Pilot study in Mumbai Metropolitan Region demonstrates viability of low-cost air quality sensors

A pilot project conducted across various locations in the Mumbai Metropolitan Region (MMR) has demonstrated the viability of low-cost sensor devices (LCSD) in collecting actionable data on air pollution

A pilot project conducted across various locations in the Mumbai Metropolitan Region (MMR) has demonstrated the viability of low-cost sensor devices (LCSD) in collecting actionable data on air pollution. Experts have proposed that these LCSDs may be used to expand currently deficient monitoring systems in Mumbai and beyond at a fraction of the cost of regulatory-grade continuous ambient air quality monitoring systems (CAAQMS).

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The study was conducted in partnership between Indian Institute of Technology (IIT)-Kanpur and the Maharashtra State Pollution Control Board (MPCB), using a fleet of low-cost devices produced by four indigenous companies: Respirer Living Sciences, Airveda Technologies, Personal Air Quality Systems (PAQS) and Oizom Instruments. While a low-cost monitor can be purchased for about ₹60,000, a regulatory grade device can cost above ₹20 lakh each.

“This is a prohibitively expensive cost, as a result of which our existing framework of CAAQMS is a bit patchy. But we can’t spend all our money on measurement. We also have to invest in mitigation. CAAQMs only give a broad picture of air pollution, while low-cost devices can be used for a more hyper-local picture. But first we needed this study to make sure that the information they provide is reliable,” said Sudhir Srivastava, chairman, MPCB.

Data for the study, ‘Technical Assessment of Low-Cost Sensor based PM2.5 and PM10 Monitoring Network in Maharashtra’ was gathered between November 2020 and May 2021, when 40 such low-cost monitoring sensors were installed alongside or co-located with MPCB’s 15 existing CAAQMS in Colaba, Worli, Sion, Bandra, Vile Parle, Kurla, international airport, Powai, Kandivli, Mulund, Borivli, Vasai, Mahape, Nerul and Kalyan. The low-cost monitors, restricted to capturing data on particulate matter pollution (PM2.5 and PM10), were solar powered and connected to the Internet for real-time data transmission.

The monitors were allowed to operate for seven-months, and the data acquired from them was compared with that of the nearby CAAQMS. The findings reveal that sensors from three start-ups saw a percentage error (measured with respect to true value measured by CAAQMS) of less than 25% for uncalibrated values. After calibration, the error was reduced to less than 15% for three types of sensors and 20% for the fourth type. A range of machine-learning techniques were also used to develop the base calibration model, which experts said can be continuously improved over time to reduce the rate of error produced by low-cost devices.

“It has been a matter of debate whether cheaper tools can be used to complement regulatory-grade devices in India, as has already been done in cities such as Beijing, London and across the US. Our study shows that this is possible, but only when the correct calibration model is applied. Of the 40 low-cost devices we used, more than half showed an acceptable error rate. To be sure, this data is not a substitute for regulatory-grade devices, but can be used by citizens and authorities to take mitigative action with reasonable confidence of accuracy,” said Dr SN Tripathi from the Centre for Environmental Science and Engineering at IIT-K, who led the study.

The experiment also provides insights into the granular nature of air pollution across Mumbai. The average pollution load calculated by the sensor-based network for PM 2.5 concentration across each location in MMR varied largely month by month. For instance, at Vile Parle it was 80 ug/m3 in November, which dipped to 26 ug/m3 in May. Some coastal locations like Colaba too recorded high pollution levels with a PM 2.5 concentration of 56 ug/m3 in December. The monthly mean was the highest at 124 ug/m3 at Kalyan in the month of January.

Commenting on these findings, Dr VM Motghare, joint director (air quality) at MPCB said, “We [Maharashtra] have the highest number of non-attainment cities and macro planning is needed to monitor the implementation of the National Clean Air Programme. So, the data from these low-cost sensors will really help us in identifying the hotspots in the state.”