With ESA's Sentinel-5 Precursor (S-5P) satellite, it is now possible to access near-real time data pertaining to trace gases and atmospheric pollution for the entire planet. Having gotten to know about this from University of Luxembourg's MOOC, I am in awe with this abundantly useful Earth Observation Programme.
While the pandemic is unfortunate, the resulting nationwide lockdown presented an opportune moment 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 there would be a significant dip in the pollution levels - after all, I had come across few news reports which were indicative of this - the residents of Jalandhar being able to view Himalayan mountain range from their rooftops, and clear blue skies greeting residents of New Delhi (a rare occurrence for both these cities).
Nitrogen Dioxide (NO₂) is a major and harmful air pollutant which is emitted upon the combustion of fossil fuels by industrial plants and automobiles as well as from forest fires, microbiological activity in soil and lightning. Reddish-brown in color, this gas is highly reactive and thus, its lifetime in the atmosphere spans, at most, a day. Hence, if concentrations are detected in the troposphere, one can be certain that the source of the emission lies in the vicinity. That being said, these are not Surface-level emissions (measured in parts per million i.e. ppm) and factors such as Wind speed and Temperature can affect the concentration and spread of the gas as it flows up the atmosphere. As a result, it would be somewhat inaccurate to infer that the source of emissions lie directly underneath the tropospheric NO₂ hot spots (measured in molecules per m²).
While the methodology of extracting Tropospheric Nitrogen Dioxide Vertical Column Density level can be observed here, below is the map-based output comparing Tropospheric NOâ‚‚ levels in April 2020 with that of April 2019 over virtually the entire Indian territory (a portion of West Bengal and the North Eastern-states were unfortunately out of swath-range in a single Sentinel-5P L2 Offline imagery product).
Below are some observations and inferences:
There has been a considerable drop in NOâ‚‚ levels, the biggest benefactor being North India followed by West India. NOâ‚‚ levels over South India have reduced considerably as well
East India still has several NOâ‚‚ hot spots though, albeit of a lower intensity than prior to the lockdown. I believe these can be attributed to the continuing industrial activity in the mine-to-metal and mine-to-power value chain - these are deemed as essential services and hence, are exempted from the Nationwide lockdown. Nonetheless, it is evident from the lower NOâ‚‚ levels that the magnitude of economic activity has reduced
Notice the few hotspots around the coastline in the state of Gujarat in West India - I believe these are from the continuing operations at the world's largest Petroleum Refinery in Jamnagar and the allied industries in its vicinity - again deemed as essential services
The tiny hot spot on the west coast of Sri Lanka lies exactly over Colombo - the industrial hub of that country
As you would concur, Remote Sensing is a such a useful method to determine macro-level phenomena! Besides Nitrogen Dioxide, the TROPOMIÂ instrument on-board Sentinel-5P is able to acquire data related to pollutants such as Ozone, Sulphur Dioxide, Aerosols, Carbon Monoxide, Methane and a few other trace gases.
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