My intuition has always been that there is less air pollution at night because there are fewer cars on the road and fewer factories humming at that time of day.
Pollution Time of Day Guesses
Apparently, I’m not alone. I asked 139 people to guess what time of day has the least air pollution (lowest PM2.5), and nighttime came in first.
I also know people who arrange their schedules to work out in the morning to avoid the worst pollution. But how accurate are our intuitions?
Pollution Data by Time of Day
To get to the bottom of it, I analyzed thousands of hours of PM2.5 data from the US Embassy in Beijing. When I finally got the answer, I was surprised:
Pollution was actually worst at night! And it was the lowest in the afternoon, right around rush hour. So if you’re planning a picnic or exercising outside in Beijing, you’re usually best off between noon and 6 pm.
What about other cities?
Is that how PM2.5 generally works, or is it unique to Beijing’s activity or climate? Fortunately, US consulates in several other cities publish their historical data.
Shenyang
In terms of climate, Shenyang is pretty similar to Beijing, and its daily PM2.5 patterns are very similar (although nighttime isn’t quite as bad as Beijing):
Chengdu
The pattern in Chengdu is similar. PM2.5 is lowest in the afternoon and highest in the morning.
Shanghai
The picture starts changing when you get to the south. In Shanghai, PM2.5 is lowest in the afternoon (like in the north), but there’s also a dip in the very early morning:
But also note that the scale is really zoomed in for Shanghai and Guangzhou (below). The difference between the lowest and the highest averages for Shanghai is only about 4 micrograms, as opposed to about 25 micrograms in Beijing.
Guangzhou
Guangzhou’s pattern was virtually identical to Shanghai’s: a dip in the very early morning and a dip in the afternoon.
Here’s the pattern across all five cities at once. You can see that the hourly variation in northern cities is much larger than the variation in southern cities (Guangzhou and Shanghai).
Air Quality in the Morning vs. Nighttime
But that’s all China. Surely there are lots of factors that go into air pollution, like geography, climate, and industry. So is that pattern something unique to China, or is it more common around the world?
Researchers analyzed 17 million hourly PM2.5 datapoints from 3,110 sites around the world. They also found that pollution was the worst at night (around midnight) and lowest in the afternoon (around 4pm).
Why Is Pollution Best at Rush Hour?
How does it make any sense that pollution is the lowest during rush hour? The answer has to do with the sun, wind, and something called the “mixing height.”
At night, the air cools down, wind is less strong, and the air tends to settle closer to the ground. That means all of our air pollution is closer to us on the ground.
But in the day, the sun warms the surface of the Earth, and that warm air rises. Wind also tends to be stronger. That makes the air spread out over a larger area, which means our pollution gets more spread out.
But keep in mind that the air is NEVER safe on average in any of the five cities at any time of day. So take “safest” with a grain of salt!
Open Data
All of the data is available from the US Embassy and consulate websites. Thanks to Josh Malina for collecting and analyzing the consulate data.
Caveat: Gas Pollutants
It’s important to point out that this analysis uses data on particulate pollution. There are other types of pollution out there. One big one is ozone pollution, which tends to be highest around midday.
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This is really interesting and contradics other information I’ve read online. As an asthma sufferer who likes the outdoors, it’s really important for me to know when it’s safest to go out. It makes sense that polution is lower in the afternoon as it will be warmer and the air will rise and expand, carrying those particulate matter with it. Thank very very much for sharing this important data.
Would you say this also goes for villages, where most pollution is caused by people burning various materials to warm their houses up, and not by cars?
Thanks a lot, man. Very useful data. I’ll workout in the afternoon from now on
Hi Thomas
You did an appreciable work with proof of concept. Indeed it helped to understand the different timings of concentration of pollutants.
thanks