How Delhi’s Air Quality Monitors Work: Methods, Errors & CAG Findings Explained

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• The Supreme Court has asked authorities to justify whether Delhi’s air-quality monitoring equipment is appropriate for the city’s conditions. 
• Delhi currently operates 40 Continuous Ambient Air Quality Monitoring Stations (CAAQMS), each functioning as a compact, automated laboratory housed in a temperature-controlled cabin. 
• These stations, positioned across the city for representative measurement, monitor eight key pollutants — PM2.5, PM10, NO₂, SO₂, CO, O₃, ammonia and lead — as mandated by CPCB’s 2012 guidelines. 

• Inside each dust-proof, air-conditioned unit, racks of analysers, pumps and data loggers process samples drawn through inlets mounted on masts above the station roof.

How Delhi’s AQI Stations Measure Pollutants

• Delhi’s air-quality monitors use specialised, CPCB-approved techniques to measure each pollutant. 
• Particulate matter (PM2.5 and PM10) is tracked using Beta Attenuation Monitors, which gauge how dust collected on filter tape reduces beta-ray transmission. 
• Gaseous pollutants are measured through optical and chemical methods: 
  • sulphur dioxide via UV fluorescence, 
  • ozone by UV photometry, and 
  • carbon monoxide with Non-dispersive infrared (NDIR) absorption. 
    • NDIR absorption is a gas sensing technology that measures the concentration of a specific gas by analyzing how much infrared light it absorbs.
• Nitrogen oxides are detected through chemiluminescence, while ammonia is measured using optical spectroscopy. 
  • Chemiluminescence is the emission of light as a result of a chemical reaction.
  • Optical spectroscopy is a scientific technique that studies the interaction of light with matter to determine a sample’s physical and chemical properties.
• These instrument-based techniques comply with the National Ambient Air Quality Standards to ensure uniform, reliable data nationwide.

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Factors That Distort Air-Quality Readings

• AQI accuracy depends on equipment reliability and the volume of validated data recorded daily. 
• Stations often miss CPCB’s 16-hour data requirement due to shutdowns caused by calibration, power cuts, extreme weather or transmission failures. 
• A recent CAG report found many Delhi stations failed to log complete data or measure key pollutants like lead, weakening daily AQI assessments. 
• Technical issues also distort readings: high humidity inflates particulate measurements, instruments drift without frequent calibration, and poor station siting near buildings or vents skews airflow. 
• Together, these operational and environmental challenges reduce the precision of Delhi’s air-quality readings.

What Research Reveals About PM Measurement Errors

• Multiple studies show that Delhi’s particulate readings — especially from Beta Attenuation Monitors (BAM) — can significantly overestimate pollution under certain weather and loading conditions. 
• A 2021 CSIR–NPL and AcSIR study found that beta gauge accuracy declines sharply when relative humidity (RH) exceeds 60%, causing particles to absorb moisture and appear heavier. 
• The study reported more than 30% overestimation, with bias rising up to fivefold during high-pollution events when particle mass loading is high. 
• Seasonal effects worsen errors, particularly in winter and post-monsoon months.
• Researchers advised using site-specific correction factors, which lowered biases from 46% to below 2%. 
• The U.S. EPA similarly warns that heavy particle accumulation can disrupt airflow and destabilise readings. 
• These issues help explain why Delhi’s stations experienced data dropouts on Diwali night, when sudden pollution spikes overloaded the instruments.

Ensuring Reliable Air-Quality Data: Calibration, Compliance & Oversight

• Calibration and Maintenance Are Crucial
  • Continuous Ambient Air Quality Monitoring Stations (CAAQMS) must follow strict calibration schedules.
  • CPCB’s 2012 guidelines mandate maintaining detailed calibration records for every particulate monitor.
  • Regular checks are essential because even minor instrument drift affects readings — especially for gases measured through sensitive optical methods.
• Major Gaps in Data Reporting
  • The CAG audit exposed serious shortcomings in Delhi’s monitoring network:
    • None of DPCC’s 24 stations measured lead (Pb), despite its mandatory inclusion in AQI calculations.
    • Monthly AQI data was incomplete for 12% of months (2014–2021), meaning many stations failed to produce the minimum required valid data.
• Need to Upgrade and Reposition Stations
  • CAG recommendations include:
    • Relocating stations obstructed by buildings, trees or improper siting.
    • Upgrading or replacing equipment unable to measure all mandated pollutants.
    • Ensuring daily data availability for all pollutants to provide a complete air-quality picture.
• Third-Party Audits for Accountability
  • Experts, including Anumita Roychowdhury (CSE), stress the need for regular independent audits to verify:
    • Whether stations follow CPCB protocols,
    • Equipment calibration accuracy,
    • Data generation and reporting standards.

Source: IE | CNBC