The most serious pollutants in the urban atmosphere are ozone, nitrogen oxides (NOx), sulfuric oxides (SOx) and particulate pollution. Ground-level ozone, or smog, is created by chemical reactions between NOx and volatile organic compounds (VOCs) in the presence of sunlight.

High temperatures increase the rate of this reaction. Vehicle emissions, emissions from industrial facilities, gasoline vapors, and chemical solvents are the major sources of NOx and VOCs. Particulate pollution is made up of microscopic solids or liquid droplets that can be inhaled and retained in lung tissue causing serious health problems.

Large shade trees can reduce local temperatures by 3 to 5 °C.

It is also important to shade parked cars.  16% of hydrocarbon emissions are evaporative emissions of fuel and volatilized plastics from parked vehicles.

Edinburgh Road South - Where Are The Shade Trees?

Evaporative emissions and the exhaust emissions produced during the first few minutes of engine operation significantly affect local microclimate. If cars are shaded in parking lots, evaporative emissions greatly reduced.

Cars parked in parking lots with 50% canopy cover emit 8% less through evaporative emissions than cars parked in parking lots with only 8% canopy cover.

The volatile components of asphalt pavement evaporate more slowly in shaded park lots and streets. The shade not only reduces emissions, but also reduce shrinking and cracking of asphalt so that maintenance intervals can be lengthened.

Trees also act as filters intercepting airborne particles and reducing the amount of harmful particulate matter. The particles are captured by the surface area of the tree and its foliage. Large,  broad-leafed trees with dense foliage collect the most particulate matter.

Guelph Urban Forest Friends
http://www.guffguelph.ca