People often recommend planting trees to make cities greener, cleaner and healthier. But during heat waves, city trees can actually boost air pollution. Indeed, a new study finds, up to 60 percent of the smoggy ozone in a city’s air on hot days may trace to chemicals emitted by trees.
The findings might seem the opposite of what you would expect, notes Robert Young. He’s an expert in city planning at the University of Texas at Austin who was not involved in the new study. Indeed, he notes, “everything has multiple effects.” The new findings do not mean cities should discourage tree planting, he says. Instead, cities may need stricter controls on other sources of pollution, such as tailpipe emissions from cars and trucks.
City trees offer a host of benefits. These include helping soak up stormwater that might otherwise drain into rivers (carrying pollution with it). Trees also provide cooling shade. They even soak up carbon dioxide, a greenhouse gas. At the same time, these trees release oxygen into the air.
But oxygen is far from the only gas that trees and certain other green plants release into the air. One of these chemicals is a hydrocarbon known as isoprene (EYE-so-preen). It can react with combustion pollutants, such as nitrogen oxides. The result is the formation of ozone. A component of smog, ozone can irritate the lungs and aggravate airway diseases, such as asthma.
Cars and trucks are major sources of nitrogen oxides. And these oxides don’t interact only with isoprene. They also react with certain scented compounds that trees can spew. Among these are monoterpenes (MON-oh-tur-peens) and sesquiterpenes (SES-kwih-tur-peens). These terpene reactions can help create lots of other very tiny airborne pollutants.
Galina Churkina works in Germany at Humboldt University of Berlin. She and her team wanted to probe how much the chemicals released by trees could affect city air.
To do this, the researchers turned to a computer. They asked it to model the likely reactions between plant chemicals and nitrogen oxides in air throughout the Berlin metropolitan area. To do that, the researchers fed in local weather data for two summers. One was 2006, when there was a heat wave. The other was 2014, when temperatures were milder.
An average daily high there in summer tends to max out at roughly 25° Celsius (77° Fahrenheit). On such a day, chemicals emitted by area greenery would likely have contributed to making about 6 to 20 percent of the ozone in the city’s air. But at peak spikes during a heat wave, when temperatures soar to more than 30 °C (86 °F), tree-chemical emissions also spike. As a result, they are now likely to be responsible for up to 60 percent of the ozone in air.
Churkina says her team was not surprised to see the seemingly contrary relationship between plants and pollution. She adds that “its magnitude was, however, quite amazing.”
Her team shared its new findings June 6 in Environmental Science & Technology.
The results, Churkina says, suggest that city tree-planting programs should not ignore the role this greenery may play in aggravating summer air pollution. Adding more trees will improve quality of life only if those cities also undertake plans to sharply cut vehicle pollution in summer and to boost their reliance on clean energy sources for electric power, she says.
asthma A disease affecting the body’s airways, which are the tubes through which animals breathe. Asthma obstructs these airways through swelling, the production of too much mucus or a tightening of the tubes. As a result, the body can expand to breathe in air, but loses the ability to exhale appropriately. Asthma is a leading cause of hospitalization and the top chronic disease responsible for kids missing school.
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
combustion (adj. combustible ) The process of burning.
component Something that is part of something else (such as pieces that go on an electronic circuit board or ingredients that go into a cookie recipe).
compound (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.
greenhouse gas A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
hydrocarbon Any of a range of large molecules containing chemically bound carbon and hydrogen atoms. Crude oil, for example, is a naturally occurring mix of many hydrocarbons.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.
monoterpene A type of molecule with 10 carbon atoms and 16 hydrogen atoms that may produce a scent.
nitrogen oxides Pollutants made up of nitrogen and oxygen that form when fossil fuels are burned. The scientific symbol for these chemicals is NOx (pronounced “knocks”). The principle ones are nitric oxide (NO) and nitrous oxide (NO2).
oxide A compound made by combining one or more elements with oxygen. Rust is an oxide; so is water.
oxygen A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).
ozone A colorless gas that forms high in the atmosphere and at ground level. When it forms at Earth’s surface, ozone is a pollutant that irritates eyes and lungs. It is also a major ingredient of smog.
pollutant A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.
smog A kind of pollution that develops when chemicals react in the air. The word comes from a blend of “smoke” and “fog,” and was coined to describe pollution from burning fossil fuels on cold, damp days. Another kind of smog, which usually looks brown, develops when pollutants from cars react with sunlight in the atmosphere on hot days.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.
JOURNAL: G. Churkina et al. Effect of VOC emissions from vegetation on air quality in Berlin during a heatwave. Environmental Science & Technology. Vol. 51, June 6, 2017, p. 5859. doi: 10.1021/acs.est.6b06514.