Reduction of air pollution
As cities strive to meet air quality standards, the ways in which trees contribute to clean air should not be overlooked. The most important pollutants in the urban atmosphere are ozone (O), nitrogen oxides (NO), sulfur oxides (SO) and other types of pollution on a smaller scale. Smog is produced by chemical reactions between NO and volatile organic compounds (VOCs) in the presence of sunlight. High temperatures increase the speed of these reactions. Vehicular and industrial emissions, gasoline vapors and chemical solvents are the main sources of NO and VOCs. Polluting particles less than 10 or 25 micrometers "Micrometer (unit of length)") in size (PM10 and PM25) are formed by microscopic droplets of solids or liquids, which adhere to lung tissues causing serious health problems. Pollution mostly begins as smoke, which causes serious problems for people with heart and respiratory diseases and irritation for healthy people. Trees are an important, effective and economical solution to reduce pollution and improve air quality.
With healthy and extensive urban forestry, air quality can be drastically improved. Trees help reduce air temperature and the heat island effect in urban areas. This decrease in temperature not only reduces energy consumption, but also improves atmospheric quality since the formation of ozone requires high temperatures.
• - As the temperature increases, the formation of ozone increases.
• - Urban parks reduce temperature and therefore ozone production.
• - Large tree shadows can reduce ambient temperatures by between 3 and 5 °C.
• - In Sacramento County, California, it was estimated that raising canopy cover to five million trees would reduce summer temperatures by 3°C. This reduction in temperature would decrease peak ozone levels by 7% and smoggy days by 50%.
The reduction in temperature due to the shade of trees in parking lots (parking lots) reduces the amount of evaporative emissions in parked cars. Parking lots without tree shade can be seen as miniature heat islands, where temperatures can be even higher than in surrounding areas. Wooded areas can significantly reduce air temperatures. Although the main mass of hydrocarbon emissions comes from vehicle exhaust pipes, 16% of those emissions come from evaporation that occurs in fuel distribution systems when vehicles are stopped and engines are still hot. These evaporative emissions and combustion gases from the first minutes of an engine's operation are highly harmful to the local microclimate.
• - Cars parked in parking lots with 50% tree area emit 8% less evaporation than vehicles parked in parking lots with only 8% tree area.
• - According to the positive effects of trees in parking lots, cities such as Davis "Davis (California)"), California, established ordinances that determine that parking lots must have a 50% tree-lined surface.
Volatile components of asphalt evaporate more slowly in shaded parking lots and streets. Shade not only reduces emissions but also increases the durability of the asphalt and therefore the need for maintenance. Less maintenance means less hot asphalt (fumes and vapors) and less machinery operating (hydrocarbon emissions). The same principle applies to roofing on an asphalt base.
Trees also reduce pollution by actively fixing air pollution particles. The stomata, pores on the surface of the leaves, capture polluting gases that are then absorbed into the water inside the leaf. Some tree species are more susceptible to the absorption of pollution, which can affect the normal development of the plant. Ideally, trees should be selected according to a high capacity to absorb pollutants and a high resistance to the negative effects they cause.
A 1991 study throughout the Chicago region determined that trees remove approximately 17 tons of carbon monoxide (CO), 93 tons of sulfur dioxide (SO), 98 tons of nitrogen dioxide (NO), and 210 tons of ozone (O).
Those who work with urban parks are sometimes interested in knowing the amount of carbon that is fixed from the air and stored in trees as wood in relation to the amount of carbon dioxide (CO) that is released into the atmosphere during the work of the machinery necessary to maintain the trees, which is powered by fossil fuels.
In addition to absorbing harmful gases, trees also function as filters, intercepting particles from the air and reducing the amount of important harmful particles, being captured by the surface of the tree and its foliage. Those particles remain temporarily on the surface of trees until they are washed away by rainwater, blown away by strong winds, or until they fall to the ground within a drop of leaf transpiration. Since trees are a temporary reservoir of harmful particles, if they did not exist, those temporarily retained particles would remain in the air, damaging human health. Increasing the tree area increases the retention of air particles and therefore improves air quality.
• - Large trees with evergreen foliage are the ones that remove the greatest amount of particles from the air.
• - The 1991 Chicago study determined that trees fixed approximately 234 tons of particles less than 10 micrometers (PM10) in size.
• - Large healthy trees with trunks of at least 75 cm in diameter remove approximately 70 times more air pollution per year (1.4 kg/year) than healthy, small trees with less than 10 cm in trunk diameter (0.02 kg/year).