After Coronavirus pandemic started spreading in the country, the lockdown imposed by the Indian government across Indian states has shown a positive effect on air quality across India. COVID-19 lockdown that started on 25 March, has made the air-quality improved in the country with a remarkable reduction in pollution levels across major Indian cities, especially northern India.

According to a report by the European Space Agency (ESA), Mumbai and Delhi have witnessed a significant reduction of around 40 to 50% of nitrogen dioxide (NO2) compared to the same period last year.

An another report by NASA says that -- after just a week of reduced human activities, NASA satellite sensors observed aerosol levels at a 20-year low for this time of year in Northern India. This is due to the country-wide lock-down mandate that resulted in decreased activity at factories and severely reduced car, bus, truck, and airplane traffic.

Aerosols are tiny solid and liquid particles suspended in the air that reduce visibility and can damage the human lungs and heart.

The data via ESA's Copernicus Sentinel-5P satellite, show averaged nitrogen dioxide concentrations over India from 1 January to 24 March 2020 and 25 March (the first day of the lockdown) to 20 April 2020 – compared to the same time-frame as last year.

ESA’s Director of Earth Observation Programmes, Josef Aschbacher, said, "Another interesting feature we can see from these images is the maritime traffic across the Indian Ocean. We can clearly see a faint trail of nitrogen dioxide emissions left in the atmosphere as commercial ship traffic appears almost the same as last year. The shipping lanes appear as a straight line because the ships follow more or less the exact same route."

Moving on to NASA's report, the american space agency's satellite sensors observed aerosol levels at a 20-year low for this time of year in northern India. Every year, aerosols from human-made (anthropogenic) sources contribute to unhealthy levels of air pollution in many Indian cities.

Aerosol optical depth (AOD) is a measure of how light is absorbed or reflected by airborne particles as it travels through the atmosphere. If aerosols are concentrated near the surface, an optical depth of 1 or above indicates very hazy conditions. An optical depth, or thickness, of less than 0.1 over the entire atmospheric vertical column is considered “clean.” The data were retrieved by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite.

The first five maps below, made available by NASA Terra satellite, show AOD measurements over India during the same March 31 to April 5 period for each year from 2016 through 2020. The sixth map (anomaly) shows how AOD in 2020 compared to the average for 2016-2019.


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