Delhi-NCR hit by summer ozone once again, but levels lower than previous summers: CSE analysis

New Delhi and South Delhi areas worst affected by ground-level ozone. Central Delhi and Gurugram face worsening crisis 

The region in the grip of multi-pollutant crisis  

  • Ground-level ozone has exceeded standards nearly on all days of summer (March-April)
  • While geographical spread of stations reporting ozone exceedance is lowest in past five years, duration of exceedance in specific stations on the rise
  • Ground-level ozone hotspots located in areas with low levels of NO2, CO and PM2.5. Ground-level ozone has become a yearlong problem in the region
  • Summer air a toxic cocktail of gases and particulates – particulate, ozone, nitrogen oxides and even high CO pollution
  • This demands urgent policy focus on ozone mitigation and stringent action on vehicles, industry, power plants, and all other combustion sources that emit gases that form ozone.

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New Delhi, June 4-5, 2023: On the eve of the World Environment Day, Centre for Science and Environment (CSE) has issued an alert on rising ozone pollution and multi-pollutant crisis that Delhi and the National Capital Region are facing during summers. If unchecked, this can become a serious public health crisis in coming years. 

CSE has continuously alerted about the growing problem of ground-level ozone. “The policy and public attention that is nearly fully drawn towards particulate pollution, has neglected mitigation of toxic gases. Inadequate monitoring, limited data and inappropriate methods of trend analysis have weakened the understanding of this growing public health hazard. Learn from the advanced economies that after controlling particulate pollution have fallen into the grip of rising NOx and ozone crisis. India should prevent this trap. But the standard practice of Central Pollution Control Board to average out the data of all stations to determine daily AQI cannot capture the public health risk from this short-lived and hyper-localised pollutant. This underestimates the severity of the local build up and high toxic exposures in the hotspots,” says Anumita Roychowdhury, executive director, research and advocacy, CSE.

“Due to the very toxic nature of ground-level ozone, the national ambient air quality standard for ozone has been set for only short-term exposures (one-hour and eight-hour averages), and compliance is measured by the number of days that exceed the standards. Compliance requires that the standards are met for 98 per cent of the time of the year. It may exceed the limits on two per cent of the days in a year, but not on two consecutive days of monitoring. There should not be more than eight days in a year when the ozone standard is breeched, and not on two consecutive days,” says Avikal Somvanshi, senior programme manager, Urban Lab at CSE.

Why ozone needs special attention? Health evidence suggest that ozone is emerging as a serious public health issue in India. The 2020 State of Global Air report states that age-standardized rates of death attributable to ground-level ozone is among the highest in India and the  seasonal 8-hour daily maximum concentrations have recorded one of the highest increases in India between 2010 and 2017 – about 17 per cent. This requires deeper understanding of what is going on in different cities and regions to inform mitigation. 

Complex chemistry of ground-level ozone makes it a difficult pollutant to track and mitigate. Ground-level ozone is not directly emitted from any source. It is produced from complex interaction between nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon monoxide that are emitted from vehicles, power plants, factories, and other combustion sources and undergo cyclic reactions in the presence of sunlight to generate ground-level ozone. VOCs can also be emitted from natural sources, such as plants. Ozone not only builds up in cities but also drifts long distances to form a regional pollutant that makes both local and regional action necessary. This not affects public health but also crop production and food security. 

This highly reactive gas has serious health consequences. Those with respiratory conditions, asthma, chronic obstructive pulmonary disease, and particularly children with premature lungs and older adults are at serious risk. This can inflame and damage airways, make lungs susceptible to infection, aggravate asthma, emphysema, and chronic bronchitis and increase the frequency of asthma attacks leading to increased hospitalisation. 

The investigation: This assessment has traced trends during summer (March-May) between 2019 to 2023 May (up to May 30th). The analysis is based on publicly available granular real time data (15-minute averages) from the CPCB’s official online portal Central Control Room for Air Quality Management. The data has been captured from 58 official stations under the Continuous Ambient Air Quality Monitoring System (CAAQMS) spread across Delhi-NCR. Delhi (40), Gurugram (4), Faridabad (4), Noida (4), Ghaziabad (4), and Greater Noida (2). This analysis tracks exceedances at each station in core NCR. Breach of the standard by even one station is considered exceedance. Days with multiple stations exceeding the standard indicates the severity of the spatial spread and number of people exposed. Given that the data is capped at 200 µg/m3 by CPCB, it is not possible to determine how high the concentration really goes. 

The study has considered global good practice and taken on board the USEPA approach of computing eight-hour averages for a day and then checking for the maximum value among them to capture the daily ozone pollution level. USEPA assesses city-wide or regional AQI based on the highest value recorded among all stations of the city or the region. Thus, trends have been calculated in terms of number of days when the daily level has exceeded the 8-hr standard (referred as exceedance days hereafter).

While analysing the data it has also been noted that the ozone data available on CPCB portal never exceeds 200 μg/m3, while data for the corresponding time on Delhi Pollution Control Committee may show higher levels. Therefore, due to this capping of data it is not possible to understand the nature of peaking in the city. This needs to be addressed as there are two sets of standard for ozone – 8-hourly standard of 100 μg/m3 and one hourly standard at 180 μg/m3. Capping makes assessment of one-hourly standard challenging. 

Key highlights of the analysis 

Ground-level ozone exceedance is reported on nearly all days of summer: This summer ground-level ozone exceedances were reported on 87 days between 1 March and 30 May. Over the last five years it has been noted that ground-level ozone has been all season problem but it is exceptionally worse in the months of April and May. This summer has relatively less battered by heatwaves compared to previous few summers and this is reflected in limited geographical spread of ground-level ozone exceedance. The dangerous build-up of ground-level ozone can happen anytime during the year, but it is usually in small pockets during non-summer months. For it to have wider spatial spread hot and sunny weather conditions are needed which are generally present in summer – especially during April-May. 

Geographical spread of ground-level ozone pollution in Delhi-NCR during March-April lowest in past five years, but the duration of the exceedance on the rise: Ground-level ozone usually exceeds the safety standard on all days of summer in some location in Delhi-NCR every year. The spatial spread (number of stations exceeding the standard across the core NCR) has been lower this year. On an average 10 stations have exceeded the standard daily this summer, which is 33 per cent lower from the mean of previous four summers. 

Even though, the spatial spread of ground-level ozone has decreased this summer, its duration has increased. This summer, at the stations which reported exceedance the rolling 8-hr average stayed above standard for 4.9 hours on average, which is up from 4.6 hours observed the last summer. 

New Delhi and South Delhi neighborhoods are worst affected by ground-level ozone pollution: Nehru Nagar in south Delhi is the most chronically affected in the core Delhi-NCR. It has exceeded the standard in this location for 75 days this March-May. It is followed by Sri Aurobindo Marg, Dr KS Shooting Range and Mandir Marg as the worst polluted. 

Ghaziabad, Gurugram and Greater Noida are also seriously affected by the ground-level ozone pollution. Faridabad has least instances of ground-level ozone exceedances in the region. The locations that have recorded the most ground level ozone exceedence include Nehru Nagar, Sro Aurobindo Marg, Dr KS Shooting range, Mandir Marg, Alipur and Patparganj in Delhi. Sanjay Nagar and Vasundhara in Ghaziabad, Gwal pahari in Gurugram, and Knowledge Park in Greater Noida. 

Locations with the lowest ground-level ozone pollution in the core Delhi-NCR include Punjabi Bagh, Ashok Vihar, Siri Fort, Chandni Chowk, North Campus, and Pusa in Delhi. Sector 16A and 11 and New Industrial Town in Faridabad, and Sector 1 in Noida. 

Central Delhi and Gurugram are facing worsening trend: Mandir Marg in New Delhi registered highest increase in number of exceedance days compared to the average of last four summers. It registered a jump of 63 additional exceedance days. It was followed by Gwal Pahari in Gurugram and Patparganj in East Delhi, both registered an increase by 51 eceedance days. 

Sector 16A in Faridabad and Sirifort in Delhi registered most reduction in exceedance days compared to average of previous four summer. Their exceedances were down by over 40 days this summer. JLN Stadium, Narela and Sonia Vihar in Delhi and Indirapuram and Loni in Ghaziabad were other locations that registered significant improvenment. The locations that have recorded the most increase in ground level ozone include Mandir marg, Alipur, Burari Crossing, CRRI Mathura Road in Delhi. Gwal Pahari in Gurugram; Sanjay Nagar and Vasundhara in Ghaziabad and sector 116 in Noida. 

However, the locations that show the most improvement in ground level ozone exceedance days include Siri Fort, JLN Stadium, Narela, Sonia Vihar, Bawana, Ashok Vihar, and Dwarka Sector 8 in Delhi; Sector 16A in Faridabad; and Indirapuram and Loni in Ghaziabad. 

Ground-level ozone hotspots are located in the areas with low levels of NO2, CO and PM2.5: The spatial distribution of ground-level ozone is inverse of the NO2, CO and PM2.5. Mandir Marg and Nehru Nagar in Delhi and Sanjay Nagar in Ghaziabad are exception to this phenomena as these stations report both high NO2 and ground-level ozone. Hotspots for CO and PM2.5 are completely distinct from the hotspots for ground-level ozone. This bears out the fact that while ozone is created in polluted areas with nitrogen oxide being the catalyst, it also gets mopped up in high NO2 areas as it further reacts. But the ozone that escapes to cleaner areas with less NO2 builds up faster as unavailability of NO2 hampers its dissipation. Locations with the highest NO2 pollution in the core Delhi-NCR include Anand Vihar, ITO, Mandir Marg, East Arjun Nagar, Nehru Nagar, Patparganj, Okhla Phase II in Delhi; Sector 125, Noida, and Knowledge Park Greater Noida.  

Regional hourly ozone peak level is down by 24 per cent compared to lockdown times but many stations have breached CPCB’s cap this summer: Since CPCB caps the data at 200 µg/m3 it is not possible to access precisely how high the ground-level ozone concentration can go up to, but for to get a relative understanding in this study hourly data averaged across all station and all days of May has been analysed. This indicative analysis shows that compared to May of 2020 ground-level ozone is not lingering in the air post sunset and the hourly peak is also on an average down by 24 per cent. The re-emergence of morning and evening rush-hour traffic is helping in neutralising ground-level ozone at sunrise and sunset as increased NO2 levels cannibalise it. 

The maximum 8-hour average at Patparganj in Delhi hit 191 µg/m3 on 5 May 2023. This was the highest level recorded at this location ever and also the highest this summer among the stations of the core NCR. In previous summers 190 µg/m3 has been breached by stations at RK Puram, Aya Nagar, Dr KS Shooting Range, Sri Aurobindo Marg and Nehru Nagar. These stations registered peaks in 180 µg/m3 range this summer as well. Knowledge Parks in Greater Noida are peak hotspots outside Delhi.  The locations with highest daily peak ground-level ozone pollution in the core Delhi-NCR include Patparganj, Nehru Nagar, Mandir Marg, Alipur, Mundka, Sr Aurobindo Marg, Dr KS Shooting range; Knowledge Park III -Vin Greater Noida; and Sanjay Nagar in Ghaziabad. 

Given the data cap of 200 µg/m3 enforced by CPCB at the 15-minute granularity, it is stunning to observe that multiple stations are still logging 8-hourly averages exceeding 180 µg/m3 every year. This underscores the magnitude of the pollution. 

Night-time ground-level ozone continues to persist: Ground-level ozone should ideally become negligible in the night air but Delhi-NCR has been witnessing a rare phenomenon where ozone levels remain elevated hours after sunset. This was found to be very wide-spread during the lockdowns of 2020 summers and it continues to linger this summer as well. This May night-time ozone was noted on 28 days with 3 stations on average reporting it every night. Night-time ozone has been considered when hourly concentration has exceeded the level 100 µg/m3 between 10PM and 2AM at any station. Night-time ozone is most frequently in East Arjun Nagar in East Delhi where it was reported on 42 nights this summer. Vasundhara in Ghaziabad, Knowledge Park III in Greater Noida, Gwal Pahri in Gurugram and Sector 116 in Nodia also reported most instances of night-time ground-level ozone. 

The locations with most night-time ground-level ozone pollution instances include East Arjun Nagar, Mandir Marg, DTU and Alipur in Delhi; Vasundhara in Ghaziabad; Gwal Pahari and Vikas Sadan in Gurugram; Sector 116 and Sector 125 in Hoida; 

Ground-level ozone has become a yearlong problem: Even though the ground-level ozone exceedance is the worst during summer months, it remains a year-long problem as a few locations continue to record exceedance throughout the year. There have been only nine days this year so far that have registered no exceedance among any air quality monitoring stations of the core Delhi-NCR. Foggy and cold conditions of January conventionally inhibit formation of ground-level ozone but ozone was found to be exceeding at multiple stations on 26 days this January. On the annual scale, last year exceedance were reported on 334 days, they were only 312 days in 2021, 304 days in 2020 and 286 days in 2019. 

Summer air is a toxic cocktail for multiple pollutants - high particulate, NO2 and CO pollution: It is not just ground-level ozone pollution that is poisoning the summer air, significant levels of PM, CO and NO2 are noted as well. Seasonal level of both PM10 and PM2.5 is above their respective 24-hour standard, while CO and NO2 levels are under the daily standard but are higher than the mean of pervious four summers. 

Carbon-monoxide (CO) pollution is widespread and year long in Delhi-NCR: CO comes almost entirely from vehicles especially petrol vehicles and is impacted by the high intensity traffic in the region. This analysis has considered CO as in addition to being a highly toxic gas it also contributes to the ozone formation. It may be noted that India has one of the most strintent CO standards in the world. CO also has only short-term standards (1-hour and 8-hours) due to its a highly toxic nature. Therefore, it’s analysis is also done in terms of exceedance days and at station level. Results point out that CO exceedance is recorded every day of the year, with its geographical spread increasing significantly during aumtum and and winter months. 

This summer Sector 125, Noida recorded 84 exceedance days which is the highest for any station in the core Delhi-NCR. It was followed by DU North Campus and IGI Airport T3 in Delhi. Ghaziabad and Gurugram are also seriously affected by the CO pollution. Najafgarh and Aya Nagar in Delhi and New Industrial Town in Faridabad are the least affected locations in the region. 

The locations with the highest CO pollution in the core Delhi-NCR include IGI Airport T3, CRRI Mathura Road, North Campus DU, Anand Vihar and Siri Fort in Delhi; Sector 125; Knowledge Park III -V in Greater Noida; and Indirapuram in  Ghaziabad. The locations with the most CO exceedance include North Campur, IGI Airport T3, Anand Vihar, CRRI Mathura Road, Okhla Phase 2, Vivek Vihar, Vikas Sadan, MDC National Stadium in Delhi; Sector 125 in Noida; and Indirapuram in Ghaziabad. 

The locations with the highest PM2.5 pollution in the core Delhi-NCR include NSIT Dwarka, Shadipur, Anand Vihar, Bawana, Jahangirpuri, RK Puram, Narela, Wazirpur in Delhi; and Sector 51 in Gurugram. 

Ozone monitoring is limited on a nation-wide scale -- less than half of the cities with air quality monitoring have ground-level ozone monitors: There are 476 cities that have official air quality monitoring stations either under NAMP (manual monitors) or CAAQMS (realtime monitors) or both. Of these only 200 cities have ground-level ozone monitors. CO monitors are present in 207 cities. All the ground-level ozone and CO monitors are under the CAAQMS program. 50km radius of ground-level ozone monitoring and CO monitoring only covers 75 per cent of population in 129 districts and 134 districts of India. 

Act now

The clean air action plan and especially summer action plan cannot ignore ozone mitigation anymore. This requires very stringent action to control a range of toxic gases – that are help to form ozone – from vehicles, industry, power plants and all other combustion sources in the entire region.  Stringent measures are needed to control NOx, CO and a range of volatile organic compounds. 

Simultaneously develop a robust public information and dissemination system to alert public about ozone exceedance wherever ozone build up is happening for exposure management. 

Delhi and NCR mirror the national problem. But the national assessment of the problem is inadequate mainly because of limited monitoring of ozone. This requires massive scaling up of monitoring capacity to track ozone pollution across the country. 

For more details, interviews etc, please contact: Sukanya Nair,, 8816818864