Choosing the right trees to improve urban air
No one disputes that Ronald Reagan was dead wrong when he said in 1981 that “trees cause more pollution than automobiles do.” However, scientists have known for nearly two decades that the biogenic emissions from certain trees can increase the levels of some pollutants, particularly ozone, in urban air. Trees also take up pollutants and cool urban air, but until now, only government scientists using sophisticated computers have had access to models for determining when the pros of planting different tree species outweigh the cons.
In a paper published this week on ES&T’s Research ASAP website (es050581y), a team of U.K. scientists describe the first method for evaluating the impact that different tree species have on urban air quality that can run on a PC. Their Urban Tree Air Quality Score (UTAQS) is intended to help urban planners in areas where air pollution is an issue to make the best possible decisions about which tree species to plant. “Native broadleaf tree species such as oak, elm, and maple tend to be the worst under our scoring system,” says Nick Hewitt, the ES&T paper’s corresponding author, who is with the environmental science department at Lancaster University (U.K.). However, he stresses that he and his colleagues are “definitely not saying chop down the oak trees.”
“Trees do benefit air quality if you plant the right kind,” Hewitt explains. The study’s results should hold true for any mid-latitude city in a temperate climate, including U.S. cities, he says. Just as important, Hewitt adds, is the fact that the procedure, which was developed by a team of researchers he co-led with his Lancaster University colleague Robert MacKenzie, can be tailored to provide appropriate recommendations for cities in any ecosystem.
This work goes beyond previous efforts to ‘rank’ the potential contribution of various tree species by considering the impact of the emissions using an air quality model,” says Alex Guenther, a senior scientist at the National Center for Atmospheric Research (NCAR).
The main reason that trees are able to help clean pollutants such as ozone and NOx, out of the air during the day is a side effect of having their stomates open during photosynthesis, when the plants take in CO2 and produce carbohydrates and oxygen, explains David Nowak, a project leader at the U.S. Department of Agriculture’s (USDA) Northeastern Research Station in Syracuse, N.Y. “For example, when the stomates are open and there’s a higher level of ozone outside the leaf than inside the leaf, there’s a diffusion gradient and the ozone goes into the leaf. When it goes into the leaf, it reacts with the surface and is taken out of the atmosphere,” he says.
However, the volatile organic compounds (VOCs) produced by trees, particularly the isoprene emitted by deciduous trees, which shed their leaves annually, have been shown to increase levels of ground-level ozone in an urban area. In 1988, William Chameides of the Georgia Institute of Technology used Atlanta as a case study to show that the impact of such biogenic emissions from trees on urban ozone levels could be significant. Hewit says that Chameides’ research inspired him. “In a hot, southern city, ozone apparently is strongly influenced by trees. For many years I wondered, does that apply to a northern city [like one in England]?” he notes.
To find out, he and his colleagues used Birmingham, an industrial city with air pollution levels on a par with other large U.K. cities, as a case study. First, they painstakingly identified the specific tree population of the Birmingham metropolitan area, Hewitt says, stressing that any model is only as good as the data used to create it.
They then fed the tree population data into a well-known atmospheric chemistry model called [eee] CiTTyCAT, which was developed at Cambridge University (U.K.) and is “widely used throughout the U.K. for air quality research work,” MacKenzie says. “UTAQS measures the effects of individual tree species in an urban forest on the city’s air quality. The score weights the impact of a tree species on different pollutants by comparing tree-induced changes in model pollutant concentrations to air quality standards. The score is compiled relative to air quality standards because 1 part per billion (ppb) of ozone is not equivalent to 1 ppb of NO2 in health terms,” he explains.
Tree planting for carbon sequestration
Research in this area is important because state and local governments around the world are beginning to consider tree planting to address a variety of environmental issues, including air quality, says Lisa Tilney, Trees and Air Quality project coordinator for the National Tree Trust, a nonprofit group. For example, the Kyoto Protocol allows countries to meet their carbon emission targets by planting forests to soak up carbon instead of making emission cuts.
Regions out of compliance with the U.S. EPA’s ozone standards may also be able to obtain partial credit for planting trees, according to guidance that the agency released last September. However, the guidance doesn’t specify which species to plant but instead “encourages general strategic tree planting as a means to potentially reduce ozone levels,” Nowak says.
“Urban areas that have considered the impact of biogenic VOC on air quality include not only locations in North America and Europe but also in China, South Korea, Thailand, South Africa, Columbia, Argentina, Chile, Brazil, [and] Australia,” Guenther says, noting that other tools have been developed to aid urban planners in choosing trees based on their biogenic VOC emissions.
For the UTAQS to work, a user must know the VOC emission rates of trees in their city. This VOC emission information is widely available for trees that grow in temperate regions, Hewitt points out, but far fewer published data exists for trees found in tropical regions.
No one knows exactly why some deciduous trees emit high amounts of isoprene, but it tends to be produced by hardwood species during the daytime, when they are actively photosynthesizing. Scientists hypothesize that such trees generate isoprene as a kind of antifreeze to protect the leaves from drying out during hot weather, Nowak says.
Because the nine genera of deciduous trees known to produce the highest levels of isoprene include the varieties that Hewitt’s rating system has found to increase air pollution, Nowak says that this UTAQS result is no surprise. However, MacKenzie stresses that “the UTAQS score does not simply reflect the isoprene emission rate . . . our score incorporates effects on NO2 and particulates (PM).”
Temperature is also important
Nowak contends that the UTAQS results would be improved if temperature effects were included. “The transpiration, cooling, and shading provided by the trees can cause the air temperature to drop, which in turn can reduce the levels of air pollutants by altering emission rates and changing the photochemistry of ozone formation,” he explains.
By incorporating temperature data into photochemical models, Nowak says that his agency’s studies show that planting trees—including those that emit high levels of isoprene, such as oaks—always reduces air pollution in urban areas in the eastern U.S. north of Baltimore. Temperature data are beyond the scope of what they set out to achieve with the UTAQS score because analyzing such data requires more computing power than is possible with a PC, MacKenzie says.
The UTAQS model is nonetheless important for helping city planners who don’t have access to more complex models to make decisions, points out Sir Geoffrey Allen of the U.K.’s Natural Environment Research Council’s Urban Regeneration and the Environment program, which funded the study. “If you want to plant a tree along a motorway, a coniferous variety will obviously be a better choice,” he explains.
“City authorities in the U.K. are already applying what we’ve done,” Hewitt says. “But we hope and expect that this ES&T paper will generate interest around the world,” he says.
