Air Quality Monitoring
Kartavya Desk Staff
Source: IE
Subject: Environment
Context: The Supreme Court has sought details on the equipment used in Delhi’s air-quality monitoring stations and whether these instruments are suitable for the city’s extreme meteorological conditions.
About Air Quality Monitoring:
What it is?
• Air Quality Monitoring is the systematic measurement of pollutants in the atmosphere to assess compliance with national air standards, identify health risks, understand pollution sources, and support regulatory actions.
• In India, it is governed by the National Ambient Air Quality Standards (NAAQS), 2009.
Types of Air-Quality Monitoring Devices:
• Continuous Ambient Air Quality Monitoring Stations (CAAQMS):
• Automated, temperature-controlled stations used for real-time monitoring. Track eight pollutants: PM2.5, PM10, NO₂, SO₂, CO, O₃, NH₃, and Pb. Used extensively in Delhi (40 stations).
• Automated, temperature-controlled stations used for real-time monitoring.
• Track eight pollutants: PM2.5, PM10, NO₂, SO₂, CO, O₃, NH₃, and Pb.
• Used extensively in Delhi (40 stations).
• Manual Monitoring Stations (e.g., Gravimetric Samplers):
• Measure pollutants using manual collection methods, especially for PM, metals, benzene, and PAHs. Provide periodic data, not real-time information.
• Measure pollutants using manual collection methods, especially for PM, metals, benzene, and PAHs.
• Provide periodic data, not real-time information.
• Low-Cost Sensors (LCS):
• Compact devices useful for trend analysis, public awareness, and dense spatial mapping. Less accurate; require calibration against reference-grade instruments.
• Compact devices useful for trend analysis, public awareness, and dense spatial mapping.
• Less accurate; require calibration against reference-grade instruments.
How It Works?
• Particulate Matter (PM2.5 & PM10): Measured primarily using Beta Attenuation Monitors (BAM):
• Measured primarily using Beta Attenuation Monitors (BAM):
• A beta radiation source passes through clean filter tape. Air is drawn; particles accumulate; beta signal decreases. Reduction in signal = pollutant mass concentration. Used widely in Delhi under CPCB guidelines.
• A beta radiation source passes through clean filter tape.
• Air is drawn; particles accumulate; beta signal decreases.
• Reduction in signal = pollutant mass concentration.
• Used widely in Delhi under CPCB guidelines.
• Sulphur Dioxide (SO₂): Measured by UV fluorescence; SO₂ emits faint fluorescence under UV light.
• Ozone (O₃): Measured by UV photometry, tracking ozone’s absorption of ultraviolet light.
• Carbon Monoxide (CO): Measured by Non-Dispersive Infrared (NDIR) absorption, based on CO’s absorption of IR waves.
• Nitrogen Oxides (NOx): Measured by chemiluminescence, detecting light produced when NO reacts with ozone.
• Ammonia (NH₃): Measured using optical spectroscopy based on its absorption spectrum.
Key Features of Continuous Monitoring Systems:
• Automated Real-Time Measurement: Provide minute-to-minute readings essential for AQI calculation and forecasting.
• Temperature-Controlled, Dust-Proof Cabins: Ensure instrument stability in diverse environmental conditions.
• Standardised Protocols: Operate under CPCB’s 2012 guidelines ensuring uniform calibration, sampling, and quality-control procedures.
• Remote Data Transmission: Data is relayed to CPCB/SPCB servers and displayed publicly on AQI dashboards.
• Multi-Pollutant Capability: Each station tracks eight regulated pollutants simultaneously.
Limitations:
• High Humidity Interference: Beta-gauge monitors overestimate PM levels when relative humidity >60%, as particles absorb moisture and appear heavier.
• Calibration & Instrument Drift: Infrequent calibration leads to instrument drift, impacting accuracy of PM and gaseous pollutant readings.
• Location Constraints: Stations placed near buildings, trees, or vents face distorted airflow, causing skewed readings.
• Data Availability Gaps: CPCB requires 16 hours of valid data/day; many Delhi stations fail due to power outages, equipment failure, and maintenance issues.