Using computer models, NASA researchers have found that since February Covid-19 pandemic restrictions have reduced global nitrogen dioxide (NO2) concentrations by nearly 20 per cent.

Nitrogen dioxide is an air pollutant that is primarily produced by the combustion of fossil fuels used by industry and transportation.

Since the Covid-19 pandemic began, space- and ground-based observations have shown that Earth's atmosphere has seen significant reductions in some air pollutants.

However, scientists wanted to know how much of that decline can be attributed to changes in human activity during pandemic-related shutdowns, versus how much would have occurred in a pandemic-free 2020.

"We all knew the lockdowns were going to have an impact on air quality," said study lead author Christoph Keller with Universities Space Research Association (USRA) at NASA's Goddard Space Flight Center in the US.

The model simulation and machine learning analysis took place at the NASA Center for Climate Simulation.

Its "business as usual" scenario showed an alternate reality version of 2020--one that did not experience any unexpected changes in human behaviour brought on by the pandemic.

The difference between the model simulated values and the measured ground observations represents the change in emissions due to the pandemic response.

The researchers received data from 46 countries --a total of 5,756 observation sites on the ground--relaying hourly atmospheric composition measurements in near-real time.

On a city-level, 50 of the 61 analysed cities show nitrogen dioxide reductions between 20-50 per cent.

"In some ways I was surprised by how much it dropped," said Keller.

"Many countries have already done a very good job in lowering their nitrogen dioxide concentrations over the last decades due to clean air regulations, but what our results clearly show is that there is still a significant human behaviour-driven contribution," Keller added.