There is a common perception that rain can clear smoke and reduce air pollution. In fact, even the Indonesian government has urged people to pray for rain during severe smog episodes this year.
Let’s delve into why this perception exists and explore whether rain truly does clear smoke and improve air quality.
1. Sometimes after it rains in Beijing, the sky is gloriously blue.
When that happens, many people like say, “Wow, the rain washed away the pollution and smoke! There air quality has improved”
2. But then sometimes it rains and rains, and the sky still looks full of smoke and air pollution. Why didn’t the rain clear the smoke?
People seem not to notice this occurrence as much. Maybe that’s because air pollution and smoke is often the norm than fantastically blue skies.
Does Reduce Particulate Pollution and Clear Smoke?
Researchers in Lanzhou systematically tracked what happened to pollution 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 pollutants by just 8.7%. For light to medium rain, the effect on PM2.5 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 it’s Just a Lanzhou thing?
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 (r = -.04) than for wind (r = -.37). Researchers in Europe have found similar effects for rain showing rains limited effect on reducing air pollution and smoke.
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.