When the effects of Covid-19 can be seen from space

The images of the decrease in air pollution following the health crisis linked to the novel coronavirus in China are striking because they illustrate how human activities have an impact on the quality of the air we breathe.

Sn the maps produced by NASA, based on data from European and American satellites, what do we see? An orange "cloud" represents the amount of nitrogen dioxide (NO2) contained in the air over China in January 2020. Nitrogen dioxide is released into the air primarily by vehicles and industrial facilities, and can cause respiratory problems. A month later, the “cloud” has almost disappeared from the same area… Involved? A sharp drop in the country's activity, linked to the Covid-19 epidemic and the confinement of part of the Chinese population.

CNRS teams have also highlighted the decrease in the amount of carbon monoxide (CO) in the air over China and northern Italy in February 2020 compared to previous years, thanks to data from another satellite: IASI.

Carbon monoxide measured by the IASI satellite mission in China (left) and Italy (right). As this gas persists in the atmosphere for several weeks, the impact is not limited to confined areas but also extends to the surrounding area.
Maya George (LATMOS / CNRS), Author provided

Satellites regularly witnesssudden or extreme events : this was the case recently with the monitoring of atmospheric plumes emitted by intense fires of Australia. This may also be the case during strong volcanic eruptions (we remember the Eyjafjöll eruption in 2010 which paralyzed part of the air traffic) or pollution peaks (ozone, methane, ammonia) above mega-cities or industrial sites.

Satellites scan the composition of the atmosphere

Acquired using instruments placed in orbit around the Earth more than 800 kilometers above our heads, these observations are invaluable for monitoring the physicochemical balance of our atmosphere at every point on the globe and in near real time.

All alterations in the atmospheric balance have major impacts on our environment and of course, our health and our lifestyles. From the 1980s, observations made it possible to detect and understand the deterioration of the ozone layer, that part of the stratosphere that protects us from the sun's ultraviolet rays. The industrial practices responsible for this phenomenon could be modified so that the anomaly - which could have been fatal in the long term to all life on Earth - is "corrected". The trend has been reversed and we expect the ozone layer to return to normal by 2050.

Today, emissions into the atmosphere of additional greenhouse gases (mainly carbon dioxide and methane), linked in particular to industries using fossil fuels and deforestation, cause the temperature to rise, disrupting thus the fragile balance of our entire Earth system. Also, the emissions of pollutants or more or less fine particles are responsible for the degradation of air quality, the consequences of which on human (or more generally animal) health and ecosystems have been proven.

However, the solutions to reverse these harmful trends have not yet been implemented and the contribution of observations to understand more and more precisely and quantify must serve our decision-makers to find the right compromises between environmental, economic and political measures.

Take inventory of emissions using satellite observations

The European Union has set up a vast environmental monitoring program called Copernicus. This includes a strong space component piloted by the European Space Agency (ESA), “the sentries” responsible for continuously observing the essential climatic variables of our planet. For the atmosphere, it is in particular a series of spectrometers on board either in low orbit (around 800 km altitude) which allows observation from all points of the globe at least twice a day, or in geostationary orbit (at 2 km altitude) for continuous monitoring of a fixed area, in this case Europe. A spectrometer is used to measure different elements in the atmosphere.

The measurements obtained from space thus complement the measurements made on the ground which have the advantage of providing often more precise data in a very local way, while the satellite measurements provide better temporal spatial coverage, useful for example for monitoring the transport of pollution plumes. The other advantage of satellite observations is their capacity to inform us about the atmospheric composition in isolated regions and very little covered by ground measurement networks.

These satellite observations are therefore important and serve in particular to restrict emissions inventories, validate our knowledge of the physico-chemical processes of the atmosphere by being compared with model simulations, and improve the forecast and monitoring of pollution peaks. by being assimilated into the dedicated models.

Instruments at work in observing the atmosphere

To observe atmospheric constituents, passive space instruments measure atmospheric spectra. These spectra result from the interaction between the radiation (solar or emitted by the Earth or the atmosphere) and the molecules contained in the air which each have their own "signature".

The use of these signals makes it possible to restore the gas concentrations as a function of the altitude. Indeed, all of the molecular absorption lines that constitute a spectrum are so many characteristic fingerprints of each molecule: the position in the spectrum indicates the identity of the molecule, and the length provides information on the concentration of this gases in the atmosphere. By covering the spectrum from ultraviolet to far infrared, we thus ensure to obtain the signatures of a maximum of chemical compounds but also to cover different layers of the atmosphere from the ground.

The spectrum of helium is characteristic of this chemical element.
NASA, CC BY

For example, the European instrument TROPOMI is a spectrometer which covers a wide band of spectra in the ultraviolet and infrared domain. This enables the acquisition of a wide range of pollutants such as nitrogen dioxide, ozone, formaldehyde, sulfur dioxide, methane and carbon monoxide with precision and spatial resolution unmatched from space. . This instrument therefore delivers a daily map of the a most air pollutants.

Also in the near infrared, the measurement of the MICROCARB project, whose launch is scheduled for 2021, is for its part dedicated to the measurement of the main greenhouse gas: carbon dioxide. The spectrometer analyzes the solar radiation reflected by the Earth and which, passing through the atmosphere twice, is partially absorbed by molecules in the atmosphere. The solar spectrum is thus modified and absorption lines appear at wavelengths specific to the molecules encountered. Thanks to technology, these very fine gas absorption lines can be identified, thus allowing the carbon dioxide content to be returned with very high precision.

With the advancement of space and digital technologies, it is now possible to acquire a precise global atmospheric monitoring system in order to better understand and predict pollution phenomena. With the arrival of Copernicus services in Europe, the use of spatial data in synergy with ground data and models will no longer be limited to scientific research and will allow operational applications dedicated to monitoring the atmospheric composition on the whole globe. This is the objective of the CAMS service (Copernicus Atmospheric Monitoring Service) piloted by the European Center for Weather Forecasting (ECMWF).

With the launch of the MERLIN and Microcarb satellites, followed by a European sentinel dedicated to CO2, the precise quantification of the concentration of the two main greenhouse gases will make it possible to contribute to a better estimate of the carbon footprint on Earth and its evolution for the coming years, hopefully with political measures to meet the ecological challenges. of our planet.The Conversation

Carole deniel, Responsible for atmospheric composition and Climate programs, National Center for Space Studies (CNES)

This article is republished from The Conversation under Creative Commons license. Read theoriginal article.

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