CSE rings the alarm bells on ozone pollution in Delhi-NCR

Ozone pollution is rising, getting more widespread and has become a multi-seasonal problem, says its new analysis 

  • Ozone not just a summer problem – CSE analysis finds ozone levels exceeding the standard even during winter, making winter smog more toxic
  • Despite the pandemic and lockdowns, more days and locations witness higher and longer duration ozone spells
  • Unusual elevation of ozone noticed even during night -- ozone is produced from other gases photochemically under the influence of sunlight
  • Top 10 high ozone locations in Delhi-NCR include stations from south Delhi as well as smaller cities in the region
  • The situation demands refinement of the clean air action plan to add strategies for ozone mitigation, with stronger action on vehicles, industry and waste burning  

See the complete CSE analysis report Click here:

New Delhi, July 30, 2021: Ozone, a highly reactive gas and dangerous for those suffering from respiratory conditions and asthma, is becoming more widespread in Delhi and the National Capital Region (NCR) across all seasons – says a new analysis by Centre for Science and Environment (CSE) released here today. 

Contrary to the notion that ozone, produced photochemically from interaction between gases in the presence of sunlight, is predominantly a summer phenomenon, the analysis finds the gas has emerged as an equally strong concern during winter as well. 

“Ozone is now a round-the-year problem requiring urgent attention of clean air programmes,” says CSE executive director Anumita Roychowdhury. “The risk from it is often not well understood, as the current regulatory practice of spatially averaging the data from all monitoring stations blunts the curve, and underestimates the magnitude of threat from the worst affected locations and also during different seasons.” 

“As ozone is a highly reactive gas and even short-term exposure can aggravate respiratory conditions leading to emergency hospital admission, it requires a different approach to risk assessment and communication,” she adds. 

“This analysis shows that the number of days not meeting the ozone standards has begun to increase even during winter months, making the winter smog more toxic,” says Avikal Somvanshi, programme manager in CSE’s Urban Lab team of the Sustainable Cities programme. The assessment has traced trends during different seasons – summer (March-June), winter (October-February) and monsoon (July-September), between 2018 and July 18, 2021

Granular real time data (15-minute averages) have been sourced from the Central Pollution Control Board’s (CPCB) official online portal Central Control Room for Air Quality Management. The data has been captured from 81 official stations under the Continuous Ambient Air Quality Monitoring System (CAAQMS) spread across Delhi-NCR (40 stations in Delhi, four each in Gurugram, Faridabad, Noida and Ghaziabad, three in Meerut, and two in Greater Noida while 20 smaller towns in NCR have one station).

From this station-level data, the analysis has calculated the trends in terms of number of days when the daily level has exceeded the eight-hour standard. More than 55 million data points have been sourced and analysed from the CPCB portal for this analysis. 

Key highlights of the analysis 

City-wide averages do not tell the whole story – location-specific averages give a clearer picture of the magnitude of the problem

The CSE analysis finds that the city-wide average largely remains within the standard with just occasional exceedances. But even from this yardstick, the ‘good’ category days (50 per cent below the daily eight-hour concentration standard) have started declining in Delhi. In 2020, Delhi had 115 ‘good’ days, which was 24 days less than in 2019. 

A location-wise analysis presents a different story: it shows that exceedance of the eight-hour average standard is quite widely distributed in the city. The methodology used here is consistent with the USEPA (United States Environmental Protection Agency) method of considering the highest concentration among all the monitors within each reporting area. 

Ozone build-up is a hyper-local phenomenon; there can be 100 microgramme per cubic metre (µg/m3) difference in ozone levels between two neighboring stations. On April 27, 2021 Delhi saw variations as high as 150 µg/m3 between locations: while areas adjoining Yamuna, Lyuten’s Delhi and west Delhi showed low ozone levels, south, central, and north Delhi recorded higher levels -- south-east Delhi had the highest levels that day.  Says Somvanshi: “At the moment, it is not possible to compute the peak pollution in the city as the data that is available from CPCB is capped at 200 µg/m3 . This also makes daily AQI reporting ineffective.” 

Frequency and spread of the ozone problem

Having tracked individual stations, the analysis has captured three prominent trends – one, there is exceedance in at least one station every day over the years; two, exceedance in multiple stations has been changing over time; and three, there have been ‘extreme’ cases when all or over 16-20 stations have reported exceedance together on the same day.  The findings:

  • Exceedance at least in one station per day over the years: The eight-hourly standard for ground-level ozone exceeded at least in one station in Delhi on 290 days in 2020. This is up by 30 days compared to 2019 and 26 days more than 2018. This year has already recorded 169 days of exceedance, an increase of 21 days for the same period last year. In the NCR, Gurugram saw the maximum jump in number of exceedance days between 2019 and 2020 -- its tally of 201 days in 2020 was an increase by over 300 per cent from previous year. Noida also registered a 54 per cent jump. Faridabad recorded no change between 2019 and 2020. Ghaziabad and Greater Noida saw a decline, with the latter showing the biggest drop of 28 per cent. Meerut does not have adequate data for 2019 to carry out comparative trend analysis, but with 109 days of exceedance in 2020, it was the least polluted major city in the NCR.
  • Days with multiple stations exceeding the standard are on the rise: Not only are some parts of the city breaching the standard daily, but multiple locations have also started doing the same, indicating wider distribution of the risk. The year 2020 had 30 per cent more days when six or more stations exceeded the standard compared to 2019. The first half of 2021 has registered 108 days when six or more stations exceeded the standard, higher than in the same period in previous years.
  • Extreme events declining: Number of extreme events (more than 20 stations exceeding the standard on a single day) has declined. The most extreme event was recorded during the summer of 2019 coinciding with one of the hottest and longest heat waves in north India. Both 2020 and 2021 summers have been lighter on heat waves in comparison. In 2019, the maximum ozone levels were not recorded on the hottest days; in fact, the peak of 28 stations exceeding the standard fell on June 20, 2019, a good week after the heat wave ended. During summer of 2020 and 2021, despite the decline in ozone precursors, substantial increase in the number of days exceeding the standard was noted in six or more stations. This points to ozone build-up happening at higher frequency with a wider geographical spread.
  • Cluster of ozone hotspots in south Delhi and Lutyen’s Delhi: From the perspective of the number of days exceeding the standard in a year, south, central and north Delhi have recorded higher number of days. In south Delhi, Dr K S Shooting Range (233 days), Siri Fort (150 days), Nehru Nagar (174 days), and Sri Aurobindo Marg (126 days) have more than 120 days of exceedances during 2020. They make up four of the top five most polluted spots within Delhi. In Lyuten’s Delhi, J L N Stadium (116 days) and National Stadium (82 days) had relatively high numbers of exceedance days – both with over 80 days in 2020. North, east and west Delhi also had a few stations recording high exceedance (Sonia Vihar had 142 days of exceedance), but these are not closely clustered as seen in south Delhi-Luyten’s Delhi sub-region. 

Ozone is a problem of all seasons – even winter and monsoon

The CSE analysis busts the myth that ozone is only a summer problem and is not a concern during colder months. Most of the increase in annual number of days exceeding the standards is due to the uncharacteristic increase in ozone during monsoon and winter.  The winter of 2020-21 shows a dramatic 84 per cent increase in the number of days with six or more stations exceeding the standard. In fact, there is a 32 per cent increase in number of days when more than 10 stations exceeded the standard during the 2020-21 winter, compared to the previous two winters. 

In comparison, summer data is quite stable over years – in 2021, summer had 90 days with six or more stations exceeding the standard, which is identical to 2019 summer and 10 days more than 2020 summer. This is despite the fact 2021 summer was milder with no severe heat waves reported. Moreover, 2020 monsoon saw 140 per cent more days with six or more stations exceeding the standard compared to 2019 monsoon. If days without exceedance is considered, 2020 monsoon had only 36 such days, while the number for 2019 monsoon was 45 days and for 2018 monsoon 60 days. 

  • Ozone and winter smog: How ozone behaves during the winter smog episode is still the least understood phenomenon. Analysis of data on ozone, PM5 and NO2 levels in R K Puram for the smog season of 2020 shows that ozone remained high throughout the season. There was a minor drop noted on the peak smog day when the solar radiation was considerably lower. But as soon as the wind improved and removed the PM2.5, ozone levels spiked up. Elevated level of ozone makes the smog more toxic.
  • Meteorology aids in ozone formation both during summer and winter: Sunshine plays a critical role in photochemical reaction between gases to form ozone. Ozone data from two stations – R K Puram and Siri Fort -- were correlated with solar radiation, temperature and NO2 to understand spatial differences and the difference between summer and winter. Hourly peak ozone level perfectly corresponds with the peak solar radiation though not proportional. Quantum of ozone generated at 600-700 W/mt2 of solar radiation during summer was identical to ozone generated at 100-300 W/mt2 of solar radiation during winter. However, temperature does not seem to have as much impact on increasing the concentration. Similarly, NO2 and ozone that has a very strong negative relationship during summer which somewhat weakens during winter. In R K Puram, the rolling eight-hour average remained above the standard for 31 hours between May 5 and 6, 2021. A similar summer trend was noted at Siri Fort.
  • Pandemic effect – ozone problem persisted and duration of ozone exceedance also increased: While particulate and NOx levels dropped quite significantly, ozone levels recorded high levels in several locations. Ananda Vihar is most striking -- it had less than five days of exceedances during previous two summers (2018 and 2019) but during the pandemic summer it registered 49 days of exceedances. Also the duration of elevated levels for number of hours increased during the lockdown phases. For instance, the stations noting exceedance of the 8-hr ozone standard during the summer of 2019 recorded continuous elevation for an average 5.3 hours during the day. But this duration jumped by 20 per cent during the 2020 summer lockdown. This needs assessment of behavior of NOx and VOC levels.
  • Night-time ozone higher during pandemic lockdown and night curfews: Ozone generally disintegrates after sunset as in the absence of sunlight NOx further reacts with ozone to neutralize it. But elevated ozone level has been noticed during night time. This is consistent with what has been found internationally. With the gradual setting in of inversion towards the evening and reduced traffic, ozone levels rise. Even during the pandemic when number of day with night time levels breaching 100 μg/m3 after 10 PM has been noticed. Stations high night time levels were noted on an average 19-20 nights during the summer. Dr Karni Singh Shooting Range with 54 nights was the most affected in nighttime ozone, followed by Siri Fort (44 nights) and Nehru Nagar (34 nights).
  • Ozone pollution in major NCR cities: Ozone is also an emerging problem in the NCR. Ghaziabad has the most number of days exceeding the standard. In Faridabad Sector 16A station has recorded 152 days of exceedance during 2020. But there is variation between stations. Station at New Industrial Town recorded just three instances of exceedance in 2020. In Gurugram, Vikas Sadan recorded 139 days of exceedance and was the most polluted part, while neighboring Terigram recorded just 35 days. Noida has shown the least number of days with exceedance, with three of its four stations recording less than 20 days of exceedance during 2020.
  • Top 10 locations include south Delhi as well as small towns: Even smaller towns of NCR including Bulandshahr in Uttar Pradesh and Bhiwani in Haryana appeared in the top 20 list of ozone afflicted towns and cities. South Delhi locations dominate the list with four mentions in top 10. This also include stations from Gurugram, Ghaziabad and Faridabad, Dr K S Shooting Range was the most polluted location in NCR.
  • Daily trend and Air Quality Index bulletin must capture the intensity and spread of ozone pollution across locations: The current method of spatial averaging of ozone for city-wide averages for city-wide trends and AQI do not capture the most polluted part of the day. The CPCB’s AQI bulletin considers only an 8 AM-4 PM concentration average of ozone to compute and compare with the eight-hour standard. But this approach fails to include the hours with higher elevation. Ozone concentration generally peaks in the late afternoon in Delhi and the usual peak hour is either 4-5 PM or 5-6 PM during summer. Therefore, exclusively using 8 AM-4 PM averages leaves out the most polluted hours from the calculations. For instance, on July 16, 2021, the eight-hour average for RK Puram for 8 AM-4 PM stood at 96 μg/m3, just below the standard. But the eight-hour average for 10 AM-7 PM rose to 103 μg/m3, above the standard. In fact, data shows that even the 4 PM-midnight average can exceed the standard, something CPCB’s AQI bulletin currently fails to inform. 

Act now

Given the serious health risk associated with ozone, we must include its mitigation in the National Clean Air Programme and in the comprehensive action plan for Delhi-NCR.   

Take strong action on high emitters of NOx and VOCs including vehicles and industry: If action plan focus only on controlling particulate pollution, ozone can grow as a bigger threat that will be difficult to undo. The recent source inventory studies in Delhi have shown that the transport sector is the highest contributor of NOx and VOCs (that produce ozone) in Delhi followed by industry and other sources. According to SAFAR emission inventory transport is responsible for 62.5 per cent of NOx load and 90 per cent of VOC; and industry 24 per cent of NOx and 9 per cent VOC. ARAI-TERI inventory indicate that transport is responsible for 81.4 per cent of NOx load 80 per cent of VOCs. 

Include ozone in the winter smog mitigation plan: It is deeply worrying that ozone levels have been found to be exceeding the mark of 100 μg/m3even during winter and is highly sensitive to solar radiation. Not only the number of days exceeding the standard in different locations during winter is high it can also spike after the dimming effect of particulate matter is reduced with dissipation of the smog episodes. Reducing the gases will also reduce secondary particles formed from these gases. The IIT Kanpur source apportionment study has shown that secondary particulate can be as high as 25 per cent of the particulate concentration during the winter in Delhi. 

Refine the approach to ozone trend analysis and AQI reporting to provide ozone appropriate advisory on days when ozone is the lead pollutant: As ozone is a very harmful gas it is important to calibrate AQI to report the most polluted eight-hour average of the day, ideally making it an hourly alert. Also the current practice of only city averaging needs to change to include alerts based on the worst affected area as is the global good practice. Also any capping of data needs to be avoided for real time reporting to enable assessment of peak levels in the city both for 8 hour average and one hour average standards. 

For interviews and other help, please contact Sukanya Nair of The CSE Media Resource Centre – 8816818864, sukanya.nair@cseindia.org



See the complete CSE analysis report Click here
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