A team of researchers from the Delhi University have zeroed in on five trees, which they claim are the biggest contributors to clean the national capital’s air.

Plants have always been known to clean our air not just by sucking up and trapping harmful gases, such as carbon dioxide, but also by trapping the particulate matter.

“But not all plants have the same ability to bring down pollution and clean the air. Our research shows that there are some trees such as peepal, saptaparni and jamun which help to clean the city’s air more than others,” said Dr Vijay Kumar, assistant professor of botany at Shivaji College under Delhi University.

A 16-member team comprising three assistant professors and 13 students collected data on air pollution and dominant tree colonies from five areas — Mandir Marg, Civil Lines, Anand Vihar, RK Puram and Punjabi Bagh — from September 2015 to September 2016.

The five spots were chosen as the air quality monitoring stations of the Delhi Pollution Control Committee are located in these areas. The sixth station could not be accessed because of security issues as it is located inside the Indira Gandhi International Airport.

“We compared the levels of five pollutants — PM2.5, PM10, NOx, SOx and ozone — in these areas as measured by the monitoring stations and then studied the dominant tree colonies,” he added.

It was found that while areas such as Mandir Marg and RK Puram have lower pollution levels, others such as Anand Vihar and Civil Lines are highly polluted.

“On inspection, it was found that areas which were dominated by trees such as peepal, saptaparni, jamun, devdar and champa registered lower levels of pollution. Some areas such as the Civil Lines, which are dominated by trees such as the vilayti kikar, registered high levels of pollution,” said Kumar.

Several parameters such as the tree’s height, its canopy size, leaf size, shape and orientation of leaves, leaf characteristics such as texture, stomata, trichomes (leaf hairs), dust accumulation over unit area were considered and studied in laboratories.

“We found that these five trees were able to trap more pollutants, including PM2.5 and PM10, than others. Their leaf structures were such that they helped to trap more dust and other pollutants,” said Dr Kumar.

They, however, pointed out that concretisation of pavements which leave hardly any space for water to percolate down to the roots and the falling groundwater levels are taking a toll on the trees’ ability to bring down pollution.

“Trees with stunted growth, smaller leaf size and thinner canopy won’t be able to absorb the same amount of pollution as a healthy tree,” he said.

The project titled “Amelioration of Air Quality in Urban Ecosystem of Delhi – Role of Avenue Trees” also focused on the presence of birds as bio-indicators of a healthy tree.

“It was found that some trees such as the peepal not just helped to bring down pollution levels but also supported a wide range of bird species. The grey hornbill and brown-headed barbet were found in large numbers in areas which were dominated by trees such as peepal,” said Dr Virat Jolli, assistant professor of zoology from the Shivaji College and one of the researchers.

The study was funded by Delhi University. Eminent ecologist and emeritus professor of Centre for Environmental Management of Degraded Ecosystems, CR Babu, was the mentor of the research. The researchers are now working to publish the findings in a peer reviewed journal.