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Air quality in Chennai during lockdown - do we have clues to mitigate air pollution?

To fight the COVID-19 pandemic, the first phase of national lockdown that started on March 24 ended on April 15. As India enters the second phase of national lockdown that will end on May 3, there is optimism around improved air quality in Indian cities, that are otherwise among the most polluted ones in the world. Cities in north India are witness to blue skies and starry nights - with residents of Jalandhar in Punjab able to view snow clad Dhauladhar ranges (over 250 kilometres away) for the first time in over 30 years. Chennai’s air quality is affected by a variety of factors such as localised industrial pollution, vehicular emissions, dust pollution, and garbage burning. While these sources of city’s pollution have been on pause since lockdown, the emission of pollutants from power plants to the north of Chennai (Ennore) and from landfills in Kodungaiyur and Perugundi are constant.


The lockdown has provided an otherwise improbable experiment for study of air pollution and its sources in the city. With economic activities and vehicular traffic coming to a grinding halt, fuel consumption linked pollutants can be expected to come down, while little or no change can be expected in pollutants coming from power plants and landfills. The status of air pollutants during lockdown present us with a critical opportunity to find clues towards air pollution mitigation in Chennai. For finding the air quality of Chennai during lockdown, data for key pollutants i.e. particulate matter 2.5 (PM 2.5), sulphur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), and ozone was downloaded from the Central Control Room for Air Quality Management - All India, a web page maintained by the Central Pollution Control Board (CPCB). CPCB has continuous air quality monitoring stations (CAMS) in Alandur, Manali, and Velachery, while Tamil Nadu Pollution Control Board (TNPCB) has one in Manali village. 24 hour average data (for PM 2.5, SO2 and NO2) and 8 hour average data (for CO and ozone), from March 24 to April 15, for the year 2019 and 2020 was downloaded, with 2019 data serving as control with which the changes during lockdown were compared.


The most significant reduction during the first phase of lockdown (1.8 fold) was seen in levels of PM 2.5. On 15 out of 22 days of the lockdown, daily average PM 2.5 levels from the four CAMS were below the World Health Organization (WHO) prescribed standard of 25 µg/m3, a significant improvement over 2019 levels during the same period (figure 1).  However, a look at the data from individual CAMS quickly reveals that the PM 2.5 levels in 2020 at Alandur and Velachery are marginally higher than in 2019, though the values are still within the WHO standard. This could be a result of unpaved dusty roads from where wind can bring up dust particles into the air. PM 2.5 are emitted directly from sources like unpaved roads, construction sites, or fires and are formed in the atmosphere due to complex chemical reactions between oxides of nitrogen and sulphur that are emitted from sources like power plants, automobiles and industries. The overall reduction of PM 2.5 levels in Chennai during lockdown are most likely a result of halt in construction activities and in vehicular traffic.

Figure 1: Reduction in PM 2.5 levels during lockdown. Values within the shaded area represent the values that are below the WHO standard of 25 µg/m3.


A similar reduction of 1.8 fold was seen in the levels of NO2 during the lockdown period for the whole city. Of the four CAMS locations, the most significant reduction in NO2 was seen in Velachery whereas Alandur, Manali and Manali village had no significant reduction. Since vehicular emissions are primarily responsible for NO2 levels, it is not surprising that its levels have reduced the most in Velachery, normally a high traffic density area. NO2 is also emitted from power plants and all other equipment that burn fuel, which is the most likely reason for no significant change in NO2  levels in Manali and Manali village that have power plants in the neighbourhood.

No significant reduction was seen in levels of SO2 and ozone in Chennai during the first phase of lockdown, while CO levels had reduced by 1.6 fold as compared to 2019 levels. Vehicular emissions and fuel burning are responsible for CO levels and with vehicular transport paused during lockdown, we have seen an expected reduction in CO levels in Chennai. SO2 is primarily emitted from coal burning and with power plants to Chennai’s north continuing to function, the levels of SO2 are expectedly the same as last year. Ozone is a secondary pollutant formed in the atmosphere due to chemical reactions aided between oxides of sulphur and nitrogen, CO, and other atmospheric gases, aided by energy from the sun, and there was no significant change seen in ozone levels either. Of the three CAMS locations measuring ozone (except the one in Manali), ozone levels in Manali village are higher as compared to last year.

While analysing the data from the CAMS equipment maintained by CPCB and TNPCB, some limitations need to be considered. One of the primary considerations during the lockdown is regular upkeep of the equipment. There is no information on calibration and maintenance status during lockdown when no employees are allowed to visit the locations of CAMS equipment. This information on equipment calibration and maintenance is a concern even without lockdown.

In conclusion, the reduction of vehicular transport in Chennai during the first phase of lockdown has had the greatest impact on improving Chennai’s air quality, though only modestly. It is Chennai’s power plants and landfills that are robbing the residents of an opportunity to breathe cleaner air. Air pollution is a known environmental and health hazard, and the lockdown has been partially beneficial in avoiding them. The lockdown has helped ease vehicular traffic in the city, and for continuity of work, many sectors have shifted to work from home depending on the feasibility. If work from home becomes a practice post lockdown as well, we have some hope of breathing cleaner air. A work from home system for four days a month could be considered as a way forward. There are significant economic, environmental and health costs if we miss this unprecedented opportunity offered by lockdown to mitigate air pollution.

However, sustained public pressure, political will and policy level changes are required to address pollution from power plants and landfills.

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