 lighting strikes contributed to 62 fast-moving wildfires in the province.[8,9]

The village of Lytton, known for its normally high temperatures and dry climate by Canadian standards, was destroyed by one of the wildfires. 1,000 people were evacuated, and two people died.

Wildfires, in turn, generate smoke and PM2.5 which can drift for thousands of miles from their source when carried on prevailing winds. For example, smoke from 26,000 individual Amazon wildfires in 2019 could be detected 11,000 miles away in Papua New Guinea and Australia.

Temperature inversions

When it comes to pollution, warm temperatures aren’t the only concern. Some of the world’s worst pollution days cans coincide with winter, especially when a region is prone to temperature inversions.

Temperature or thermal inversions can occur over cities or mountain valleys when warm air forms over cooler air on the ground. Temperature inversion caps and traps pollution in an area, preventing pollution from dispersing to other locations.

Inversions can also be influenced by heat island-induced circulation in urban areas. The heat island effect occurs when buildings, roads, and city infrastructure absorb heat more than surrounding trees and water bodies. This leads to higher temperatures in urban areas than in outlying, greener areas.

According to 2014 and 2015 studies in the Journal of Applied Meteorology and Climatology, layers of air in an inversion interact with heat and pollutants in urban areas to serious air pollution.[10,11] If the urban area happens to be in a valley, air circulation is complicated by urban heat and poor ventilation options for dispersing pollutants out of the island.

Cities located in valleys or near mountain ranges such as Salt Lake City, Los Angeles, Denver, and Mexico City can be subject to severe pollution through temperature inversions.[12,13,14,15]

Winter temperatures

Cold temperatures often mean people must turn to combustion to warm their homes. Cooking and fuel fires can lead to days of extremely heavy particle matter pollution for people who burn low-cost wood, coal, and dung.

During the winter of 2020 to 2021, winter fuel choices led to hazardous air quality conditions for urban citizens in Mongolia, Afghanistan, and Kyrgyzstan in Central Asia.

During winter, any home with a wood-burning fireplace or wood burner can be a source of indoor and outdoor air pollution. However, a recent study warns that wood burners can pose a threat to indoor air quality, tripling the amount of air pollution inside a home.[16]

A 2020 study published in Atmosphere reviewed wood burner use in South Yorkshire, England.[17] The study concluded that when burners were lit, average particle levels rose from 27 micrograms per cubic meter (μg/m3) to 195 μg/m3.

The U.S. air quality index considers 195 μg/m3 to be “ Very Unhealthy”.

Pictured: U.S. Air Quality Index. Source: IQAir and U.S. EPA

The takeaway

Air pollution and weather, wind, and temperature all interact to improve or, all too often, worsen air quality

Knowing environmental conditions and how they interact with airborne pollution can help in better understanding what’s contributing to poor air quality.