First-of-its-kind ultra fine emission inventory reveals top polluted hotspots in Mumbai

“Our study introduces a first-of-its-kind ultra-fine surface emission inventory for a megacity like Mumbai, compiling data with 2020 as the base year. This detailed dataset encompasses seventeen major and minor sources of eight pollutants, providing a basis for targeted mitigation actions under the National Clean Air Programme (NCAP),” said Mangaraj.

The eight pollutants include particulate matter (PM2.5 and PM10), carbon monoxide (CO), nitrogen oxides (NOx), sulphur dioxide (SO2), volatile organic compounds (VOCs), black carbon (BC), and organic carbon (OC), all of which contribute to air quality deterioration. “While other pollutants such as CO, SO2, and OC warrant attention and can have adverse effects on air quality and public health, PM2.5, PM10, BC and NOx stand out as concerns for Mumbai due to their significant contributions to air quality index and associated health impacts,” said Sahu.

Sources and the scale of emissions in the megacity

The inventory showed annual emissions levels with PM2.5 at 44 gigagrams per year (Gg/yr) and PM10 at 72.5 Gg/yr illustrating the presence of particulate matter — small pollutant particles, ranging from 2.5 microns to 10-micron size, that can easily enter the lungs and cause health ailments — contributing to Mumbai’s air quality concerns. “In a densely populated and urbanised region such as Mumbai, sources such as vehicular emissions, road dust, industrial activities and open burning of municipal solid waste contribute to elevated levels of PM10 and PM2.5, making it a major concern for air quality management,” said Sahu.

To contextualise the severity of PM emissions, as of 2022, the United Kingdom (UK) government reported PM10 emissions of approximately 127 thousand tonnes (or 127 Gg/yr) and PM2.5 emissions of around 65 thousand tonnes (or 65 Gg/yr). Annual emissions of PM10 and PM2.5 have decreased over time due to strict regulatory measures and changes in industrial practices in the UK.

In the study, source-wise contributions to PM10 and PM2.5 were identified from various sectors. For PM10, the major sources include windblown road dust (19%), road transport (20%), and industrial activities (19%). Residential sources also contribute 14% to PM10 emissions. On the other hand, PM2.5 emissions are predominantly from road transport, (~30%), followed by industrial sources (22%), and residential emissions (10%). Other sources include thermal power plants, windblown dust, and various minor contributors such as municipal and household waste burning.

Meanwhile, annual emissions for other pollutants in Mumbai showed CO levels at 359.7 Gg/yr, NOx at 175.6 Gg/yr, SO2 at 132.9 Gg/yr, VOCs at 223.4 Gg/yr, black carbon at 13 Gg/yr, and organic carbon at 12.5 Gg/yr. High levels of CO can lead to cardiovascular diseases, NOx contributes to respiratory issues, SO2 can aggravate asthma, VOCs are linked with respiratory irritation and organ damage, black and organic carbon affects lung function and contributes to respiratory symptoms.

Mangaraj added that in addition to contributing to the formation of ground-level ozone, NOx is an excellent precursor. “In Mumbai, the high density of vehicles and industrial activities leads to significant NOx emissions, contributing to air pollution. Addressing these pollutants through targeted emission reduction strategies and air quality management measures is crucial for improving overall air quality and safeguarding public health in the city,” she said.

Mumbai’s 10 "critical" polluted hotspots

Ten hotspots were highlighted in Mumbai, each characterised by unique sources contributing to local air pollution, out of 100 hotspots across MMR assessed under the larger framework of the study. The 10 hotspots have multiple sources in each area and vary in their primary sources of emissions, which include traffic, industrial activities, power generation, and waste burning.

“By analysing the spatial emission data, we have identified potential pollution hotspots where the geographic information systems (GIS) based spatial techniques are used to visualise the emissions pattern across the city and highlighted sectors dominating and influencing air quality in each region,” said Beig.

Worli is significantly impacted by traffic emissions and nearby industrial activities. Goregaon experiences high emissions from road transport and area-based commercial activities. In Borivali (West), vehicular emissions and commercial zones are major contributors. Industrial emissions and some residential burning contribute to poor air quality for Chembur. Bandra East suffers from a combination of traffic congestion and emissions from small-scale industries. Trombay is heavily dominated by emissions from large-scale industries and thermal power plants. At RL Kelkar Marg (Sion), both vehicular traffic and industrial activities contribute. Shivaji Nagar's (Govandi) air quality is predominantly influenced by industrial outputs, waste burning, and road traffic. RC Marg (Mahul) is affected by high emissions from local industries and vehicular movements. Lastly, Mulund (East) is notable for its local industrial emissions and the presence of road dust.

In Mumbai, the transport sector and windblown road dust, powered by a vehicle count of 3.8 million and a large amount of time spent in vehicular travel, stand out as major contributors to hotspot pollution. With 1,479 registered industrial units, power generation is dominated by the coal-fired Trombay Thermal Power Station, which has high annual coal consumption (approximately 2.4 million tonnes from 13 units). The household sector contributes through the use of various fuels, with a notable portion of the population (20.4 million) still relying on kerosene, wood, and cow dung. The city's slums (almost 2,500 clusters), where approximately 41.3% of the population resides, also add to the pollution from similar domestic fuel usage. Other sources include street vendors, crop residue burning, diesel generators, aviation, and municipal solid waste burning. Most importantly, the construction sector adds to the particulate matter (PM10 and PM2.5) load with the use of materials including brick kilns and concrete.

“We found that apart from traditional sources like industry, transport sector, windblown road dust, these hotspots need abrupt mitigation measures, especially towards the pollution from emerging sources such as municipal solid waste burning, and slum cooking which pose a challenging threat to the megacity. They require ward-wise strategies to reduce emissions. Improving waste management practices, enhancing green spaces, and promoting non-polluting cooking fuel are needed urgently,” said Mangaraj.

Impact on public health and policy support

The inventory paints a worrying image indicating a potential public health crisis, said the authors. "Residents of Mumbai are exposed to substantial health risks. This is a critical juncture for public health policy. By weaving scientific evidence into policy measures, we can protect the health of citizens and enhance their overall quality of life," said Beig.

Exposure to elevated PM10 and PM2.5 levels could subject Mumbai's residents to a gamut of health issues, especially those affecting respiratory and cardiovascular health, and could even lead to premature mortality, Beig said. "NOx emissions heighten the risk of respiratory conditions, including asthma, and impair lung function, while VOCs are known precursors to air toxins with carcinogenic potentials," he added.

Meanwhile, the inventory emerges as a tool for policymakers with source-wise details for each hotspot in the city. "For a non-attainment city, identifying and acting upon emission hotspots is crucial through a ward-wise pollution abatement approach," said Sahu. The NCAP's goal is to reduce PM concentration by 40% by 2026 from 2017 levels.

Comparative analysis with previous studies

When compared to the last comprehensive study by the National Environmental Engineering Research Institute (NEERI) in 2009-10, this new inventory shows an alarming rise in certain pollutants, reflecting the increased industrial activity, vehicular emissions, and urban sprawl across MMR over the last decade.

NEERI’s assessment continues to be the baseline followed by the Brihanmumbai Municipal Corporation (BMC) and Maharashtra Pollution Control Board (MPCB). However, a senior official from MPCB said, “There has been a more in-depth air quality assessment, emissions inventory and source apportionment study for Mumbai by NEERI developed last year but the findings are still being analysed by us and it is yet to be made public.”

Beig said that the findings from the present study introduce an ultra-fine (approximately 400 metres by 400 metres across MMR) surface emission inventory covering 17 sources and eight pollutants with 2020 as the base year, while NEERI's emission inventory was at a very coarse resolution (2km by 2km) with limited activity data for sectors conducted for the base year 2007, focusing on five pollutants such as PM10, CO, SO2, NOx, and HC.

“Discrepancies between the two studies primarily stem from variations in base years, technological emission factor used, the wide spectrum of sources considered, and spatial resolution of information developed. Also, over a gap of 13 years, there have been substantial changes in the urban landscape and the proliferation of air pollution sources in Mumbai,” he said.

“Our approach allows for a more precise identification of pollution sources by integrating micro-level activity data for both organised and unorganised sources and employing a bottom-up approach, we have been able to pinpoint emissions down to specific locales within the city, a first in the region's air quality studies,” said Sahu.

Roadmap for mitigation measures

The inventory brings to the fore pollution abatement strategies for Mumbai. Phasing out older vehicles, especially the "super-emitters" — aged heavy commercial vehicles (HCVs), light commercial vehicles (LCVs), and diesel buses, could make a substantial dent in vehicular emissions, the study said. “Such a move would address the disproportionate pollution these vehicles contribute,” said Mangaraj.

Enhancing public transportation is equally important. "By improving public transit facilities and incentivising their use, we can reduce reliance on two-wheelers and commercial cars, addressing transport emissions," added Sahu. Moreover, introducing flexible or staggered working hours and remote work policies could lessen traffic congestion, lowering emissions from transport.

Small-scale industries spread across regions such as Bandra East, Dharavi, BKC and isolated patches across South Mumbai require relocation and stringent emission controls. "Continuous monitoring of emissions from relocated industries is non-negotiable. For large-scale industries, this can be done if the continuous air quality monitoring systems (CEMS) are properly in place," said Beig. The Trombay thermal power plant needs targeted intervention. "Substituting a portion of coal with crop waste for power generation can cut down emissions while reducing coal dependency," said Beig.

In Mumbai's densely populated slum areas, a switch to cleaner cooking fuels could markedly improve air quality. "Addressing the widespread use of solid fuels in slums is critical for reducing domestic emissions," added Sahu.

Efforts to curb dust from construction and infrastructure development are another key area for mitigation. "Converting unpaved roads to high-quality paved surfaces, while ensuring groundwater percolation, coupled with regular maintenance, can significantly reduce dust," Mangaraj recommended.

Taken together, these targeted actions are steps towards safeguarding the health of Mumbai's residents and restoring the city's environmental integrity. Each measure represents a piece of the puzzle in Mumbai's overarching strategy to combat air pollution and promote public health.