[Prashant Kumar | The Conversation]
In a bid to tackle its record air pollution, Delhi has banned cars from travelling on alternate days over the first two weeks of this year. It’s better than nothing, but it won’t be enough.
Delhi’s landscape, weather, energy consumption culture, and growing urban population combines to elevate concentrations of air pollutants, including the especially harmful ultrafine particles. I recently published a study with colleagues from the UK and India looking at the factors behind Delhi’s air pollution issues. We were interested in why the city’s air is so much worse than in coastal megacities in India and elsewhere.
Coastal cities such as Mumbai have at least a chance to “replace” smog with relatively unpolluted sea breezes, whereas there are limited avenues for flushing away polluted air in landlocked Delhi. In fact, the regions surrounding the capital are sometimes even more polluted than the city itself. Most of Delhi’s brick kilns, for example, are actually found in upwind areas surrounding the city.
The city’s “breathability” is further inhibited by dense built-up areas. This is exacerbated by emissions from late evening and overnight traffic which tends to linger at ground level due to the much cooler air above it.
Pollution intensifies during winter when farmers burn off husks from paddy fields upwind of Delhi. The very light wind speed and colder winter season leads to stable atmospheric conditions that let pollution hang around at “breathing” height.
What can Delhi do about it?
The city’s alternate-day car ban, where vehicles with odd-numbered plates are only allowed to drive on odd dates and vice versa, is intended to keep 20% of cars off the road. After exemptions for women drivers, VIPs, emergency vehicles, disabled drivers and so on, this is equivalent to about 6% of Delhi’s total vehicles.
But expectations may be too high. Even in the best case scenario the concentration of polluting particles in the city’s air would only be halved. “Only”? Well Delhi needs a staggering 30-fold reduction to meet the WHO’s safety limits.
This car ban may help cut emissions of the smallest particles because combustion of fuel in vehicles produces these directly. However, it’s highly unlikely that it will help with the larger particles (“PM10”), which come more from tyre wear and road dust flung back into the atmosphere, and less from tailpipe emissions.
The 6,000 extra compressed natural gas busses introduced during this period to compensate for the banned cars easily will offset the road dust and tyre wear gains.
Delhi’s pollution problem won’t be seriously addressed until ultra-clean electric public transport becomes the norm and, more importantly, the city’s metro system is substantially expanded.
For now, however, people must be protected in whatever ways they can be. Simple remedies such as “greening” unpaved roadside areas through a natural or artificial grass canopy could possibly help in limiting coarse dust particles during dry and windy seasons. Natural measures, such as the development of wetlands and trees are also effective.
There is also a cultural context here. Even the best science and technology won’t reduce emissions and improve air quality by themselves.
It is a complicated, pick-and-mix set of problems that will prove difficult to combat without innovative, encompassing and quick action.
Prashant Kumar, Reader in Wind Engineering, University of Surrey