Supreme Court Scrutinizes Delhi's Air Quality Monitoring System
Last week, the Supreme Court directed Delhi authorities to examine the equipment used in air quality monitoring stations across the national capital. This judicial intervention comes at a critical time when deteriorating air quality index (AQI) readings have made accurate pollution monitoring increasingly vital for public health protection.
The court specifically asked authorities to explain whether the current monitoring equipment is properly suited to Delhi's unique environmental conditions. This questioning highlights the city's heavy reliance on AQI data to assess daily pollution exposure risks for its millions of residents.
How Delhi's Air Monitoring Network Operates
Delhi's air quality monitoring infrastructure consists of 40 Continuous Ambient Air Quality Monitoring Stations (CAAQMS) strategically distributed across the city. These compact, temperature-controlled cabins function as automated laboratories, operating under guidelines established by the Central Pollution Control Board (CPCB) in 2012.
Each dust-proof, air-conditioned station contains sophisticated equipment including analysers, pumps and data loggers. Sampling inlets are mounted on masts above station roofs to capture representative air samples. According to CPCB requirements, every station must track eight critical pollutants: PM2.5, PM10, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead.
The Science Behind Pollution Measurement
Different pollutants require specialized measurement techniques, all approved under India's National Ambient Air Quality Standards (NAAQS) of 2009.
For particulate matter, stations use Beta Attenuation Monitors (BAM) that employ beta rays from Carbon-14 or Krypton-85 sources. These instruments measure the reduction in beta rays passing through filter tape after air samples collect dust particles, calculating PM2.5 and PM10 concentrations based on signal decline.
Gaseous pollutants are measured primarily through optical methods:
- Sulphur dioxide detected through UV fluorescence
- Ozone measured by UV photometry
- Carbon monoxide analyzed using non-dispersive infrared absorption
- Nitrogen oxides measured through chemiluminescence
- Ammonia tracked using optical spectroscopy
Factors Affecting Monitoring Accuracy
Several technical and environmental factors can compromise the accuracy of AQI readings. The CPCB mandates that stations must collect at least 16 hours of reliable daily data for at least three pollutants, including either PM2.5 or PM10.
However, instruments frequently shut down due to calibration requirements, power fluctuations, or extreme weather conditions, causing stations to miss this crucial data threshold. A recent CAG report revealed that many Delhi Pollution Control Committee (DPCC) stations failed to generate adequate valid data monthly, with several stations not measuring required pollutants like lead at all.
Specific accuracy challenges include:
- High humidity interfering with particulate monitors
- Instrument drift from insufficient calibration
- Poor station placement near buildings, trees or exhaust vents
- Data transmission failures disrupting real-time updates
Scientific Studies Reveal Measurement Biases
Recent research has examined the reliability of particulate measurements under Delhi's extreme conditions. A 2021 study by CSIR-National Physical Laboratory and AcSIR researchers found that biases in beta gauge readings vary significantly with ambient conditions.
The study reported that when relative humidity exceeds 60%, PM2.5 measurements can show more than 30% overestimation, with this bias increasing during winter and post-monsoon periods. During high-pollution episodes, the measurement bias could increase by a factor of up to five when particle mass loading becomes exceptionally high.
Researchers recommended implementing site-specific correction factors to improve accuracy, noting that such corrections could reduce measurement bias from 46% to under 2%. The U.S. Environmental Protection Agency's operating procedures also acknowledge that high filter loading can disturb airflow inside instruments, destabilizing measurements.
Such technical limitations became evident on Diwali night (October 20 this year) when pollution spikes caused data blackouts at several Delhi AQI stations.
Ensuring Data Quality and Reliability
Maintaining accurate air quality data requires strict adherence to CPCB protocols and frequent instrument calibration. The 2012 guidelines emphasize that calibration records for each particulate matter monitor must be meticulously maintained by laboratory staff.
The CAG report highlighted significant gaps in Delhi's monitoring system, noting that none of DPCC's 24 stations were measuring lead levels despite this being a mandatory requirement. The audit found that monthly AQI data was available for less than 21 days during 12% of months between April 2014 and January 2021.
Key recommendations from the CAG report include:
- Relocating CAAQMS to suitable locations or removing obstacles
- Augmenting or replacing equipment that cannot monitor all required pollutants
- Ensuring sufficient data generation for all pollutants from every station
Anumita Roychowdhury, Executive Director at the Centre for Science and Environment and member of the expert group that devised India's National Air Quality Index in 2014, emphasized that regular third-party audits are essential to ensure station efficacy and data reliability.
As Delhi continues to battle severe air pollution, the accuracy of its monitoring systems remains crucial for implementing effective mitigation strategies and protecting public health. The Supreme Court's recent intervention underscores the urgent need to address these measurement challenges and ensure citizens receive reliable information about the air they breathe daily.
