Air quality monitoring in Delhi

Why in News?

The Supreme Court recently asked authorities to explain the equipment used in Delhi air-quality monitoring stations and whether it is suited to the city’s conditions.

• Monitoring Stations - Delhi’s network comprises 40 Continuous Ambient Air Quality Monitoring Stations (CAAQMS).
• They operate under Central Pollution Control Board (CPCB) guidelines issued in 2012.
• Tracking Pollutants - Each station must track 8 pollutants - PM2.5, PM10, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia, and lead.
• Monitoring Methods - Particulate matter is measured using Beta Attenuation Monitors (BAM), which rely on the beta ray attenuation principle.
• A small Carbon-14 or Krypton-85 source emits beta rays through a clean patch of filter tape.
• After air is drawn through and dust collects on the tape, the instrument re-measures the spot.
• Fewer beta rays pass through, and the decline in signal is used to calculate PM2.5 and PM10 concentrations.
• For gaseous pollutants, stations mainly use optical methods because gases absorb or emit specific wavelengths of light.
• Sulphur dioxide is measured by UV fluorescence, which detects the weak glow SO2 emits under ultraviolet light.
• Ozone is measured by UV photometry, which tracks how much UV light it absorbs.
• Carbon monoxide is measured using non-dispersive infrared (NDIR) absorption, based on how much infrared light CO absorbs.
• Nitrogen oxides are measured through chemiluminescence, where instruments detect the faint light produced when the gases react with ozone inside the analyzer.
• Ammonia is measured using optical spectroscopy based on its characteristic absorption of light.
• These methods are approved under the National Ambient Air Quality Standards (NAAQS) of 2009 to ensure comparable data across India.

Recent findings of the study

• Recent scientific studies have examined the reliability of particulate measurements in Delhi, including a 2021 analysis of BAM performance under the city’s extreme meteorological conditions.
• Researchers found that biases in beta gauge readings vary with ambient high RH, particle mass loading, boundary layer height and ventilation coefficient.
• When relative humidity exceeded 60%, PM2.5 measurements by the beta gauge method show more deviation, leading to high mass concentration, with overestimation rising in winter and post-monsoon periods.
• The study reported that more than 30% overestimation was observed especially when RH reaches over 60%.
• During high-pollution episodes, the bias could further increase by a factor up to ~5 when particle mass loading also becomes high.”

Reference

The Indian Express | Air Quality Monitor