Ozone levels increased during lockdown, says CSE’s new analysis of air pollution levels in 22 cities across India

Analysis brings out many learnings on summer pollution from the lockdown period in Indian cities 

To access the analysis graphs:

  • While PM2.5 and NO2 levels plummeted, invisible ozone became the most prominent pollutant in several cities.
  • Instead of the official method of considering only the fixed eight-hour average (from 8 PM to 4 PM) to assess ozone levels, CSE adapted the global best practice of calculating the maximum eight-hour average of the day. This indicated higher level of exceedence in cities.
  • While particulate levels hit the lowest possible range during lockdown phases, the share of PM2.5 fraction in the overall particulate concentration increased.
  • As traffic nearly stopped, the daily NO2 curve flattened in all cities.
  • Crop fires in April were quite extensive in Punjab and Haryana, but their impact was blunted by the overall very low background and local pollution. 

New Delhi, June 24, 2020: New analysis of summer air quality trends during the national lockdown that started on March 25, 2020, reveals a mixed trend. While the PM2.5 and NO2 curves fell and flattened dramatically in cities – a phenomenon that hogged the national attention -- tropospheric ozone pollution (henceforth ozone) increased and even breached standards in several cities: a fact that was not noticed as widely. 

This has emerged from a new analysis of 22 mega and metropolitan cities in India, done by Centre for Science and Environment (CSE). 

CSE researchers point out that ozone is primarily a sunny weather problem in India that otherwise remains highly variable during the year. It is a highly reactive gas; even short-term exposure (one hour) is dangerous for those with respiratory conditions and asthma. That is why ozone has a short-term standard – one hour and eight hours, as opposed to 24 hours for other pollutants. 

Ozone is not directly emitted by any source but is formed by photochemical reactions between oxides of nitrogen (NOx) and other volatile organic compounds (VOCs) and gases in the air under the influence of sunlight and heat. Ozone can be controlled only if gases from all sources are controlled.  

Says Anumita Roychowdhury, executive director-research and advocacy, CSE: “This pandemic-led change in air quality has helped us understand summer pollution. Normally, every year, winter pollution is what draws our attention. The characteristics of summer pollution are different: there are high winds, intermittent rains and thunderstorms, and high temperature and heat waves. This is in contrast to winter -- with its inversion, lower mixing height of air, and cold and calm conditions that trap the air and the pollutants in it.” 

Studies from the Indian Institute of Technology, Kanpur in 2015, and a joint study by The Energy Research Institute (TERI) and the Automotive Research Association of India (ARAI) in Delhi in 2018 have shown that during summer, Delhi experiences relatively higher contribution of dust compared to winter; also, the share of secondary particulates that form from the gases in the atmosphere, is comparatively lower. 

Says Roychowdhury: “This pandemic has shown that big reduction is possible only if all regions clean up together and at a scale and with speed across all critical sectors including vehicles, industry, power plants, waste, construction, use of solid fuels for cooking and episodic burning. There is a need for an agenda for a ‘blue sky and clear lungs’ for the post-pandemic period to sustain the gains. This action must also ensure the co-benefit of reducing both particulate and gaseous emissions, including ozone.” 

The dataset for trend analysis

CSE has analysed trends in PM2.5, PM10, NO2, and ozone in 22 cities across 15 states and the National Capital Territory of Delhi for the period January 1, 2019 to May 31, 2020. This also includes spatial trend analysis of ozone in selected cities. 

The most granular data (15-minute averages) has been sourced from the Central Pollution Control Board’s (CPCB) official online portal, the Central Control Room for Air Quality Management - All India (https://app.cpcbccr.com/). This has analysed over 23 million data points recorded by 116 air quality monitoring stations or about 50 per cent of the existing network under the Continuous Ambient Air Quality Monitoring System (CAAQMS) of the CPCB. 

All cities with three or more CAAQMS stations are included in this analysis; more have been chosen to ensure geographical and demographical representation. However, this study does not include modeling to isolate the impacts of lockdown or any other impacts. 

The cities covered by the CSE analysis: Delhi-NCR (including Faridabad, Ghaziabad, Gurugram and Noida), Kolkata, Chennai, Mumbai, Ahmedabad, Ujjain, Bengaluru, Hyderabad, Jaipur, Jodhpur, Patna, Vishakapatnam, Amritsar, Howrah, Pune, Guwahati, Lucknow and Kochi. 

The highlights of the analysis 

Ozone is a problem during spring and summer in the north, central and the arid parts of western India. It can even increase during warmer winters in the southern and coastal cities. 

Ozone exceedence: Even when PM2.5 and NO2 levels plummeted to the lowest in most cities during the lockdown and caught public attention, invisible ozone raised its ugly head on several days and in several cities. For analysis of ozone trends, CSE has adopted the global best practice of considering the maximum rolling eight-hour average during 24 hours, as opposed to the fixed time slot of 8 AM to 4 PM daily that the CPCB has adopted. The CPCB’s method fails to capture the worst part of the day that poses a higher health risk -- most of the time, ozone peaks after 4 PM.    

If the maximum eight-hour average during 24 hours is considered (as the US Environment Protection Agency does to capture the health risks), then more than two-thirds of the lockdown days in Delhi-NCR cities and Ahmedabad had at least one station that exceeded the standard. In Ahmedabad, the city-wide maximum eight-hour average exceeded the standard on 43 days; in Ujjain, it exceeded on 38 days. 

The city-wide maximum average in Gurugram exceeded the standard on 26 days -- at least one station exceeded the standard on 57 days. The city-wide eight-hour maximum average in Ghaziabad exceeded the standard on 15 days, with at least one station exceeding on 56 days. In Noida, the city-wide maximum average exceeded the standard on 12 days; at least one station exceeded on 42 days. In Delhi, the maximum eight-hour average exceeded the standard on four days, and at least one station exceeded the standard on 67 days. 

In Kolkata, the city-wide average exceeded on eight days; at least in one station exceeded on 17 days. Chennai and Mumbai did not register a single day of exceedance at the city-wide level, but at least one station in both exceeded the standard on 61 days and five days, respectively.     

Ozone became the prominent pollutant to lead the daily AQI on several days: The CPCB publishes the daily Air Quality Index (AQI) bulletin to inform about the severity of the pollution every day. With all pollutants down during lockdown, ozone, even at a comparatively lower level, became the most prominent pollutant of the day in several cities and led the daily AQI. It may be noted that for AQI, the CPCB considers the fixed time slot of 8 PM to 4 PM which may not be the worst average of the day.

Despite this, in Kolkata, ozone was the leading pollutant in AQI on 24 days even without exceeding the standards on those days. This is because other pollutants were lower on those days. Similarly, the AQI was led by ozone in Delhi on 10 days (eight of these days were categorised as ‘moderately polluted’, which is higher than the standard); Gurugram on 14 days (seven were categorised as ‘moderately polluted’ and one as ‘poor’); Ahmedabad on 43 days (36 were ‘moderately polluted’), and Jaipur on 13 days (one day was categorised as ‘moderately polluted’ and the rest were ‘satisfactory’).   

Ozone builds up in the cleaner areas: CSE mapped ozone concentrations spatially in the mega cities and found that ozone pollution is the highest in stations with the lowest NO2 pollution – ozone levels build up in the greenest parts of the city where the NO2 levels are very low. This is because ozone is formed when NOx, VOCs and gases react with each other under the influence of sunlight and temperature. A high NOx level can again react with ozone and mop it up. The ozone that escapes to cleaner areas has no NOx to further cannibalise it – and as a result, ozone concentration builds up in these areas. 

For example, Nehru Nagar near the Lodhi Garden area in Delhi; Colaba in Mumbai; Victoria in Kolkata; Hombegowda Nagar (Lalbagh Gargen Area) in Bengaluru; and Central University in Hyderabad are ozone pollution hotspots. The NO2 hotspots like Mundka in Delhi; Vasai West and Sion in Mumbai; Rabindra Bharati University in Kolkata; Jayanagar in Bengaluru; and Zoo Park in Hyderabad show very little ozone build-up.  


Blue skies: As noticed by everyone, particulate pollution dropped dramatically during the lockdown. Average PM 2.5 levels during the lockdown for all cities were found to be lower than the average for the same period last year. Amritsar, Chennai, Howrah, Jaipur, Jodhpur, Kolkata and Patna observed their cleanest days since January 1, 2019. Rest of the cities had registered relatively cleaner days during the monsoon season. The lockdown provided the cleanest contiguous 54-days period for Ahmedabad, Amritsar, Chennai, Howrah, Kolkata, Jodhpur, and Visakhapatnam since January 1, 2019. For Jaipur and Patna, the lockdown average was only 1-3 per cent higher than their monsoon 2019 levels. 

Cleaner regions: The clean-up happened across regions. At the regional level, in the cities of the Indo-Gangetic Plains (IGP), the lowest daily average observed varied between 7-31 µg/m3 (microgrammes per cubic metre). Within IGP, the towns of NCR registered their lowest daily average in the range of 15-24 µg/m3. In the arid western zone (Rajasthan and Gujarat) cities, the lowest range was in 9-20 µg/m3. The Deccan Plateau cities’ lowest daily average was in the range of 12-18 µg/m3. The coasts were the cleanest with the west coast cities’ lowest daily average at 8 µg/m3 and the east coast cities’ at 7-8 µg/m3. Only one city in the northeast was a part of the analysis, and its lowest daily average was 8 µg/m3.  

Rapid rise in PM2.5 with lockdown relaxation: Rise in pollution levels during lockdown 4.0, when restrictions were relaxed, has been rapid and significant for cities that had embraced complete relaxation. The average level for lockdown 4.0 compared to the average of the cleanest of the other lockdown phases was higher in Chennai by 118 per cent; in Faridabad by 185 per cent; in Jaipur and Amritsar by 109 per cent; in Visakhapatnam by 108 per cent; in Pune by 80 per cent; in Delhi by 43 per cent; in Gurugram by 56 per cent; and in Ghaziabad by 52 per cent. 

Though overall particulate pollution was lower, share of the tinier PM2.5 is higher: In mega cities, the percentage share of PM2.5 varied between 43 per cent in Delhi to 46 per cent in Kolkata; in the summer of 2019, the average ratio was 36 per cent in Delhi. This is because of higher reduction in PM10 and overall reduction in both the pollutants. But it is also clear that the winter air is a lot more toxic. During the winter of 2019, the share of PM2.5 in mega cities ranged between 65 per cent of the PM10 concentration in Delhi to 47 per cent in Bengaluru. In cities of the IGP, the share ranged between 34 per cent in Amritsar to 45 per cent in Howrah during the lockdown. This is nearly the same range as in the summer of 2019. But the winter share was very high – varying between 52 per cent in Howrah to 73 per cent in Patna. 

Land-locked cities like Bengaluru (45 per cent PM2.5), Hyderabad (44 per cent PM2.5), Guwahati (57 per cent PM2.5) and others have shown a higher percentage of PM2.5 on average, compared to IGP and coastal cities. Bengaluru and Amritsar had a near identical PM2.5 average during the lockdown period, but the percentage level of PM2.5 in Bengaluru was 45 per cent compared to 34 per cent for Amritsar – which means the air was more toxic in Bengaluru even though the averages appeared to be the same. 

Distinct regional trends: Overall, the IGP is the most polluted region in the country during summer. Within IGP, the NCR tops the chart in pollution levels; Lucknow and Patna follow closely. The arid western zone (Rajasthan and Gujarat) is the next most polluted region. The coastal cities’ pollution problem is distinct from IGP and the rest of the inland cities, but due to inadequate monitoring infrastructure, it is not well assessed. Pollution is a big problem in smaller cities like Ujjain and Guwahati, but these are not monitored adequately. In smaller metropolitan cities, even clean air was not “good” as per the AQI categorisation.   


Lowest range: In 2019, Amritsar, Ahmedabad, Delhi, Jaipur, Jodhpur and Noida saw a rise in their average NO2 levels between the spring and summer seasons. But this year, they have all registered a significant drop (39-63 per cent). Bengaluru, Chennai, Delhi, Guwahati, Howrah, Jodhpur, Kolkata and Lucknow observed their lowest daily NO2 average since January 1, 2019 during the lockdown.  

At the regional level, in the cities of IGP, the lowest daily average observed varied between 5-15 µg/m3. Within IGP, the towns of NCR registered their lowest daily average in the range of 8-15 µg/m3. In the arid western zone (Rajasthan and Gujarat) cities, the lowest range was in 8-13 µg/m3. The Deccan plateau cities’ lowest daily average was in the range of 4-13 µg/m3. The coasts were the cleanest, with the west coast cities having the lowest daily average of 2-5 µg/m3 and the east coast cities at 5-11 µg/m3. The one city in the northeast that was a part of the analysis had its lowest daily average at 3 µg/m3

In the lockdown period, 15 out of the 22 cities studied witnessed the cleanest contiguous 54 days since January 1, 2019 – the conditions were even better than in the monsoon months. In Amritsar, Hyderabad, Gurugram, Jodhpur, Patna, Pune, and Ujjain, the lockdown period was not as clean as their monsoon season. 

Daily peak pollution flattened: The hourly NO2 curve was flattened in all cities, with morning peak almost disappearing and evening peak reducing by 22-90 per cent. 

How the levels rose: Say CSE researchers: “As soon as lockdown 4.0 came in with more relaxation and the traffic returned on our roads, the average NO2 levels increased rapidly from the cleanest lockdown phase. This increase was higher in Chennai by 77 per cent, Noida by 66 per cent, Kochi by 539 per cent, Jaipur by 43 per cent, Visakhapatnam by 89 per cent, Pune by 81 per cent, Delhi by 49 per cent, Gurugram by 81 per cent, Hyderabad by 45 per cent, Bengaluru by 38 per cent, and Ghaziabad by 49 per cent.”        

The priority action agenda – CSE recommends

For ‘blue sky and clean lungs’, implement the following priority action at scale and with speed: 


  • Do not delay the BS VI emission norms for vehicles; instead, use this opportunity for a double-win strategy – scrap heavy vehicles and replace them with BS VI-compliant vehicles and support this with financial stimulus. But scrappage must be planned well so that old vehicles do not turn up in other cities/regions.
  • Move at scale to introduce cleaner battery vehicles for para-transit and public transport – mandate and pay for the change starting with autorickshaws, taxis, buses and trams. Link this with economic stimulus and clean electricity. 

Mobility and transport

  • Use financial stimulus package to augment public transport services in cities. Public transport is critical for the city to run. It is time we recognised this and made it a part of our planning.
  • Cycle and walk must be part of the ‘new-normal’ – incentivise people to reduce travel; take cycles to work. We must plan/implement cycle-walk for our cities.
  • We must plan to do everything to also reduce travel needs – adopt work-from-home, staggered timings and a roster-based attendance system. 

Clean fuel transition in industry: natural gas

  • Need natural gas for combustion in industries: But coal is cheaper than gas, as it has been placed under GST. The tax is lower and industries get the credit. Coal has also been placed under open general license (OGL) to enable its import. On the contrary, natural gas is heavily taxed.
  • Bring natural gas under GST to reduce the tax burden and to incentivise clean fuel over its dirty counterparts. Remove coal from OGL so that imports can be regulated and use can be monitored. 

Clean power

  • All power plants need to meet the new standards by 2022. CSE’s recent report (https://www.cseindia.org/coal-based-power-norms-coal-based-10125) finds that many power plants in the NCR did not meet the 2019 deadline and will not meet the 2022 deadline either. This is Delhi, for instance, has closed all its coal power plants.
  • Need a first-run policy – only those plants that meet the emission notification should be allowed to sell power.
  • Need affordable-reliable power policy as industry needs to switch from dirty fuel to cleaner fuel. Power plants must meet the 2015 standards within the deadline; accelerate natural gas-based power and renewable power.
  • Ensure all households and eateries have access to clean energy – LPG, natural gas or renewable power. 

Circular economy around waste

  • Eliminate waste burning: Ensure that no municipal waste is burnt and adopt household-level segregation and recycling facilities for all streams of municipal waste. Move towards a zero landfill policy.
  • Stringent control on construction and demolition waste: Need enforcement of rules and facilities for dust control at construction sites and recycling facilities for construction and demolition waste and quick uptake of the recycled material by the construction industry.


For more on this or to speak to an expert, please contact Sukanya Nair of The CSE Media Resource Centre, sukanya.nair@cseindia.org, 88168 18864.



To access the analysis graphs (.pdf)
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