Wind, Not Rain, Reduces Air Pollution

How Wind Affects air Pollution PM2.5 and Dust

It’s wind, not rain, that reduces air pollution. It sounds so simple, yet on rainy days, many people believe it’s the rain that’s “washing away” air pollution.

Myth: The rain washes away all the air pollution.

Data shows otherwise. Rain is poor at clearing air pollution from the skies. Most of the time, it’s wind that blows pollution away and lowers overall air pollution levels.

 

Blue Skies After a Storm? Thank the Wind.

An analysis of 6 years of pollution and weather data in Beijing found that wind was a much stronger factor than rain. Fine particulate pollution (PM2.5) correlated strongly with wind (= -.37) but almost not at all with rain (= -.04).

If so, why do so many people believe rain reduces pollution? A likely reason for this confusion is that wind normally accompanies rain. Rain often comes from changing weather fronts, which bring both rain and wind. It’s this same wind that blows out the pollution.

From our perspective on the ground, it’s easy to think the pollution has been “washed” away. In reality, most of the pollution actually gets blown away by the winds, resulting in lower air pollution levels and blue skies.

Before the storm:

Beijing Polluted Sky

After the storm:

Beijing Blue Sky

 

Data Shows Wind Reduces Air Pollution

Researchers in Lanzhou charted how a cold front and its wind helps reduce air pollution levels. Here’s what happened to pollution when a cold front moved in one day in November:

Cold Front Effect on Air Pollution

When the wind picked up, PM2.5 fell approximately 66% from top to bottom. Compare that to the effect of rain. Moderate rainfall decreased particulate pollution by just 1.45%. Even the heaviest rains reduced fine particulate by just 8.71%.

Effect of Rain on Air Pollution PM2.5

Wind Direction Matters

When the wind kicks up, it can blow away pollution—especially the small particles of PM2.5. In Beijing, that works the best when the wind comes from the less-polluted areas to the north. But when the wind comes from the south, pollution actually gets worse.

Where does Beijing pollution come from? Effect of wind direction

Why would southern wind hurt Beijing air quality?

  1. The areas south of Beijing are heavily industrialized.
  2. Southern winds trap air against the mountains to the north and west of Beijing. Essentially, southern winds force us in Beijing to stew in our own pollution.

For more on the effect of wind, check out this analysis of wind and pollution in Shanghai.

Bottom Line: How Wind Reduces Air Pollution Levels

Data shows that rain has a relatively small impact on reducing air pollutants (0-30%), but the wind that comes with a storm can have a larger effect Wind direction can also have an impact, depending on if the wind is coming from a more or less polluted area.  Simple steps can be taken to protect against air pollution both indoors and outdoors.

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