‘For Mumbai pollution, construction dust, unfavourable meteorology obvious culprits but can't rule out climate change’

While Delhi has remained India's air pollution capital, the country’s tinseltown Mumbai on the Konkan coast enjoys relatively clean air most of the year. But on Friday morning, residents of Mumbai also woke up to a hazy sky.

The city’s overall air quality index (AQI) was 171, according to the System of Air Quality and Weather Forecasting and Research data, a few points short of slipping into the ‘poor’ category. This was worse than the AQI of the previous couple of days.

The city’s PM2.5 concentration on Friday was about 17 times the safe value recommended by the World Health Organization, according to IQAir, a Swiss air quality information platform.

But this is not the first time Mumbai experienced a period of unhealthy air quality. Mumbai's PM2.5 concentration has more than doubled this year compared to 2019, a new report showed.

This is despite the vast Arabian sea that surrounds the city on three sides, orchestrating a system of moist breezes that can cleanse the onshore air, and the lush green hill stations just a few hours away, where the air quality is significantly better.

So, what has caused the coastal city that so far didn't have to worry about winter pollution, to gasp for breath the past few years?

Like Delhi, Mumbai’s civic authorities have also rolled out a slew of measures to ease the pain for residents. For instance, they aim to fit 350 BEST buses with vehicle-mounted air filters, set up “virtual chimneys at traffic congestion spots, special streetlights and air purification systems in select gardens”, according to a report in the newspaper The Times of India.

But how effective are these? Just as much as a bandage is on a bullet wound, said Abhishek Chakraborty, assistant professor in the environmental engineering department of the Indian Institute of Technology Bombay (IIT Bombay).

Down To Earth spoke to the scientist, who has been working on source apportionment of environmental pollution, real-time air pollution monitoring and control strategies, to understand what has caused the haze in Mumbai and what can be done to improve the air quality.

His immediate prescription: Data gaps in the national emissions and air quality databases need to be fixed and scientific modelling should precede any policy decision to control pollution, so that funds are not wasted. Edited excerpts:

Preetha Banerjee: What is causing the smog in Mumbai?

 

Abhishek Chakraborty: Metrology is a big factor. In the post-monsoon winter time, a condition of stagnation prevails across India because of boundary layer suppression, onset of winter and slowing of wind speeds among others.

In Mumbai, the metro construction has gathered speed over the last two years since post-COVID-19 lockdowns. Because of that, a lot of Mumbai is actually virtually becoming a kind of a construction site. This is on top of the multiple roads and flyovers being built or modified. These construction activities generate a lot of pollution and can be a reason for the spike this year.

But this scale is still unexpected because, unlike landlocked Delhi, we have a sea that can mitigate this effect. So, I cannot rule out the role of climate change either.

PB: Which areas in the city are particularly affected?

AC: I think the entire city is affected. Powai, where the IIT Bombay campus is situated, is supposed to be one of the cleaner areas. But still I'm seeing that AQI in Powai is also consistently over 150 to 200. So, more congested areas like Andheri or Bandra, or those which are more inland and away from the coast, may be the MMR region and Navi Mumbai, may be more polluted.

PB: Since when has the air quality in Mumbai noticeably worsened?
AC: From 2021 onwards, there has been a continuous uptick in concentrations of air pollutants, with a continuous spike in April and November. But this year, it is particularly pronounced.
The AQI has been going up in a pretty accelerated manner.

PB: So far we understood that Mumbai has enjoyed a geographical advantage because of the sea. But why has it been ineffective this year?

AC: My hunch is that climate change has impacted the flushing abilities of the sea steered by the land and sea breeze system. During day time, the hotter, more polluted air over land rises up and is replaced by sea breeze, which is cleaner. Similarly, at night, pollutants in the air are flushed offshore as land breeze moves towards the sea.

So in both the scenarios, the air quality should improve. But I think this sea-land air breeze engine has become weaker. This can be because the ocean surface is also getting very warm due to climate change.

Then, we know that the ocean also has a lot of sea salt and biogenic, biological material.
So maybe the evapotranspiration or the sea salt generation rate has increased. And so that is also contributing to some extent because when sea salt interacts with pollutants, then they produce many secondary results.

Moreover, plants emit a lot of biogenic Volatile Organic Compounds at higher temperatures, which can again react with different atmospheric oxidants, producing ozone and different secondary particulates. These fine particles are very efficient at causing smog. So I think climate change is likely playing a role as well although more research is needed for a conclusive attribution.

PB: How efficient is the region’s air quality monitoring and data availability?

AC: I think the crux of the problem lies here. First, the monitoring network is inadequate, although it may be enough according to the spatial distribution recommended by CPCB, however, spatio-temporal resolution is quite poor for a vast and diverse city like Mumbai.

Mumbai has 21 monitoring stations for its 23 million population. Some more stations are proposed, but the number is still low.

The other thing is, what is the point of this monitoring network, even if you make it more sophisticated or more widespread, if nobody is there to assess the data and monitor the trends?
This data should be used to build policy and also keep tracking its effectiveness. Then, go back to the discussion table and figure out the flaws in a particular policy and refine it.

But right now, in Delhi and in every other city, what we are getting is basically a knee-jerk reaction in the name of policy, just because there is a lot of noise around it, and the authorities feel the urge to do something.

But what is the scientific basis for this? For example, the proposal to fit 350 buses with air purifiers, how did they arrive at that number? What is the specification of the purifier? How much air is it going to suck in and throw out? Then, the odd-even policy of Delhi.

Is there any scientific modeling study that shows that this is going to have a significant impact?

The data available has many problems too. First, Continuous Ambient Air Quality Monitoring Station (CAAQMS) data is public but the Continuous Emission Monitoring System (CEMS) which collects industrial pollution data is not public. Why not? And who is auditing the CEMS and CAAQMS? Are they properly calibrated and maintained?

My group analyses CAAQMS data and we experience tonnes of issues with it. Like there are many times when the data is just static, without variations for hours or days. This isn’t possible.
Then suddenly there is a peak and that peak continues for hours.That peak has so high value that maybe something happened and that instrument kept on reading the same value. And then there is data missing for the entire hours or days and the complete data set is blank when you download it. Then sometimes, we see the variation is minimal, which again is contrary to what should be happening in the atmosphere.

PB: What is your suggestion?

AC: A city like Bombay or any large urban space, what we need to do is, first of all, decongest vehicular traffic. Traffic is terribly slow in Mumbai and whenever these vehicles slow down or are waiting at the traffic junction, they emit more than when they are running at higher speeds.

So, once you decongest, you can get rid of this additional emission.

Then, the pollution certificates of cars are a big scam in the city. A polluted vehicle can emit five to 10 times more than a well-maintained vehicle.

We have to put in place checks and balances so that people cannot cheat the system. There are now thermal cameras available which can capture the emission temperature at the traffic junction and RTO officials could check if there is a mismatch with the certificate. And in this, the onus is on the people also because the pollution board cannot go after every car individually.

Make high spatio-temporally-resolved digitised emission inventory and keep on updating the same on regular intervals. This will aid in making effective policies and keep track of the progress.

Then, more research is needed to tackle secondary pollution, which is caused by some primary emissions. We should specifically target those primary emissions that have more potential to produce pollution after atmospheric reaction. That part is completely missing.

Finally, we have to understand that we are playing a long-term game. The United States Environmental Protection Agency used all their financial and technical might – they employ more than 10,000 PhDs – and even then, took 40-45 years to clean up the US air.

For India, too, we need that kind of thorough planning and methodical approach, for which scientific rigour is of utmost importance. The National Clean Air Programme has onboarded academic institutions. So, why the policy makers and regulators take advantage of that?

For instance, IIT-Bombay is handling three cities in Maharashtra. What is supposed to happen is, the pollution control board and ULB will take into consideration our suggestions on the monitoring network or policy, how to seek the policy effectiveness and then modify it, refine it.
This has to be done in a regular and systematic manner.

But in reality, the interaction is very minimal and irregular. I think there has to be a system inside the NCAP to manage funding depending on policy efficacy or interactions with academics. May be the institutions should also be financially empowered aprt from ULBs and regulatory bodies..

PB: What are some of the projects you are working on?
AC: We have research planned on the effectiveness of metro rail, CNG and EV in curbing air pollution. We are also working on the secondary pollution that how this primary and secondary
pollution interacting. Some of the models may show some light on this climate change and pollution issue also.

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