I am going to be lambasted for saying this, but say I must. Trees do not reduce pollution. They may bring about a general feeling of wellness due to their green foliage. But, contrary to general belief trees do not eliminate polluting gases or particulates. On the other hand, trees on the sides of a crowded street actually concentrate these pollutants by impeding free flow of air.
People are up in arms in a Delhi against cutting of trees for widening of roads. Traffic jams or crawling traffic creates a lot more pollution that trees lining the roads are imagined to reduce. We need free flow of traffic to cut down pollution.
Trees absorb carbon dioxide, the greenhouse gas. But CO2 is not a pollutant, nor is it a health hazard. Besides, CO2 is a global phenomenon as it quickly disperses in the air. Sulphur and nitrogen oxides and particulates are pollutants and pose a risk to our health. They do so by immediate impact on our bodies before wind moves them out. They get washed down by rain and pollute water bodies and soil. Too many trees by the roadside actually trap these in the crowded streets and aggravate the problem. Besides, trees do not absorb sulphurous and nitrous gases since their metabolism has no use of these gases.
Here are scientific papers that establish that trees in urban landscapes actually aggravate Pollution:
https://www.sciencedirect.com/science/article/pii/S1618866716303740
http://theconversation.com/do-trees-really-help-clear-the-air-in-our-cities-48202
This is not to say that all trees should be cut down to aid free flow of air, but not to blindly believe whatever is presented to us by activists, media, NGT or Courts. Trees should be planned and planted to a plan, aesthetics being a contributing factor . Trees provide cool shade, they soothe our eyes and bring about a feel good emotion. They must be planted so that traffic flows freely, sight of motorists and pedestrians is not obstructed. Reduction in pollution should certainly not be an objective.
Meanwhile let those roads be widened. Widening cuts down pollution, reduces accidents and eliminates pedestrians’ deaths.
http://www.iflscience.com/environment/greener-not-cleaner-how-trees-can-worsen-urban-air-pollution/
