Around the world, cities provide plenty of opportunities such as better education, advanced health treatment facilities, better employment, commerce and trade as compared to rural areas. Therefore, more than half the world's people live and work together in urban communities and it is projected that by the year 2030, three out of five people will stay in cities. The unrestrained and rapid growth of cities has also brought environmental degradation and causes many serious problems such as worsening of air quality, loss of natural habitat and species diversity, and increased human health risks associated with heat waves, noise and crowding. In most urban areas of developing countries, a variety of harmful air pollutants such as particulate matter, carbon monoxide, nitrogen and sulfur oxides, and polycyclic aromatic hydrocarbons are emitted from a variety of sources, mainly the burning of wood, fossil fuels and vehicular emissions, which adversely affects the health of human beings, animals and other living creatures. In the urban environment, trees provide many economic, social and environmental benefits to people, such as aesthetic beauty, improvement of property values, erosion prevention, storm water management, noise reduction, mental health development and crime reduction. In addition, trees help cool the air by shading surfaces that otherwise would absorb the sun's energy and then reradiate it out as heat. Trees also cool the ambient air. Urban trees on average reduce air temperatures on summer days by 2-4ËF, although in some circumstances the cooling effect can be even larger. Trees also sequester carbon, helping to mitigate climate change. The efficiency of atmospheric cleansing by trees in congested cities could be improved by planting more trees other than shrubs or herbs. Generally, avenue trees act as living filters to decrease pollution through absorption, accumulation and detoxification. Trees remove gaseous air pollutants such as ozone, sulphur dioxide, and nitrogen dioxide mainly by uptake via leaf stomata, although some gases are removed by the plant surface. Once inside the leaf, gaseous air pollutants diffuse into intercellular spaces and may be absorbed by water films to form acids or react with inner leaf surfaces. Though some particles can be absorbed into the tree, most particles settle on the branches, leaves and twigs of plants and are washed out by the rain. Throughout the world, different advanced technologies such as smog-free towers in the Netherlands and adhesive roads for particulate matter in London have been applied. However, these technologies are extremely costly and unaffordable for countries like India and therefore the most eco-friendly and cost-effective way is plantation of tolerant tree species alongside highways, city streets, in parks, and in residential yards.
