Foliage on trees might be useful as low-tech pollution sensors

As noted in prior posts, significant lung damage has been associated with the PM2.5 particulates; they are a major health hazard.  Additionally, diesel is a significant source of such particulates.  Thus, researchers considered potential options for assessing air quality in an inexpensive manner.

When researchers analyzed leaves collected at several sites along streets in Bellingham, Washington, they found that the leaves along bus routes were as much as 10 times more magnetic than leaves collected on quieter residential streets.  The increase in magnetism came from iron oxide particles in emissions that were trapped on the microscopically rough surface of the leaves.

Iron oxide particles smaller than 2.5 micrometers across are typically magnetic, while those larger than 10 micrometers aren't.  Rain washes away no more than 30 percent of all the particles stuck on a leaf, and even ultrasonic vibrations can't fully cleanse the surface.  These characteristics thus make tree leaves a good candidate for pollution monitoring.  Other pluses, according to researchers: Leaves are cheap, and they provide information about the air near ground level where people are, not high above the street where most air quality monitoring equipment is installed.

Scientists still must assess how the number of iron oxide particles trapped by leaves relates to the total number of particles of different chemical classes in the air.  Because many air quality standards are based on exposures for short periods of time, such as eight-hour or 24-hour intervals, researchers must also assess how to estimate short-term air quality from leaves, which accumulate particles throughout their growth.

The results of the pilot study on this strategy can be found at