US researchers have used data from multiple satellites and ground monitors to create a “comprehensive and consistent” map of air pollution across the globe.
The data covers the two decades to 2018, thus taking account of the impact of recent programs designed to curb pollution. And the trends reveal some surprises.
Led by Melanie Hammer, the Washington University in St. Louis project looked specifically at PM2.5 – particles of 2.5 microns or less that can make their way deep into the respiratory system and are particularly problematic for people with conditions such as asthma.
These particles are created in nature, but also by human activities, including manufacturing, vehicle exhaust and wood-burning stoves. And they are not easy to accurately monitor on the ground because there isn’t a comprehensive network covering the globe, says Randall Martin, co-author of a paper in the journal Environmental Science & Technology.
To create their map, Hammer, Martin and colleagues started with satellite images of columns of atmosphere that spanned the ground to the edge of space. Using the established GEOS-Chem model, which simulates atmospheric composition, they could infer how much PM2.5 should be on the ground, at the bottom of any given column.
When comparing the predictions to actual levels measured by ground monitors, the agreement was striking but not perfect, so Hammer added the differences between the observed and predicted amounts of PM2.5 and expanded the ground-based predictions across the globe, filling in the massive gaps between monitors.
This extra step brought the observed and predicted levels of PM2.5 from 81% to 90% agreement.
Once they were able to take a good look at the most recent pollution levels around the world, the researchers saw some stark changes from previous trends, particularly in China.
“We’re used to seeing just large, increasing trends in pollution, but in China what we found, from 2011 to 2018, is that there actually is a particularly large negative trend,” says Hammer
Elsewhere in Asia, the picture isn’t as positive, and while pollution levels did not seem to be increasing in India, the country seems to be in a plateau phase. “The broad plateau of very high concentrations, to which a large population is exposed, is quite concerning,” Martin says. “It affects the health of a billion people.”
But on balance, he says, the takeaway message is hopeful. “The changes in China are very dramatic, larger than we have seen anywhere in the world over the observational record. It illustrates a real opportunity to improve air quality through effective controls.”