To better understand how trees affect air quality, it is important to know the different types of air pollutants. Certain pollutants, both in the form of gases and particles, are emitted directly into the atmosphere, in particular sulfur dioxide (SO 2 ), nitrogen oxides (NO x ), carbon monoxide (CO), suspended particles (PM) and volatile organic compounds (VOCs). The other pollutants are not emitted directly; they are formed through chemical reactions. For example, the formation of ozone at low altitudes is often the result of the reaction of emissions of NO and VOCs in the presence of sunlight. Some particles are also formed from other pollutants emitted directly into the atmosphere. Trees act on these air pollutants in three main ways:
- they locally modify ambient temperatures, microclimates and energy consumption by buildings;
- they clean the air;
- they emit different chemicals.
Effects of trees on air temperatures, local microclimate and building energy consumption
Rising air temperatures can lead to higher energy demand from buildings in the summer, greater air pollution and a higher incidence of heat-related illnesses. Trees modify microclimates and reduce air temperatures in several ways: evaporation caused by their perspiration, breaking the wind and protecting certain surfaces with their shade. Vegetated areas are capable of reducing the temperature of the surrounding environment by several degrees Celsius, especially when the cover is wooded and shrubby.
Although trees generally contribute to cooler summer temperatures, their presence can sometimes have the opposite effect. Otherwise, transpiration from trees and forest cover also affects radiation absorption and thermal storage, relative humidity, turbulence, surface albedo, surface roughness and the height of the mixing layer (i.e. height at which air and surface substances, especially pollutants, are dispersed by vertical mixing processes). These changes in local weather are likely to alter pollutant concentrations in urban areas.
A change in wind speed can have a positive or negative impact on air pollution. On the positive side, reduced wind speed decreases infiltration of cold air in buildings, which results in less energy consumption to heat them in winter (and also less associated pollutants emitted by boilers and power plants). On the negative side, the reduction in wind speed sometimes leads to less dispersion of pollutants, which increases pollutant concentrations locally. In addition, with lower winds, the height of the layer of the atmosphere in which the pollutant mixes is often reduced. This reduction in the mixing layer tends to increase the pollutant concentrations since the same quantity of pollutants is now mixed with a smaller volume of air.