You thought rain washes away pollution?

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I’ve wondered whether rain decreases pollution for a long time because I’ve noticed two things:

1. Sometimes after it rains in Beijing, the sky is gloriously blue.

When that happens, many people like me say, “Wow, the rain really washed away the pollution!”

2. But then sometimes it rains and rains, and the sky still looks like this afterwards.

People seem not to notice this occurrence as much. Maybe that’s because pollution is more the norm than fantastically blue skies.

Does rain wash away particulate pollution?

Researchers in Lanzhou systematically tracked what happened to particulate levels when it rained.

First they looked at the smallest particles (1–2.5 microns). They analyzed different sizes of rain from light drizzle (left) to extremely heavy (right):

Even during the heaviest rains (right), rain reduced small particles by just 8.7%. For light to medium rain, the effect on PM 2.5 was close to zero.

It’s easier to wash away larger particles. Here’s the effect on all particles (gray), large particles (blue), and small particles (red).

But even for large particles, the effect of moderate rain was close to 10% or less. The most violent rains had an effect closer to 30%.


Maybe that’s just a Lanzhou thing?

With Josh Malina (the nerd behind AQCast) helped us by analyzing 6 years of PM2.5 and weather data for Beijing. The correlation between rain and PM2.5 was far smaller (= -.04) than for wind (= -.37). Researchers in Europe have found similar effects for rain.


Bottom line: The effect of rain on particulate pollution is small.

Keep in mind that the effect of rain might be some what different in different cities. That’s because some cities will have a higher percentage of large particles or small particles. That’s also because rain tends to be heavier, softer, shorter, longer, calmer, or more violent from place to place.


Why is it really blue sometimes after rain?

But that still leaves a mystery: why is it so blue sometimes after it rains? I think the key is that rain isn’t just water. It’s also weather fronts and wind—moving air. When strong winds come in, they blow particulate away. From our perspective on the ground, it’s easy to think the pollution has been “washed” away, but most of it was actually blown away.

The researchers in Lanzhou found this pattern when a cold front moved into the city. Here’s what happened to pollution levels when a cold front moved in one day in November:

To my eyeball, that looks to be about a 66% drop in PM 2.5 from top to bottom.

One more thing: You might have noticed the spike before the cold front passed.

The authors say that happens when the wind starts to blow before the cold front passes. That wind mostly kicks the larger dust particles; its effect on the smaller particles is lower.


Evidence that wind matters

When the wind kicks up, it can blow away particulate—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.

That probably happens because

  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.

Choon Khin is a Smart Air engineer from Singapore, studying chemical engineering at the National University of Singapore

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