For nearly thirty years, NASA has conducted studies oriented to the analysis of the quality of the air we breathe, and more generally to the thin layer of gas that envelops the planet Earth.
In recent years, the space agency has collected and analyzed a huge amount of data with the aim of fully understanding the mechanics behind climate change and air pollution. In this article we will analyze some of these data, with a nod to the missions currently underway and those that will be launched in the near future.
The pollutant
When we speak of "air pollution" we refer to chemicals or particles that are released into the atmosphere and that have negative effects on human health. The Clean Air Act of 1970 established legislation that provides for the tracking of six of these pollutants: nitrogen dioxide (NO2), ground-level ozone, carbon monoxide, particulate matter (roughly microscopic grains of solid or liquid material) , sulfur dioxide and lead.
In this historical period, all humanity is indirectly perceiving the effects of his activity on the natural ecosystem: indirectly because instead of observing the damages, it is observing the benefits deriving from the cessation of human activity. River waters returning to their original crystalline color, blanket of smog vanished from cities and dolphins spotted in places once unthinkable, just to name a few examples.
Fortunately, many of the satellites that observe Earth can capture what the human eye cannot see, including potentially harmful pollutants in the air we breathe. From their privileged point of view (from space), these satellites help measure and track air pollution as it travels around the world.
In analyzing the data coming from one thing is clear: reducing emissions from human activities can have a profound effect on air quality.
China's recent response to the COVID-19 pandemic, characterized by a strong quarantine e restrictions on industrial activities and travel, is a particularly vivid example of this. Data from instruments of NASA's AURA satellite and European Space Agency (ESA) Sentinel-5 satellites showed a significant reduction in nitrogen dioxide (NO2) – a harmful gas emitted by power plants, industrial plants and motor vehicles – in large parts of the country during the stop period.
The pandemic represents a very advantageous and extremely useful case for terrestrial observations made from space, precisely because of the changes it imposes on human activities; however, satellite air quality data has applications in a wide range of disciplines. And above all, they can constitute a database that can be consulted in real time, such as AIRNow.
AIRNow
The AirNow system is a platform created to optimize the distribution of national reports and forecasts in real time on air quality. Measurements are made from thousands of monitoring stations in the United States, Mexico and Canada; however, these ground monitoring stations do not reach anywhere, but have limitations.
By introducing satellite observations through the MODIS instrument it is possible to fill this gap and also measure those particular areas, which allows to obtain a more accurate image of how air pollution (and in particular fine particulate matter) is distributed in a certain continent, and how its concentration varies over time.
By incorporating the Ozone Monitoring Instrument (OMI) data, NASA was able to identify a significant drop in NO2 in the past 15 years in the United States: This is proof that approving the Clean Air Act rules (i.e. gas vehicle regulations, switching to cleaner fuels and so on) was a fair choice, which is now paying off.
An example of a result obtained: to demonstrate that since 2004 NO2 levels have fallen by 40% – 50% depending on the metropolitan area (American continent) examined. In fact, the air in the United States is at the moment the cleanest since the industrial revolution.
Ozone is the bad one
While ozone in the stratosphere is critical to protecting life on Earth, surface ozone is a toxic gas for most plant and animal species. NASA combines satellite data with computer models to provide a snapshot of the chemical composition of the whole atmosphere, and the graphical representation is only part of the whole analysis process. Indeed the management of all these data represents a real obstacle for the scientists involved.
Recent events
According to the report of the Global Burden of Disease, lair pollution is the main environmental cause of mortality: is a well-known statistic in the public health sector. California fires, for example, have caused air quality problems not only in the past year, but also in the past decade. A HAQAST team was able to examine emissions and develop new methods for using existing data obtained from the VIIRS and MODIS tools. Their input was instrumental in helping the state of California understand the extent of the fires and the quantification of emissions.
Various missions are currently under development which were born with the aim of investigating the many facets that characterize this aspect of life on planet Earth. For example, the Tropospheric Emissions: Monitoring Pollution (TEMPO) mission is designed to measure different pollutants, including NO2 and ozonein the United States during daylight hours. TEMPO will give scientists the opportunity to observe how sources of pollution and chemical concentrations change over the course of a single day. The launch of the TEMPO mission is scheduled for 2022 and will be part of an international constellation of similar satellites, which includes the South Korean geostationary environment monitoring spectrometer (GEMS) and Sentinel-4 from the European Space Agency. The satellite is expected to launch in geostationary orbit at 35,786 km above the Earth's equator.
The mission Multi-Angle Imager for Aerosols (MAIA), also scheduled for shortly after 2020, will improve scientists' understanding of particulate matter, those tiny microscopic particles that persist in the air, with a focus on large metropolitan areas. Data such as these will help the health community better understand the connection between aerosol pollutants and health problems, including complications in post-birth infants and the onset of cardiovascular disease.
Barry Lefer, NASA Tropospheric Composition scientist explains that "MAIA will allow us to study these aerosols in detail, to tell us how large they are and how many of them are in that very small category which is the most harmful to human health. We will also be able to better understand what the particles are that will take us where they came from"
"I'd like to see a future where real-time Earth observation data is perfectly and continuously available to everyone, from orbit to the palm of your hand"said Haynes."It would allow anyone to make quick decisions about air quality and their health".
Let's not forget the arctic
Ozone is a highly reactive molecule composed of three oxygen atoms which occurs in nature in small quantities. The stratospheric ozone layer is a sunscreen which absorbs harmful ultraviolet radiation which can damage plants and animals and affect people causing cataracts, skin cancer and suppressed immune systems.
Ozone levels over the Arctic during the month of March reached a record low. An analysis of satellite observations shows that ozone levels reached their minimum on March 12, 2020, reaching 205 Dobson units (one Dobson unit corresponds to a thickness of 0.01 mm).
Such low levels are rare, but have already been observed in the past. Similar ozone levels occurred in the atmosphere in 1997 and 2011. For comparison, the lowest value observed in the Arctic during the months of March is usually around 240 Dobson units.
"This year's low Arctic ozone level occurs roughly once per decade, "explains Paul Newman, chief scientist for earth sciences at NASA's Goddard Space Flight Center."For the overall health of the ozone layer, this fact is worrying because ozone levels in the Arctic are usually higher in March and April".
The decrease in ozone in the Arctic was caused by a combination of factors due to atmospheric mechanics, called "waves", which are weaker than usual. These "waves" guide the movements of the air through the upper atmosphere, similar to what already happens in the lower atmosphere, but with a much more significant intensity.
In a typical year, these waves travel upward from the lower mid-latitude atmosphere to disrupt the circumpolar winds that blow around the Arctic. When they stop the polar winds, they carry ozone with them to other areas of the stratosphere, also filling the ozone tank above the Arctic.
Chlorine and bromine
The resulting mixing has a second effect, which is to heat the Arctic air. Warmer temperatures make it more difficult for stratospheric clouds to form. Is it an important fact? Yes, because these clouds allow the release of chlorine of which they are composed, a release which is the basis for the reduction of the ozone layer. Chlorine and bromine that deplete ozone come from chlorofluorocarbons and halons, the chemically active forms of chlorine and bromine derived from artificial compounds that are now prohibited by the Montreal Protocol. The mixing therefore makes the formation of stratospheric clouds more difficult, effectively blocking the release of compounds harmful to the ozone layer.
In December 2019 and from January to March 2020, the "waves" were weaker than usual and did not disturb the polar winds. The winds therefore acted as a barrier, preventing ozone from other parts of the atmosphere from entering the arctic and rebalance low ozone levels. Furthermore, the stratosphere has not heated up as described above, giving free rein to the formation of polar stratospheric clouds that have allowed the release of reactive forms of chlorine, causing further decrease in ozone.
The man has stopped entering chlorofluorocarbons into the atmosphere thanks to the Montreal Protocol, and this is good because otherwise the Arctic situation would have been far more catastrophic than it was down. The image above shows what would have happened.
Since 2000, levels of chlorofluorocarbons and other substances that deplete the ozone layer in humans have significantly decreased in the atmosphere and continue to do so. But chlorofluorocarbons are compounds that take decades to degrade, and scientists expect stratospheric ozone levels return to 1980 levels by the middle of this century.
