Amidst the devastating Covid-19 pandemic, a rare positive has been the significant global decrease in air pollution levels. Primarily, experts have measured nitrogen dioxide (NO2), one of the six major air pollutants (in addition to particulate matter, carbon monoxide, sulfur dioxide, ground-level ozone, and lead). NO2 has, like most other gases, natural and human sources.
Natural sources include lightning, oceans, and volcanoes. However, in urban regions, natural sources of NO2 account for a small fraction of the total NO2 levels; according to a 2005 report by Australia’s Department of the Environment and Heritage, natural sources of NO2 only account for 1 per cent of overall NO2 levels in cities.
Human activity is almost entirely responsible for NO2 emissions in urban regions, with road transport being the number one cause. Planes, power plants and ships, all of which burn fossil fuels, are also significant human sources of NO2. Given this, it’s unsurprising that during the stringent global lockdowns, NO2 levels have dropped significantly in urban areas, especially in India’s densely populated cities.
Satellite imagery from the European Space Agency’s Copernicus Sentinel 5P satellite measure NO2 levels globally. These measurements accurately reflect emissions sources, because unlike other gases that can travel a significant distance from where they’re emitted, NO2 has a short lifespan and dies before it can move very far.
In other words, if the Sentinel 5P satellite captures a hotspot of NO2 over Delhi, it’s highly likely it was emitted from within Delhi’s vicinity. Satellite imagery is, therefore, a highly reliable tool for measuring NO2 emissions, especially if data with high levels of cloud coverage is excluded.