Tree transpiration and tree canopies can affect air temperature, radiation absorption and heat storage, wind speed, relative humidity, turbulence, surface albedo, surface roughness and consequently the evolution of the mixing-layer height. Such changes in local meteorology can have an affect on local pollutant concentrations in urban areas. Urban trees are generally associated with contributing to cooler summer air temperatures, however in some instances they may have the opposite effect causing an increase in air temperature. Where tree stands consist of scattered tree canopies, radiation can reach and heat ground surfaces; at the same time, the canopy may reduce atmospheric mixing, preventing cooler air from reaching the area. In such cases, tree shade and transpiration may not compensate for the increased air temperatures due to a reduction in overall mixing. However reduced air temperature as a result of tree planting is believed to improve air quality because emissions of many pollutants and/or ozone-forming chemicals are temperature dependent.
Trees can remove gaseous air pollution either through uptake via leaf stomata or the plant surface. Once inside the leaf, gases diffuse into intercellular spaces and may be absorbed by water films to form acids or react with inner-leaf surfaces. Recent research suggests that the planting of trees along the sides of roads could reduce NO2 concentrations in addition to providing amenity value. Trees can also remove pollution by intercepting airborne particles. Some particles can be absorbed into the tree, though most that are intercepted are retained on the plant surface. The intercepted particle is often re-suspended to the atmosphere, washed off by rain, or dispersed through leaf fall. Consequently, vegetation is thought to be only a temporary retention site for many atmospheric particles.
