Yes, the drop was real. When Italy locked down in early March 2020 to slow the spread of COVID-19, satellites recorded a sharp fall in nitrogen dioxide (NO2) over the north of the country, above all in the Po Valley around Milan. The European Space Agency (ESA) published an animation of the change in mid-March, and independent scientists shared day-by-day maps that showed the same pattern.
The improvement was also short-lived. Nitrogen dioxide comes mostly from traffic, power plants and industry and lingers in the air only for hours to days, so it falls quickly when activity stops and returns just as fast when activity resumes. Here is what the satellites showed, how to read the maps, and what the lockdown did and did not change.
What the satellite saw over northern Italy
The Po Valley is one of the most polluted regions in Europe. It is densely industrial, heavily trafficked and ringed by the Alps and Apennines, which trap pollution in still weather. On a normal winter map of NO2, it shows up as a dark red smudge.
Italy began restricting movement in Lombardy and neighbouring provinces in early March 2020 and extended the lockdown nationwide days later. ESA’s animation, covering 1 January to 11 March 2020 with a 10-day moving average, showed the red smudge fading as those measures took hold. Claus Zehner, ESA’s mission manager for Copernicus Sentinel-5P, said: “The decline in nitrogen dioxide concentrations over the Po Valley in northern Italy is particularly evident.”
He also added a sensible caveat. Cloud cover and changing weather can cause variations in the data, but the team was confident that the reduction coincided with the lockdown and the resulting fall in traffic and industrial activity.
How Sentinel-5P measures nitrogen dioxide
The maps come from TROPOMI, the instrument carried by the Copernicus Sentinel-5P satellite. Launched on 13 October 2017, the satellite orbits about 824 km above Earth and passes over any given spot at around 1:30 p.m. local time. TROPOMI reads sunlight reflected from the atmosphere and can work out how much of several gases is present, including NO2, ozone, carbon monoxide, sulphur dioxide and methane, at a resolution of about 7 by 5.5 kilometres.
ESA’s Director of Earth Observation Programmes, Josef Aschbacher, described it as “the most accurate instrument today that measures air pollution from space.” Because the data are free and open, researchers anywhere could build their own maps within days, which is exactly what happened in March 2020.
One important detail: the maps show the amount of NO2 in the column of air between the ground and the top of the lower atmosphere, not the concentration at street level. Scientists at the Royal Netherlands Meteorological Institute (KNMI), which helps process the data, stressed that a single day says little, because wind and weather move pollution around. Averages over many days give a far more reliable picture.
Day-by-day maps: February to March 2020
Atmospheric scientist Santiago Gassó posted a series of TROPOMI maps on Twitter in March 2020. He noted that NO2 naturally swings from day to day, and between weekdays and weekends, yet the change he saw was still striking. In each map, darker orange and red mean more nitrogen dioxide.
Saturday 8 February 2020. Before any restrictions, a thick band of high NO2 covers the western Po Valley, from Turin through Milan, with smaller hotspots farther south. White patches are areas where clouds blocked the view.
Saturday 7 March 2020. Comparing a Saturday with a Saturday removes the weekday effect. A month later, the same region shows only a faint trace across Lombardy and the Veneto.
14 February 2020. Concentrations are still high across the north, with the darkest area over the Milan metropolitan region.
24 February 2020. The first local quarantines in Lombardy and the Veneto had just begun. The northern plume is still clearly visible, centred further east.
4 March 2020. With schools closed and movement slowing, the levels over the Po Valley look noticeably lower, although single-day maps should always be read with caution.
The European picture from ESA
ESA’s own visualisations compared averaged NO2 over Europe in January 2020 with the period after the Italian lockdown. They show the same trend at a continental scale.
In the first frame, a bright red and yellow patch sits over northern Italy, with others over cities such as Madrid and the industrial belt of the Low Countries and Germany. In the second, the Italian hotspot has largely faded. ESA later extended the analysis to Paris, Madrid, Milan and other cities as lockdowns spread, comparing March 2020 with the 2019 average, and noted that the percentage fall in concentrations is not necessarily the same as the fall in emissions, because atmospheric chemistry is not linear.
ESA’s time-lapse animation shows the change unfolding across January, February and March:
Venice’s clear canals: what really changed
At the same time, residents of Venice shared photos of canals so clear that fish and the canal floor were visible. Some of the most widely shared images were posted on Twitter by Alberto Folin.
A canal corner beside a stone bridge, with the bottom clearly visible through still water. Normally the constant passage of water taxis, delivery boats and gondolas churns up mud and keeps the water cloudy.
A shoal of small fish over the sandy bed of a Venetian canal in March 2020.
The clearer water was real, but the main reason was not cleaner water. With boat traffic almost halted, sediment settled on the bottom instead of being stirred up. Reduced pollution may have played a small part. Some viral claims from the same weeks, such as dolphins swimming in the Venice canals, were false: National Geographic showed those images had been taken elsewhere in Italy.
A temporary drop, and what it taught scientists
As restrictions lifted in spring 2020, NO2 over northern Italy climbed back toward normal levels. The same thing happened in China, where nitrogen oxide emissions rose sharply once businesses reopened in February and March.
The climate effect was smaller still. Global CO2 emissions fell by about 6.4% in 2020, a record drop, yet carbon dioxide builds up over decades, so its concentration in the atmosphere kept rising and reached a new high in May 2020. Emissions rebounded the following year.
What the lockdown did offer was an unplanned experiment. For the first time, scientists could see from space how much of a region’s air pollution comes from everyday traffic and industry, and how fast it can fall when those sources quiet down. That evidence matters for public health, since nitrogen dioxide irritates the airways and is linked to worse asthma and other respiratory problems. Anyone with breathing concerns should talk to a doctor about local air quality.
The lesson most researchers drew is simple: cleaner air is possible, but lasting change comes from cleaner transport and energy, not from stopping life altogether.
Sources: ESA: Coronavirus nitrogen dioxide emissions drop over Italy; Copernicus: Sentinel-5P reveals nitrogen dioxide emissions drop over Italy; ESA: Coronavirus lockdown leading to drop in pollution across Europe; Sentinel-5 Precursor (Wikipedia); Impact of the COVID-19 pandemic on the environment (Wikipedia); maps posted by Santiago Gassó; Venice photos by Alberto Folin
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