Air Pollution in India during Coronavirus Lockdown

With ESA's Sentinel-5 Precursor (S-5P) satellite, it is now possible to access near–real-time data on trace gases and atmospheric pollution across the entire planet. I first learned about this through the University of Luxembourg’s MOOC, and since then, I’ve been in awe of this immensely powerful Earth Observation Programme.

While the pandemic was unfortunate, the nationwide lockdown presented an opportunity for me to use S-5P data to extract, visualize, and compare the change in air-pollution levels in India arising from the reduction in economic activity. I hypothesized that pollution levels would dip significantly—after all, news reports at the time showed residents of Jalandhar viewing the Himalayan mountain range from their rooftops, and clear blue skies appearing over New Delhi (a rarity for both cities).

Nitrogen Dioxide (NO₂) is a major and harmful air pollutant emitted from the combustion of fossil fuels by industrial plants and automobiles, as well as from forest fires, microbiological soil activity, and lightning. Reddish-brown in colour, this gas is highly reactive and has a very short atmospheric lifetime—at most, a day. Therefore, if high concentrations are detected in the troposphere, one can be fairly certain that the emission source lies nearby.

That said, these are not surface-level emissions (which are measured in ppm). Atmospheric factors such as wind speed and temperature can influence the concentration and spread of NO₂ as it rises. As a result, it would be somewhat inaccurate to assume that emission sources lie directly beneath the tropospheric NO₂ hotspots (measured in molecules per m²).

While the methodology for extracting Tropospheric Nitrogen Dioxide Vertical Column Density can be found here, below is the map-based output comparing tropospheric NO₂ levels in April 2020 with those of April 2019 over most of India. (A portion of West Bengal and the Northeastern states unfortunately fell outside the swath-range in a single Sentinel-5P L2 Offline imagery product).

Figure 1: Tropospheric NO₂ concentrations over most of Indian territory (excluding Northeastern regions) — comparing April 2020 emissions during lockdown with April 2019 emissions prior to lockdown — derived from Sentinel-5P satellite data

Below are some observations and inferences:

• There has been a considerable drop in NO₂ levels, with North India being the biggest benefactor, followed by West India. NO₂ levels over South India have reduced significantly as well.

• East India still exhibits several NO₂ hotspots, though of lower intensity compared to pre-lockdown levels. These likely correspond to ongoing industrial activity in the mine-to-metal and mine-to-power value chains—sectors deemed essential and hence exempted from the nationwide lockdown. Even so, the reduced intensity clearly indicates a reduction in economic activity.

• Note the hotspots along the Gujarat coastline in West India. These likely represent emissions from the world's largest Petroleum Refinery in Jamnagar and its allied industries—again part of essential services.

• The tiny hotspot on the west coast of Sri Lanka lies directly over Colombo, the industrial hub of the country.

  
  

As you would agree, Remote Sensing is an incredibly powerful method for analysing macro-level phenomena. Beyond Nitrogen Dioxide, the TROPOMI instrument aboard Sentinel-5P captures data on pollution indicators such as Ozone, Sulphur Dioxide, Aerosols, Carbon Monoxide, Methane, and several other trace gases.

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