A species of moss known as Orthotrichum lyellii is about to resume its leading role as an indicator of air pollution in Portland.
O. lyellii made its debut on Portland's stage last year when the U.S. Forest Service released a study showing that the moss had been quietly collecting heavy-metal pollutants in the Rose City. Data coughed up by the moss ultimately led the Oregon Department of Environmental Quality to crack down on two glassmakers that had been emitting unacceptably high levels of toxic metals into Southeast and North Portland neighborhoods.
Now, the moss is poised to reprise its starring role in a sequel to the Forest Service study that will explore some of the mysteries hidden in the initial round of data. For example, Forest Service scientists will try to determine why lead was showing up at higher levels closer to the city's center, but at lower levels toward the suburbs.
One hypothesis: Lead appears to be higher near where older homes were demolished, which may be attributable to the use of lead paint.
The second round of the urban moss study formally begins in the fall, and it's not yet clear how long it will take. The first round took about three years.
Sarah Jovan, a co-author of the ongoing moss study, says moss is useful for detecting urban air pollution because it acts like "little sponges."
"They get all their nutrients and water from the air so they are quite efficient at absorption. They end up absorbing the air pollutants as well," she says.
Moss seems to "grow abundantly in all places of the city regardless of air quality, which would allow us to map pollutants at a very fine scale," Jovan says. "My career, so far, had been about devising ways to use moss and lichen to map pollutants in forests. So attempting our methods in a city wasn't a huge stretch."
Forest Service scientists also will try to determine whether there is any correlation between the rough data produced by the moss and the precise measurements produced by the DEQ's fancy air- pollution monitoring equipment. If successful, the scientists hope they will be able to have a reliable estimate of how much heavy- metal pollution is in the air simply by harvesting moss samples and putting them under a microscope.
The pattern of lead contamination recorded by the moss is just one of the mysteries revealed by the study. The moss also showed elevated levels of chromium and sulfur contamination in areas of North Portland south of two refineries that process used oil.
However, Jovan says more data is needed to conclusively link the two metals found in the moss with those two plants. The Forest Service collected no moss data within about a quarter-mile of the plants, and the DEQ has no air-monitoring data on chromium and sulfur pollution from that area or any point north of the oil recycling plants, such as on Hayden Island, where residents have been complaining about a horrific stink traced to the two plants for many years.
The Forest Service has been using moss and lichen for many years in its studies of air pollution in national forests. For example, a 2005 USFS study of lichen, a close relative of moss, found that air pollution from Portland was threatening ecosystems in the Columbia River Gorge. It said topographic and meteorological conditions made the gorge an "exhaust pipe" for air pollutants generated in the city.
In 2013, the USFS teamed up with DEQ to launch the current study, which attempts to improve our understanding of urban air pollution, says Jovan, a lichenologist by training. Jovan and her team of five scientists measured concentrations of 22 different metals in 346 moss samples collected across the city in December 2013.
One of the samples taken from a site in the Kenton neighborhood detected an extraordinarily high level of lead. An investigation revealed that a nearby house recently had been demolished. Across the city, Jovan says, elevated lead levels in moss appeared to correspond to recent home demolitions.
She thinks the lead may have come from leaded house paint, which had been in common use until banned in 1978.
But high lead levels also have been found in areas where scientists would expect to see higher lead amounts in soil from past use of leaded gasoline.
The scientists will conduct a forensic analysis of the lead in moss samples to determine its source, whether from paint, leaded gasoline or industrial emissions, Jovan says.
She thinks moss makes it easier to monitor air pollution than other methods. "We don't know very much about what is in our air," she says, "and the high costs of traditional monitoring with instruments makes mapping pollution at the neighborhood or even city-scale impossible."