The Urban Forests Cleaning Beijing

If you want a taste of China, then you might want to stick your head over a coal fire. A combination of geography, aging industry and population means that smog is a regular problem in Beijing, like in many other Chinese megacities.

Fixing the problem is going to be expensive and require some major changes in infrastructure, but new research shows that botanists could help in Beijing and other pollution hotspots around the world. A paper in PLOS One this month by Wu et al. examines how urban forests can help remove particulates from the air.

You’ve probably seen the stories about NASA investigating indoor plants for cleaning pollution from the air. Wu and colleagues look at something similar, building on work on the effects of trees outdoors.

The study location was the Beijing Olympic Forest Park. This is a park in northern Beijing covering 6.8km2 (2.6 sq miles). The dominant species are Sabina chinensis (Juniperus chinensis) the Chinese Juniper, and Liriodendron chinense, the Chinese Tulip Tree. Given that the park was planted to celebrate the 2008 Olympics, it’s no surprise that these are considered ornamental species. What Wu and colleagues examined was what these tree did to the flow of particulates around the park, to see if they were trapping some of the smog.

They set up sensors to record coarse (PM10) and fine (PM2.5) particulates. The PM figure here being a measure of the size of the particles in micrometres. They set up the sensors at three heights, 1.5m, 3.5m and 5.5m, to examine how the concentration of particles varied with height. They measured samples every minute and the local weather every thirty minutes between 0700 and 1900 local time over a couple of years. This allowed the researchers to examine daily and seasonal patterns in pollution.

What they found was that, generally, the leeward sides of the trees were lower in pollution than the windward sides, showing the trees were blocking the movement of pollution. The trees were better at blocking the finer PM2.5 material. These are the smaller particles than can get deeper into your lungs.

The Chinese government is serious about using plants to reduce urban pollution. Last year they announced plans for Liuzhou Forest City, a new development in southern China. If research can optimise planting to reduce pollution, then the future might look a little less like Blade Runner and a bit more like Rivendell.