How reliable is city’s pollution data?

Delhi's official air quality index dropped from 366 to 309 between November 2 and 3, but a detailed analysis of monitoring data raises questions about the reliability of pollution readings during one of the city’s most toxic weeks of the year.

- Abhishek Jha

Missing data, suspicious measurement patterns, and algorithmic loopholes in how the city's average AQI is calculated appear to have combined to produce readings that may not accurately reflect ground conditions, an HT analysis of Central Pollution Control Board data shows.

To understand the issue, it is essential to unpack the mechanism to calculate AQI, which contains builtin flexibility designed to ensure readings are available even when some stations malfunction. But this flexibility can be exploited to generate more favourable pollution assessments, particularly when data goes missing during the most polluted hours of the day or when the dominant pollutant ata station switches from one that generates a higher index to one that shows lower readings.

The analysis of 168 hours of data from October 28 to November 4 found that missing station data was not random. With more data missing during polluted hours than clean ones, the net effect would be to make Delhi’s air quality appear better than actual conditions.

What are the loopholes in AQI calculation?

The AQI calculation works through a multi-step process. The 24-hour average concentration of six pollutants and 8-hour average for carbon monoxide and ozone is converted into a sub-index at each of Delhi's 39 air quality monitoring stations. The highest sub-index among the eight pollutants is declared the AQI for that station. The average AQI across the 39 stations becomes Delhi's official reading.

However, the system allows three significant relaxations:

One: An AQI need not be calculated across all 39 stations for the city average.

Two: Only 16 of 24 hours of data is sufficient for calculating the sub-index for a pollutant.

Three: A 24-hour sub-index is not needed for all eight pollutants. If the sub-index is available for three pollutants, and one of them is either PM2.5 or PMO, the station’s AQI can be calculated.