Trees are often considered a natural solution to improve air quality since they can absorb carbon dioxide and other pollutants. However, not all trees are created equal, and they are not always a miracle cure in the battle against air pollution.
Role of Trees in Air Pollution
In Ecole Polytechnique Federale de Lausanne public research university, researcher Donato Kofel studied urban trees' positive and negative effects on outdoor air quality in Geneva Canton. In his Master's project series in environmental sciences and engineering, he developed a new way of using the geographic information systems (GIS) application in investigating the impact of Geneva Canton on the air quality in the region.
The research involves conducting an inventory of Geneva Canton containing almost 240,000 isolated trees or those outside a forest. The lone trees in the inventory account for nearly 25% of the canton's total trees. The inventory also considered various tree characteristics such as species, location, crown diameter, trunk height, and trunk diameter. These data were used in generating maps showing the total leaf area of the trees, which in turn indicate their ability to filter out particulate matter from the surrounding.
Trees naturally release biogenic volatile organic compounds (BVOCs) depending on factors such as the amount of sunlight, air temperature and humidity, tree species, and the damage or stress they encounter.
In a parallel study, Kofel also investigated the role of trees in the formation and deposition of ozone. The BVOCs naturally emitted by trees are converted into ozone through photochemical oxidation, with other compounds in the atmosphere released by man-made activities. Kofel was surprised to find out that release of BVOCs from trees has the potential to form ozone which is known to affect human health and the environment negatively.
Kofel compiled the data on the 51 most common tree species found in Geneva Canton and used them in computing their hourly BVOC emission rates. He found out that some species of oak often found along the streets and parks have the highest BVOC emission rates and, therefore, the greatest potential to form ozone.
The maps also suggest that around 25% of particulate matter created from human activities in Geneva Canton was removed by urban trees. On the other hand, the ozone-forming potential of these trees is almost ten times higher than their ozone-removing capability. Every year, they release 130 metric tons of BVOCs equivalent to 18% of the VOCs released annually by road traffic.
The study's findings reveal that humans emit enough nitrogen oxides for the right chemical reactions to occur. It means that the potential of trees to form ozone can be reduced by lowering the human emissions of nitrogen oxides.
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Impact of BVOC Emission
All plants release a wide range of volatile compounds, including BVOC. In recent years, BVOC emissions have received attention from the scientific community because they are highly reactive and may influence the chemical and physical properties of the atmosphere.
The global carbon emission in the form of BVOC is about 1.1 Pg per year, almost of the same order of magnitude as methane emissions. They are known to hurt the atmosphere's chemistry since they produce tropospheric ozone and create aerosols as well. When BVOCs rapidly react with man-made and natural compounds in the atmosphere, they could form a photochemical smog. In addition, BVOCs influence the residence time of other greenhouse gases, which could lead to the formation of secondary aerosols.
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