The Delhi AQI Mystery: Two Numbers, One City
After events like Diwali, Delhi's air quality reports often present a confusing picture: official government figures frequently differ significantly from those reported by independent, private platforms. This disparity recently led AAP leader Saurabh Bhardwaj to accuse the Delhi government of manipulating data, claiming private readings sometimes exceeded 3,000 while official averages remained much lower. The truth, however, lies in the diverse methods used to collect, process, and present air quality information.
Different Scales, Different Readings
One primary reason for the discrepancy is the use of different Air Quality Index scales. India's official AQI, reported by the Central Pollution Control Board (CPCB), follows the National Air Quality Index, which ranges from 0 to 500, classifying 401-500 as "Severe." In contrast, many private platforms, such as IQAir, utilize the US AQI methodology. The US system features different pollutant breakpoints and can extend beyond 500 for extremely high pollution levels, meaning a private platform might report an AQI far exceeding the official Indian scale's maximum category.
Averaging Periods Matter
The time over which pollution is measured also plays a crucial role. India's official AQI methodology typically uses 24-hour average concentrations for pollutants like PM2.5 and PM10. Private platforms, however, often provide hourly readings. This difference is significant during sudden pollution spikes, such as those caused by firecrackers. An hourly monitor can quickly reflect a sharp increase, whereas a 24-hour average will incorporate earlier, potentially lower, pollution levels, resulting in a lower overall figure.
Location, Location, Location
Delhi's air quality is not uniform across the city. The official Delhi AQI is a citywide average derived from multiple monitoring stations. Individual stations, particularly those near traffic-heavy roads, construction sites, or industrial areas, can record much higher or lower pollution levels than the city's average. Therefore, comparing a reading from a single highly polluted neighbourhood to the official citywide average can be misleading.
Varying Data Sources and Calculation Methods
The source of data is another factor. The Delhi government's AQI relies on continuous ambient air-quality monitoring stations operated by the CPCB and the Delhi Pollution Control Committee. Private platforms often combine data from these government stations with their own networks of privately operated monitors, including lower-cost sensors. Furthermore, how city-level figures are calculated can vary; the Delhi government might average AQI values from reporting stations, while a platform like IQAir might use the median of hourly aggregated concentration data.
Sensor Technology Differences
The equipment used to measure pollution also contributes to varying results. Government reference stations are designed for continuous regulatory monitoring with high precision. Private networks may employ lower-cost sensors, which allow for broader deployment but can have different levels of uncertainty and be affected by environmental factors like humidity. While these sensors offer valuable local data, their readings need to be interpreted within the context of their technology and methodology.
Focus on PM2.5 for Clarity
For individuals seeking to compare air quality readings, focusing on the underlying PM2.5 concentration (measured in micrograms per cubic metre) can be more useful than the headline AQI score. PM2.5 directly represents the concentration of fine particulate matter, while AQI converts these concentrations into a score based on a specific country's or platform's methodology. When comparing, it's essential to check the PM2.5 concentration, the timestamp of the reading, and the monitoring location.
Navigating Delhi's Air Quality Information
Ultimately, the official Indian AQI from CPCB and the Delhi government's monitoring system remains the relevant reference for national standards. Private platforms offer valuable supplementary information, particularly for real-time and neighbourhood-specific pollution insights. However, these two sources should not be treated as interchangeable dashboards. The simultaneous reporting of an official AQI in the 300s and private readings in the thousands is not necessarily a contradiction, but rather a reflection of which monitor measured it, when it was measured, how the data was averaged, and which AQI formula was applied.