Smog Check Your Trees : New UC Riverside Study Ranks Local Species by Their Level of Pollution
Air pollution experts are taking a serious look at the shady side of trees.
Just like cars and power plants, trees emit hydrocarbons that, when mixed with sunlight and nitrogen oxide, create ozone, the principal component of smog.
Researchers know that trees and shrubs in the Los Angeles Basin emit between 150 and 200 tons of hydrocarbons each day, accounting for 14% of all hydrocarbon emissions in the region.
But for the first time, a new UC Riverside study pinpoints the worst offenders, ranking hydrocarbon emissions from 12 landscape trees widely used in the Southland.
The $9,500 study, funded by the South Coast Air Quality Management District, found no detectable hydrocarbons in the crape myrtle, and few in the camphor and such pines as the aleppo pine, deodar cedar, Italian stone pine and Monterey pine.
The ubiquitous liquidambar, however, treasured for its stunning fall color in Southern California, clocked in at 1,233 times the hydrocarbon emissions of the camphor.
The carrotwood, a relatively “clean” tree with low seed drop used widely in such commercial plantings as parking lots, emits 1,633 times as many hydrocarbons as the camphor.
Put another way, emissions from about three liquidambar trees or just two carrotwoods during 12 hours would equal the hydrocarbons released by one 1991-model car traveling 30 miles. (Liquidambar and carrotwood trees release pollutants only 12 hours a day because sunlight is required to trigger their hydrocarbon, isoprene.)
By comparison, it would take more than 6,000 crape myrtle trees emitting hydrocarbons for 24 hours to match the amount released in the same 30-mile drive.
“Our goal was to list trees with varying hydrocarbon emission rates, some which could then be recommended for planting on a wide-scale basis to prevent an increase in smog,” said the study’s director, Stephanie Corchnoy, a UC Riverside chemistry major.
The numbers may make it impossible for Los Angeles to achieve federal health standards for ozone, said Roger Atkinson, an atmospheric chemist and associate director of the Statewide Air Pollution Research Center in Riverside.
Air quality officials hope to reduce anthropogenic (man-made) hydrocarbon emissions in the basin from the current 1,200 tons a day to 180 tons a day by the year 2010.
“By that date, biogenic hydrocarbons (those released by vegetation) would be greater than those released by man,” Atkinson said. “And there’s a big push to plant millions of trees in urban areas. My point is that it’s probably not prudent to plant millions of liquidambars.”
California Air Resources Board spokesman Bill Sessa takes a contrary view. It is “long-term speculation” that biogenic emission levels will equal anthropogenic emissions 20 years from now, Sessa said.
“Trees are not the reason we have unhealthy air in the L.A. Basin,” he said. “We’ve always taken these kinds of statistics into account, but at the moment (biogenic hydrocarbons) are not an important factor.”
Trees could increase ozone concentration by one part per hundred million by the year 2010, according to a recent AQMD computer model. The federal health standard for ozone is currently 12 parts per hundred million.
(According to the South Coast Air Quality Management District, trees and shrubs account for about 12% of the hydrocarbons emitted; cars, trucks and buses, 41%; airplanes, ships, trains and construction equipment, 5%; residential-commercial, 20%, and industrial-manufacturing, 22%.)
U.S. cities with health-threatening ozone must produce emission inventories, which include biogenic emissions, by 1992, Atkinson said.
A 1983 statewide Air Pollution Research Center study also tracked emission rates of the region’s vegetation, but did not rank tree emissions.
“The agreement between that study and Stephanie’s is quite good,” said Atkinson. “Hers is more sophisticated since chemical analysis techniques have moved forward quite a bit since then.”
Corchnoy, 21, said she studied trees suited to Southern California’s climate--those that are smog and drought tolerant. Emissions from the 12 trees (15 trees were studied but emission rates were obtained for only 12) were trapped by covering each tree with a large Teflon bag, which was inflated with purified air. Emissions from the potted trees, none more than six feet tall, were recorded for 15 minutes.
After analyzing the air, a tree’s bio-mass, or dry-leaf weight, was determined by stripping a tree of leaves (hydrocarbons are usually released through leaves and less often through blossoms). The leaves were oven-dried and weighed, a measurement that was balanced with the air analysis to give Corchnoy an overall hydrocarbon emission rate.
The two highest emitting trees Corchnoy studied, liquidambar and the carrotwood, are among the most popular in Southern California, area tree companies say.
About 44 million trees are sold each year in the state, according to the California Assn. of Nurserymen. The state ranks high in sales of ornamental trees, but figures are not available as to which species are best sellers.
Among the trees Corchnoy studied, the crape myrtle (which releases few if any hydrocarbons) tops the best-seller list at Azusa-based Monrovia Nursery Co. The company produces 40 million plants each year at its two 500-acre farms.
“(The crape myrtle) is a small-scale tree, and that seems to be what’s selling right now since people have smaller properties,” said Monrovia Nursery’s public relations director, Rodger Duer.
At least one landscape architecture firm has tracked Corchnoy’s study and is considering incorporating the findings into planting recommendations.
“We’re looking down the road to the day when potential for smog production in trees might be as important as drought and smog tolerance is today,” said Eric Norris, senior planner at RHA Landscape Architect and Planners in Riverside. “The trees most commonly used by landscape architects are some of the worst offenders--liquidambar and carrotwood.”
Andy Lipkis, founder of Los Angeles-based TreePeople, which advocates wide-scale tree planting, said his organization “will certainly suggest that people use low-emitting trees. One of our primary roles is educating the public as to using the right tree in the right place.
“I would, however, caution against branding certain trees as polluters. There have always been trees. There hasn’t always been smog. We have to remember that trees were here first.”
Lipkis said he assumed that Corchnoy’s study would show that aromatic trees, such as pines, would emit more instead of fewer hydrocarbons. Certain hydrocarbons lend trees their scent, such as the hydrocarbon alpha pinene for pines and limonene for citrus. Hydrocarbons by themselves are harmless, but researchers suspect some may repel bugs, boosting a tree’s defense mechanism.
Hydrocarbon emission rates vary widely within a single species. Many East Coast pines emit up to six times the hydrocarbons as those on the West Coast, said Corchnoy.
Potency levels among classes of hydrocarbons also fluctuate. Unfortunately, those emitted by trees are “very reactive” when coupled with nitrogen oxides and sunlight, said Basil Dimitriades, project manager for the Southern Oxidant Study, which is taking a five-year look at air pollution in Southern states.
“Up until the Clean Air Act of 1990, agencies did not appreciate the importance of biogenic emissions,” said Dimitriades, who heads the nine-university, multimillion dollar study, funded by the Environmental Protection Agency and utility companies. “Biogenics turn out to be crucial to this study.”
A flurry of studies tracked biogenic hydrocarbons in the mid-1970s. Interest in the subject has again been sparked, largely because of research done by William Chameides, director of Earth and Atmospheric Sciences at Georgia Institute of Technology in Atlanta.
Although jokes about “killer trees” surfaced in 1980 after President Ronald Reagan said “vegetation” contributes to the greenhouse effect, biogenic emissions are now taken quite seriously by air pollution officials.
In at least one U.S. city, biogenic hydrocarbons already match those created by man. A 1988 Georgia Institute of Technology study found equal amounts of both natural and man-made hydrocarbons in the 11-county area surrounding Atlanta, 57% of which is wooded.
“If there’s any place where biogenic hydrocarbons are important, it’s in the South, because more trees and higher temperatures equal more hydrocarbons,” said Chameides, who headed the study.
About one-third of the Los Angeles Basin is covered with trees, chaparral, coastal sage and grasses, according to a 1988 Statewide Air Pollution Research Center study.
The Los Angeles Department of Public Works is planning a new tree inventory for the city that will identify predominant species. An early 1970s inventory, which did not include trees found on residential property, located about 1 million trees along city streets and in parks.
San Francisco-based California ReLeaf, a coalition of 50 state tree advocacy groups, set a goal in 1989 to plant 20 million trees by the year 2000. The plan is matched by a growing trend in increased federal and state funding for such projects.
Most of the region’s trees are found in the east--the San Bernardino and San Gabriel mountains--upwind from source regions that produce many of the nitrogen oxides, lessening the trees’ ozone-producing potential.
Felling such a mountainside forest to cut pollution is a concept that perhaps best belongs in science fiction scenarios, said Lipkis and others interviewed. In that story-line, only “ideal trees” would survive.
Such trees have already racked up a lengthy list of qualifications in Southern California. Ideal trees must be aesthetic, drought resistant, provide ample shade, drop relatively few seeds, be smog-tolerant and have non-invasive root systems--a sort of “super tree” in which no dysfunction is allowed.
At the sacrifice of brilliant autumn plumage provided by liquidambar trees, the study by researchers now portends adding “non-smog producing” to that list.
Just like cars and power plants, trees emit hydrocarbons that, when mixed with sunlight and nitrogen oxide, create ozone, the principal component of smog.
TREE RANKED BY RELATIVE HYDROCARBON EMISSIONS Crape Myrtle: none detected Camphor: 1 Aleppo Pine: 5 Deodar Cedar: 10 Italian Stone Pine: 14 Monterey Pine: 30 Brazillian Pepper: 43 Canary Island Pine: 57 Gingko: 100 California Pepper: 123 Liquidambar: 1,233 Carrotwood: 1,633 Source: Statewide Air Pollution Research Center / Riverside
Smog Sources Hydrocarbons are emitted from many types of Southern California trees and shrubs in addition to usual polluting sources.
5% Airplanes, Ships, Trains & Construction Equipment 12% Trees and Shrubs 20% Residentail-Commercial 22% Industrial Manufacturing 41% Cars, trucks & Buses Source: South Coast Air Quality Management District
