Delhi’s Winter Pollution Is Rising, But Can Its Air Monitors Capture The Full Picture?

Delhi’s air pollution is climbing as winter sets in, The air quality in Delhi has deteriorated into the ‘severe’ category, with the Commission for Air Quality Management (CAQM) imposing Stage III of the Graded Action Plan or GRAP due to the dangerous levels of air pollution in the national capital.

As pollution worsens, restrictions under the Graded Response Action Plan tighten through four stages. But the readings that inform those decisions depend on where monitors are placed, how many are operating and whether they collect enough data to count towards the daily AQI.

The Rules For Placing AQI Monitors

Central Pollution Control Board (CPCB) guidelines require a monitor’s air intake to sit between three and 10 metres above ground and more than 20 metres from trees. Nearby buildings must be set back by more than twice the height they rise above the intake. A station must have unrestricted airflow on three sides, remain at least 25 metres from household chimneys and 200 metres from unpaved roads, and not be placed in “a confined place, corner or a balcony”.

The rules differ for monitors designed to capture pollution near busy roads. These are positioned beside traffic to measure the air pedestrians may breathe, with the intake three metres above street level to avoid dust thrown up from the road.

Monitors measuring ozone and nitrogen oxides must be placed farther from busier roads: at least 10 metres from a road carrying up to 10,000 vehicles a day and at least 250 metres from one with 110,000 or more.

The placement requirements are intended to ensure monitors reflect local air quality. CPCB calls a site representative if its readings show “the concentrations of various pollutants and their variations in the area”. Trees and buildings can affect airflow, absorb gases or provide surfaces where particles settle.

Delhi-NCR Has 25 Stations Beyond Delhi’s Network

The five major NCR cities around Delhi have 25 stations listed in CPCB’s bulletin: eight in Ghaziabad, seven in Noida, four in Gurugram, and three each in Faridabad and Greater Noida.

Air-quality stations are either manual or continuous. Manual monitors collect samples over 24 hours, twice a week. Continuous Ambient Air Quality Monitoring Stations, or CAAQMS, record readings through the day. The daily AQI is calculated using data from continuous stations.

In Delhi, the network is run mainly by the Delhi Pollution Control Committee, CPCB, the India Meteorological Department and the Indian Institute of Tropical Meteorology.

Population Determines Minimum Coverage

CPCB’s 2019 criteria, developed with state pollution control boards and based on 2011 Census figures, set minimum station numbers according to population. Cities with 100,000 to 500,000 people should have at least four stations, including one continuous monitor. Those with 500,000 to one million people should have six, including three continuous monitors.

Cities with one million to five million people should have at least eight stations, including five continuous ones. For cities with more than five million residents, the minimum is 16 stations, of which 12 should be continuous.

The criteria use the 2011 Census, India’s latest official population count. Several cities are likely to have grown since then. Gurugram had 886,519 residents in 2011, placing it in the category requiring three continuous and three manual stations, according to figures the environment ministry gave Parliament in 2024. The city now has four continuous monitors.

Seven other large cities in CPCB’s bulletin had populations above five million in 2011 when their suburbs were included. Mumbai has 26 continuous stations, while Hyderabad and Bengaluru have 14 each—more than the 12 required for the largest population category. Chennai has nine, and Ahmedabad has 10, including Vatva, which the bulletin lists separately.

Pune has nine stations and Kolkata seven, but the bulletin lists nearby areas separately. Adding Pimpri-Chinchwad’s eight stations brings the Pune area to 17. Kolkata’s wider area has 13 when Howrah’s five and Barrackpore’s one are included.

City Boundaries Alter The Count

The station requirement can change depending on where a city’s boundary is drawn. In a late-2023 filing before the National Green Tribunal, CPCB said Kolkata’s municipal area, home to about 4.5 million people in 2011, needed five continuous stations. For the wider urban agglomeration, including Howrah and Barrackpore and about 14 million residents, it set the minimum at 12. The area had 11 stations then and now has 13.

In 2019, the NGT recorded that India needed about 800 more continuous stations and 1,250 additional manual monitors under CPCB’s criteria.

The Commission for Air Quality Management has also set area-based targets for Delhi-NCR. In May, it said Delhi and adjoining cities—Ghaziabad, Noida, Greater Noida, Gurugram, Faridabad and Sonipat—should have “one monitoring station … per 25 sq. km.” That is one station for each five-kilometre-by-five-kilometre grid. Other NCR cities and district headquarters are to have one station per 50 square kilometres.

In October, Union environment minister Bhupender Yadav said Delhi’s expansion followed a five-by-five-kilometre grid and that the same approach would be extended to other NCR cities.

CAG Raised Concerns Over Delhi Station Sites

The Comptroller and Auditor General inspected 13 continuous monitoring stations in Delhi in September 2020. It found that all were close to trees on more than one side. Two stood beside heavily trafficked roads, and three were near tall buildings and construction sites.

In its 2022 report, the CAG said the locations indicated “possible inaccuracies in the data generated by these CAAQMSs, rendering the AQI values unreliable”. Delhi’s environment department had said in October 2021 that it would work to comply with CPCB’s norms. The audit covered the period up to March 31, 2021; the report was tabled in the Delhi Assembly on April 1, 2025.

A joint DPCC-CPCB committee found in 2022 that most stations met CPCB’s siting rules, according to an environment department action-taken report cited by the Delhi Assembly’s Public Accounts Committee. The committee flagged Anand Vihar, RK Puram and Jahangirpuri, but the stations were not moved. CAQM cited their location in densely populated, polluted areas and the need to preserve long-term data.

In March 2026, the committee said several stations still did not fully meet CPCB’s rules and recommended that the department address the remaining issues.

Data Gaps Affect Daily AQI

A station’s readings count towards the daily AQI only if it records at least three pollutants for 16 hours each. One must be particulate matter—PM2.5 or PM10, particles measuring 2.5 or 10 micrometres across or less.

The number of stations contributing to a city’s AQI can therefore vary from day to day. On November 2, Bengaluru’s AQI was calculated from four of its 14 stations. CPCB’s bulletin reported “insufficient data” for three NCR towns: Baghpat, Bahadurgarh and Jind.

Delhi’s continuous monitoring network has expanded from 18 stations in April 2017 to 38 by March 2021 and 59 in the current CPCB bulletin. CAQM has proposed 46 additional stations across NCR: 14 in Delhi, 16 in Haryana, 15 in Uttar Pradesh and one in Rajasthan. In May, it urged NCR states to install them promptly.