OZONE ALERT! Toxic ozone is spreading in an unprecedented manner this summer, says CSE’s new analysis

  • Geographical spread and frequency of ozone exceedance much higher this summer in Delhi-NCR. Mumbai second most impacted metro
  • On an average, 16 stations in Delhi-NCR exceeded the standard daily in March-April – a 33 per cent rise from previous year. New Delhi and south Delhi neighborhoods worst affected
  • Hourly peak levels up by 23 per cent compared to lockdown times
  • Ozone a highly reactive gas, adversely affect people with respiratory ailments and asthma; not directly emitted from any source, but created from reaction of other gases under the influence of sunlight
  • Clean air action needs multi-pollutant approach to drastically cut emissions of precursor gases from vehicles and industrial stacks to control the emerging threat of ozone pollution 

Access the complete CSE analyses here:

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New Delhi, June 3, 2022:The summer of 2022 -- one of the hottest in history -- has witnessed widespread ozone exceedance, making the air of Delhi-NCR more toxic, says a latest analysis released here today by Centre for Science and Environment (CSE). While higher number of hot days usually record ozone levels exceeding the standards, this summer the spread of stations is much wider across the landscape. 

Among the six big metros, Mumbai is second in order followed by Kolkata, Hyderabad, Chennai and Bengaluru – the last two have longer durations of exceedance despite lower frequency compared to the other metros. This has emerged from the latest air quality analysis by CSE, released as a run-up to the World Environment Day. This is part of the air quality tracker initiative of the Urban Lab at CSE. 

“Even before we could control the problem of particulate pollution, the toxic threat of ground-level ozone is catching up with us. Despite the warning signs, this problem has not attracted adequate policy or public attention for mitigation and prevention. Inadequate monitoring, limited data and inappropriate methods of trend analysis have weakened the understanding of this growing toxic risk,” says Anumita Roychowdhury, executive director-research and advocacy, CSE. “If not addressed early, it can blow up as a serious health crisis in the coming years,” she adds. 

Health evidence is also growing stronger. The 2020 State of Global Air report states that age-standardised rates of death attributable to ozone is among the highest in India. The seasonal eight-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. 

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. In other words, there should not be more than eight days in a year when the ozone standard is breeched, and none of those allowed exceedances can be on two consecutive days. 

“Global experience shows that there is usually a trade-off. As particulate pollution is reduced, the problem of nitrogen oxide (NOx) and ozone increases. Globally, regulators are tightening the regulatory benchmark for ozone to address the toxic threat which, given its complex chemistry, is difficult to address. India should avoid falling into this trap,” says Vivek Chattopadhayay, principal programme manager, clean air programme, CSE.  

“The standard practice of the Central Pollution Control Board (CPCB) to average out the data of all stations in the city to determine daily AQI does not work for ground-level ozone as it is a short-lived and hyper-localised pollutant. A citywide average concentration level over an extended time frame does not indicate the severity of the problem and health implicationfrom local build up and exposure for people living in hotspots,” says Avikal Somvanshi, senior programme manager,urban lab, CSE. 

Why does ozone need special attention?
Thecomplex chemistry of ozone makes it a difficult pollutant to track and mitigate. Ground-level ambient ozone is not directly emitted from any source. It is produced from complex interactions between NOx and volatile organic compounds (VOCs) that are emitted from vehicles, power plants, factories, and other combustion sources.These 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 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 2022 May (upto May 30). 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. In Delhi-NCR, 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) have multiple stations and are covered in the study. 

Given the volatile and highly localised nature of ozone pollution build-up and its variability across space, and consistent with the global good practice, this analysis has considered station level trends in terms of number of days exceeding the eight-hour standard over time. As ozone formation depends on complex atmospheric chemistry and on photochemical reaction its level varies across time and space horizon. Meteorological parameters such as sunny and warm weather, stagnant wind patterns etc have bearing on its formation. 

This analysis tracks exceedances at each station in a city. Breach of the standard by even one station in the city is considered exceedance by the city. 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 microgramme per cubic metre(µg/m3) by the CPCB, it is not possible to determine how high the concentration really goes. 

This 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 highest value recorded among all city stations. 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). A simple city-wide spatial averaging has not been considered for the trend analysis though it has been assessed. 

While analysing the data, it has also been noted that the ozone data available on CPCB portal never exceeds 200μg/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 standards for ozone – eight-hourly standard of 100μg/m3 and one-hourly standard of 180 μg/m3. Capping can make assessment of one-hourly standard challenging. This study has assessed trends only based on the eight-hourly standard. 

Delhi-NCR: Key highlights of the analysis 
Heatwaves advanced the geographical spread of ground-level ozone: This year due to early onslaught of heatwaves the spatial spread of ground-level ozone started in March itself with April being the worst so far.The dangerous build-up of ground-level ozone can happen anytime during the year, but it is usually in small pockets. For it to have wider spatial spread hot and sunny weather conditions are needed which are generally present in summer, especially during May. But this year the frequency and spread of ozone exceedance started early -- in the month of March. 

Geographical spread of ground-level ozone pollution in Delhi-NCR during March-April highest in past four years: Ground-level ozone usually exceeds the safety standard on all days of summer in some location in Delhi-NCR every year. But this year the spatial spread (number of stations exceeding the standard across the city) has been much higher this year. On an average 16 stations have exceeded the standard daily this March and April, which is 33 per cent increase from previous year March and April. During 2020 when lockdowns had reduced the precursor gases needed for formation of ground-level ozone, the number was down to 10 stations daily. 

Even though, the spatial spread of ground-level ozone has significantly increased this summer, its duration has reduced slightly. This summer, daily on an average the rolling eight-hour average stayed above standard for 4.4 hours, which is marginally down from 4.6 hours observed last year and five hours recorded during 2020 summer. Longer duration during 2020 summer was due to pandemic lockdowns which reduced evening traffic therefore lesser NO2 in the evening air which is critical for breaking down of ground-level ozone after sunset.  

New Delhi and south Delhi neighborhoods are worst affected by ground-level ozone pollution: Dr KS Shooting Range in south Delhi is the most chronically affected in Delhi-NCR. It has exceeded the standard in this location for 85 days this March-May. It is followed by JLN Stadium, RK Puram and Nehru Nagar in New Delhi as the worst polluted. Greater Noida is the major hotspot outside Delhi. Faridabad has least instances of ground-level ozone exceedances in the region.. 

East and central Delhi are facing worsening trend: Patparganj in East Delhi registered highest increase in number of days exceeding the standard compared to the average of last three years. It registered a dramatic jump of 68 additional days with exceedance. It was followed by Noida’s Sector 116 and Mandir Marg, next to the President’s Estate. 

Siri Fort and Bawana in Delhi registered the most reduction in fequency of exceedances compared to average of previous three years. Their exceedances were down by over 40 days this year. Gurugram Sector 51, Dwarka Sector 8, and Najafgarh were other locations with the maximum improvenment. 

Ground-level ozone hotspots are located in the areas with low levels of NO2 and PM2.5: The spatial distribution of ground-level ozone is inverse to NO2 and PM2.5. Nehru Nagar and JLN Stadium in New Delhi are exceptions to this phenomena as both stations report high NO2as well as ground-level ozone. Likewise, industrial areas of Mundaka and Burari Crossing report concurrent high PM2.5 and 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 it cannot react further with NO2 adequately to dissipate. 

Hourly ozone peak levels are up by 23 per cent compared to lockdown times: Compared to summer of 2020 ground-level ozone is not lingering in the air post sunset but the hourly peak this year is on an average 23 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. But presence of higher concentration of NO2 is leading to higher ozone concentration during the afternoon. Theeight-hour average at Knowledge Park III in Greater Noida and Nehru Nagar in Delhi recorded close to 190 µg/m3.

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 seven stations (on an average) reporting it. Night-time ozone has been considered when hourly concentration has exceededthe level 100 µg/m3 between 10PM and 2AM at any station. Night-time ozone is mostly found in industrial areas which generally donot report high day-time ozone. Mundka in Delhi, Loni and Vasundhara in Ghaziabad and Knowledge Park III in Greater Noida have reported most instances of night-time ozone. 

Ground-level ozone has become a year-long problem: Even though the ozone exceedance is seen to worsen during summer months, it remains a year-long problem as at least few locations continue to record exceedance throughout the year. There have been only six days this year so far that have registered no exceedance among any air quality monitoring stations of Delhi-NCR. There were 15 days of no exceedance last year during the same period (Jan-May). Similarly, 2020 and 2019 had 24 days and 24 days of no exceedance respectively. Foggy and cold conditions of January conventionally inhibit formation of ground-level ozone but ozone was found to be exceeding at multiple stations on 28 days this January. It is up from 11 days and 19 days recorded in January of 2020 and 2021 respectively. Even the months of monsoon records exceedance in some locations. 

During the summer of 2022 all key pollutants have increased in Delhi-NCR: It is not just ground-level ozone pollution that has increased this summer compared to previous summers, significant increase has been noted in PM2.5 and NO2 as well. Compared to summer of 2020, NO2 is up by 61 per cent and PM2.5 by 76 per cent. 

Ozone peril in other five big metropolitan cities
After Delhi-NCR that recorded ozone exceedance on almost all days of this summer, Mumbai with 75 days of exceedance was the second most impacted metro. Kolkata-Howrah and Hyderabad registered 43 days of exceedance each. Even though the number of exceedance days in Kolkata-Howrah has been lower than that of Mumbai, its citywide concentration is 30 per cent higher than in Mumbai for this summer season. The denser network of monitoring stations in Mumbai is able to catch instances of ozone exceedance better. Greater Mumbai has 26 monitoring stations while Greater Kolkata has only 10 stations. 

It must also be noted that apart from Delhi all the other metros in western, southern and eastern regions have reported higher instances of ozone exceedance in winter than summer. These regions also have warmer winters. In most of these cities ozone is recording exceedance round the year.  

It has been noted that Chennai and Bengaluru have longer durations of exceedance despite lower frequency compared to other metros.

Act now
Ozone mitigation demands stringent control of gases from all combustion sources including vehicles, industry, power plants and open burning in the entire region. It is therefore necessary that while designing mitigation of particulate matter the key focus of action strategy today, is also calibrated for reduction of ozone precursor gases. 

Immediately, refine the action strategy for combined control of particulate pollution, ozone and its precursor gases like NOx to maximise the co-benefits of the action plan. At the national level, the National Clean Air Programme needs to propose specific measures to control ozone precursor gases including NOx, volatile organic compound, carbon monoxide etc that are emitted largely from vehicles and industry. 

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

For more details, interviews etc, please contact Sukanya Nair of The CSE Media Resource Centre for the Global South: sukanya.nair@cseindia.org, 8816818864.



Ozone Alert!
Avikal Somvanshi & Sharanjeet Kaur
Urban Lab, CSE
Media coverage