First, it happened in China. Now, Italy.

The coronavirus struck hard, and authorities responded with sweeping interventions to keep people from spreading the disease further. As citizens hunkered down at home, businesses and roads suddenly fell empty and silent. One startling result: a decline in air pollution and greenhouse gas emissions. The Washington Post this week analyzed data from the European Space Agency’s Sentinel-5P satellite, which can measure concentrations of greenhouse gases and other pollutants in the lower atmosphere. It shows that between Jan. 1 and March 12, concentrations of nitrogen dioxide, or NO2, fell drastically, especially over hard-hit northern Italy.

Nitrogen dioxide is not one of the major greenhouse gases linked to climate change. But it is produced from combustion — by cars, power plants, and other industrial sources. So it serves as a proxy for other emissions that warm the atmosphere. It also is a pollutant that can increase the risk of asthma, inflammation of the lungs and other harmful health conditions. Several experts told The Post that the changing concentrations probably reflect the decline of driving in particular, in a country in which more than half of cars burn diesel.

“I guess this is mostly diesel cars out of the road,” Emanuele Massetti, an expert on the economics of climate change at Georgia Tech University who has studied Italy’s climate policies, said in an email.