A cleansing rain sweeps up debris on city streets, washes away dirt and grime from homes and cars, and leaves a refreshing scent and feeling in the air. That last phenomenon is for a good reason- rain, snow, and wind can significantly help reduce air pollution.

On Monday morning, December 12, 2022, several U.S. cities were among the least polluted major cities in the world. Five U.S. cities breathed “Good” air quality, according to the U.S air quality index.

List of least polluted major cities and their air quality. Five U.S. cities were on the list. Source: IQAir.

Rain and snow helped lower measurements of PM2.5 in Los Angeles and San Francisco, California; Portland, Oregon; Denver, Colorado; and Salt Lake City, Utah.

PM2.5 is a common and dangerous air pollutant measuring 2.5 microns in diameter or less. Closely linked to lung and heart disease, the World Health Organization (WHO) revised the recommended annual PM2.5 to no more than 5 micrograms per cubic meter of air (μg/m3) in 2021.

Weather affects air quality

Heatwaves, rain, fog, or snow – all weather plays a critical role in determining a region’s air quality.

When high concentrations of pollutants linger in the air, air quality is poor and endangers health. This problem can be worsened by excessive heat, fog, and stagnant air. Geography, including basins and mountains, can prove an additional challenge to dispersing pollutants.

Temperature inversions, layers of warm air that can develop atop a layer of cooler air, further complicate issues impacting concentrations of air pollution. Cities and mountain valleys may trap pollutants when warm air hovers over cooler ground-level air. Temperature inversions in summer and winter are common in many western U.S. cities like Los Angeles and Salt Lake City.

The same weather conditions introducing fresh breezes to a region helps reduce air pollution concentrations. Wind disperses pollutants by pushing particles out of a region and reducing dangerous concentrations of PM2.5.

Rain will dilute particle matter. It’s particularly effective in reducing heavier, larger PM10. These coarse particles are less likely to linger in the air and travel greater distances than PM2.5. Rain helps cause PM10 pollutants like pollen, dust, and dirt to quickly settle to the ground.

Small droplets of rain can coagulate after drawing in aerosolized particle pollutants (1).

Improved air quality in California

Heavy rain and snow doused California during the preceding weekend. California has struggled in recent years with a historic drought, so the precipitation offers a welcome respite for the state and may help improve low groundwater levels.

The precipitation also reduced measurements of PM2.5 by air quality monitors from San Francisco to San Diego, as the AirVisual platform indicates.

Heavy rains and snow significantly improved the air quality in California on Monday morning. Source: IQAir.

Los Angeles enjoyed a rare day of good air quality on Monday, where air quality monitoring stations measured an average PM2.5of just 6 μg/m3 at 2 PM.

Los Angeles air quality can be much worse in winter due to temperature inversion and excessive vehicle and industrial pollutants.

Los Angeles air quality was mostly “Good” on December 12. Source: IQAir.

San Francisco air quality was excellent on Monday, December 12. Source: IQAir.

San Francisco’s air quality was similarly impacted. At 2 PM, the city’s average PM2.5 concentration was just 1 μg/m3.

Wildfire smoke reduced in Pacific Northwest

Wildfires have plagued air quality in the Pacific Northwest for several months (2). Widespread wildfires scattered across Washington State, Oregon, Idaho, Montana, and British Columbia have led to increased concentrations of PM2.5 across the region.

Coastal rains have helped to bring a breath of fresh air to region – quite literally, as air pollutant concentrations are reduced, fires are dampened, and smoke is dissipated by the weather.

Good air quality measured by the majority of air quality monitoring stations in the Pacific Northwest. Source: IQAir.

Portland air quality was mostly “Good” on Monday morning, December 12. Source: IQAir.

By 8 AM, Portland’s average measured concentration of PM2.5 was only 5 μg/m3, or “Good” according to the U.S. AQI.

Portland’s daily average measured concentrations of PM2.5 have been elevated since November 11, falling within the ranges of “Moderate” to “Unhealthy for Sensitive Groups.”

Other cities in the region also experienced good air quality. The average PM2.5 concentration in Seattle, Washington was 3 μg/m3 at 1 PM. At 10 AM in Vancouver, British Columbia, the average PM2.5 concentration was also 3 μg/m3.

Snow improves Salt Lake City air quality

Snowfall further inland in Salt Lake City may have also helped those cities enjoy good air quality on Monday morning.

Denver has received a little rain and snow and will likely receive more Monday evening.

Salt Lake City’s “Good” air quality was noted by air quality monitors across the city on Monday. Source: IQAir.

Salt Lake City struggles with pollutant-elevating temperature inversions due to its mountainous surroundings.

Reduced concentrations of PM2.5 were measured by air quality monitors in Salt Lake City following heavy snowfall that began over the weekend (3). The city had measured an average PM2.5 concentrations as high as 30 μg/m3 on December 7; the average PM2.5 concentration for December 10 and 11 was 1 μg/m3.

The takeaway

When you see inclement weather on the horizon, it may actually be warranted to breathe a deep sigh of relief. Yes, there are inherent dangers that come with extreme weather conditions, but rain, wind, and snow can herald better air quality for cities. As seen on the U.S. West Coast this Monday, weather has improved air quality for millions and offers hope for people struggling with pervasive poor air quality.