4 Caveats With Those NASA Pollution Images After Coronavirus Lockdown

Some of my fondest career members are from my tenure as a scientist at NASA Goddard Space Flight Center. It is one of several NASA space centers and houses arguably the largest collection of Earth system scientists in the world. It was always amusing to receive questions like, “Do you forecast for shuttle launches?” or “Why does a research meteorologist work for NASA? It was an indication many people have no clue that NASA has a vigorous mission to study our planet too. One of the most fascinating components of the Earth science program is the atmospheric composition focus area, which includes air pollution. This week NASA released more stunning images showing dramatic reductions in air pollution because of COVID-19-related mitigation efforts. As an atmospheric scientist, however, I thought that it was prudent to mention four caveats as you consume this information.

The first caveat is to understand what you are actually looking at in the images. I have already observed some pretty sloppy reporting concerning them. The images show tropospheric nitrogen dioxide, which is released when fuel is burned in cars, airplanes, and other combustion engines. Therefore, it can be considered as a surrogate for air pollution. According to a NASA press release, “When first released into the lower atmosphere, or the part of the atmosphere that we breathe, nitrogen dioxide reacts with other nearby chemicals and forms ozone.” Ozone is a secondary pollutant that is very harmful to us at the surface although it is a valuable ally against ultraviolet radiation in the stratosphere. Air traffic at airports like Baltimore-Washington International and Atlanta Hartsfield International are down 60 to 70 percent, according to NASA, and automobile traffic along major arteries is also dramatically lower. It is not surprising to find that the air quality has improved during the period March 15 to April 15 of 2020 (during coronavirus “lockdown”) as compared to the four previous years (image above).

These images are acquired using satellite data from European Space Agency (ESA)’s TROPOspheric Monitoring Instrument (TROPOMI), which flies on the Copernicus Sentinel-5 Precursor satellite. This effort is part of a project called the Pandora Project, but I want to emphasize that is just one of several NASA projects or missions studying air pollution. NASA’s Aura spacecraft can also measure nitrogen dioxide, and other missions measure particulate matter, ozone, sulfur dioxide, formaldehyde, carbon monoxide, and methane.

This brings me to the second caveat, and this one comes directly from a NASA press released on April 9th. It showed data from the Ozone Monitoring Instrument (OMI) aboard Aura and that air pollution (at least in terms of this proxy) was down thirty percent in the northeast U.S. The NASA news release cautioned that “nitrogen dioxide levels observed from space serve as an effective proxy for nitrogen dioxide levels at Earth's surface, though there will likely be differences in the measurements from space and those made at ground level.” Those of us in the “satellite observation of Earth” businesses also know the measurements of nitrogen dioxide only happen on days with low or no clouds.

Even with those caveats, I am pretty certain the reduction in air pollution being observed by the satellites is real. Florian Pappenberger, Director of Forecasts at ECMWF, provided a very interesting link showing the situation in Europe, and previous reductions have recently been reported in parts of Asia too. However, there is a third caveat worth raising. I spent a good portion of my 12-year NASA career as the Deputy Project Scientist for the Global Precipitation Measurement (GPM) Mission. This mission was launched to study global precipitation and gather data that can be used to improve weather, climate, and flood predictions. Oxides of nitrogen are a key component of acid rain, which suggests that precipitation and other weather variability could be responsible for some of the removal of pollution. The process of washing air pollution from the atmosphere by rainfall is called wet deposition. Though not exactly the same tie period as the maps above, the map below shows that much of the Southeast has been wetter than normal from late February through April. On the other hand, much of the Carolinas have been drier than normal yet there are still reductions in nitrogen dioxide.

The final caveat is that COVID-19 restrictions have temporarily led to cleaner air for many parts of the U.S. and world. Unfortunately, it will likely only take a matter of days for air pollution to increase as human activities eventually normalize. However, this moment in time has taught a clear and “unpolluted” lesson: humans impact our atmosphere and climate system. Period.

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