Hedge Your Bets!

Author: Anil Ramaprasad

Air pollution has been in the news every day for the past few months and there is a strong sense among people to act and to find solutions to the problem. A common reaction to air pollution is to recommend mass planting of trees across the city. Trees are great, don’t get me wrong, and play a vital role in mitigating air pollution but one has to understand, specifically, how it does it.

The composition of what we call air pollution is diverse. If we just consider the long list that the pollution control board alone monitors, one would see a list of twelve pollutants (6 gasses, 3 Particulates, and 3 metals). Trees are great for the environment and act as carbon sinks, temperature buffers, increase water holding capacity of the soil, are home to a wide range of biodiversity including birds, etc. But to expect trees alone to solve our pollution crisis would be unfair. Particularly when we’re dealing with relatively large set of pollutants like the ones mentioned above. Trees cannot absorb all these other pollutants, unlike Carbon Dioxide which is absorbed by the tree.  

Our issue is at present the one of dealing with particulate matter (Specifically at 2.5 micron size), given these have overshot national threshold levels in Bengaluru consistently over the past few years. Trees can remove particulate pollution by intercepting these airborne particles some of which get absorbed into the tissues of the tree, though most particles that are intercepted are retained on the plant surface. PM is removed by plants through a process known as dry deposition. Dry deposition is when particles in the atmosphere deposit themselves on a surface, decreasing the atmospheric concentration of PM. Much of the fine fraction (PM2.5) becomes permanently incorporated into leaf wax or cuticle, while a portion of the coarse fraction is re-suspended as a function of wind speed. The remainder of the coarse fraction is eventually washed off to the ground by precipitation. It is quite clear from the scientific literature that dry deposition of PM occurs, but studies differ on how much PM tree canopies remove.

Illustration of PM removal by trees
Illustration of PM removal by trees
  1. One key parameter controlling how much PM is removed is the concentration of the pollutant: At higher atmospheric concentrations of PM, the rate of dry deposition or absorption is greater.
  2. Another is the leaf area: More leaf area offers more surface area on which dry deposition or absorption can take place.
  3. Finally, the amount of mixing of the atmosphere matters as well, with better mixing associated with more canopy removal of PM.

We also need to keep in mind that trees can clean & cool the air only within a close radius, within about 100 feet radius. So city officials need to plan the tree plantation better in places where population density and air pollution overlap.

In Bangalore we have high exposure due to traffic emissions in near-road environments, which has a big impact on health and well-being for city-dwellers. But typically trees are much higher than what is an average human being’s breathing height. A better solution therefore turns out to be to plant a thick hedge that separates people from the pollution exposure. Strategically placed roadside hedges can play in reducing pollution exposure for pedestrians, cyclists and people who live close to roads.

In Bangalore we have high exposure due to traffic emissions in near-road environments, which has a big impact on health and well-being for city-dwellers. But typically trees are much higher than what is an average human being’s breathing height. A better solution therefore turns out to be to plant a thick hedge that separates people from the pollution exposure. Strategically placed roadside hedges can play in reducing pollution exposure for pedestrians, cyclists and people who live close to roads.

 Walkers and joggers at Agara Lake. Agara Lake is skirted by three large roads and has no hedges between the traffic and the walkway
Walkers and joggers at Agara Lake. Agara Lake is skirted by three large roads and has no hedges between the traffic and the walkway.

Hedges are thicker at the ground level, and act as physical barriers, blocking the pollutants. This is particularly important for vulnerable populations, who spend time regularly in high exposure areas. For example, if your child’s playschool is close to a main road in the city, chances are that they face extremely high exposure while there.

Illustration of hedges and trees removing air pollution
Illustration of hedges and trees removing air pollution

Recent studies have shown that hedges along busy roads can reduce Black Carbon (Black carbon is the sooty black material mostly emitted from diesel engines, and other sources that burn fossil fuel. It makes up a significant portion of PM 2.5) by up to 63%.

6 thoughts on “Hedge Your Bets!

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