Highest city air quality index, Tamil Nadu

The highest National AQI among the cities in the geography that appear in CPCB's daily AQI bulletin, for the 24 hours ending 16:00 IST.

Methodology

Source: CPCB's daily National Air Quality Index bulletin, published about 16:05 IST for the 24 hours ending 16:00 IST. Each city row's Index Value is CPCB's National AQI (0–500): for every participating continuous monitoring station CPCB computes a sub-index for each pollutant measured (PM2.5, PM10, NO2, SO2, CO, O3, NH3, Pb; at least three, one of them PM2.5 or PM10), takes the highest sub-index as the station AQI, and reports the arithmetic mean of the city's station AQIs. CPCB itself notes that averaging across locations 'is not a scientifically sound approach' and is done for simplicity. IHO takes the largest Index Value among the geography's city rows; the locator names the city (or cities, when tied). The value is copied from the bulletin, not computed.

Caveats

The maximum depends on how many cities report: a state with more monitored cities is more likely to show a high maximum. CPCB caps the index at 500, so the most polluted days are flattened at 500. Coverage is urban and uneven: only cities with continuous monitoring stations appear, the network is concentrated in the Indo-Gangetic plain, Maharashtra, Rajasthan, Karnataka and Haryana, and several states and union territories have one city or none on a given day. Cities with insufficient data drop out of a day's bulletin and come back, so the set of cities behind a state's figure changes from day to day. The network grew from about 10 cities in May 2015 to about 100 in 2020 and about 250–270 in 2024–26, so long-run comparisons, especially for India as a whole, partly reflect which cities were added (many smaller towns from 2021 onward) rather than a change in the air. A city's Index Value is the 24-hour average to 16:00 IST over whichever of its stations reported that day; some bulletins mark stations whose data ended at 15:00.

Comparability

Comparable day to day within a geography; across geographies and years it is affected by the number of cities monitored. The city behind the maximum can change from day to day.

How signals are computed from this series: methodology.