“We are not only scientists; we are men, too. We cannot forget our dependence on our fellow men. I mean not only our material dependence, without which no science would be possible, and without which we could not work; I mean also our deep moral dependence, in that the value of science must lie in the world of men, that all our roots lie there. These are the strongest bonds in the world, stronger than those even that bind us to one another, these are the deepest bonds – that bind us to our fellow men” – Prof. J. Robert Oppenheimer, Speech to the Association of Los Alamos Scientists – November 2, 1945
What do we breathe?
We used to hear that we breathe (dio)oxygen. Although, this gas is an important element present in the atmosphere, we actually breathe other gases (the so-called trace gases) and particles (also named aerosols). The increase of these additional compounds directly affects our air quality (or pollution toxicity), but also drives the current climate change that we have been provoking through the green-house gases (e.g. the famous CO2 or carbon dioxide). Continuous increase in human population combined with activities based on fossil fuel consumption change then our air at different levels. Our ways of living, in spite of their incredible advantages, are clearly leading to a new atmosphere composition. Different from two centuries ago… But our societies are also more vulnerable than in the past. This leads to several consequences such as: 1) occurrence of summer smog over most of cities in the world, including developed countries, 2) rapid and efficient pollutants transport from one region to others make local emissions an international concern, 3) alteration of the chemical composition of the troposphere (i.e. low part of the atmosphere) on a global scale. We cannot ignore these changes in our atmosphere. There is a need to accurately measure and monitor them, in order (hopefully) to identify the right mitigation actions and to protect our planet.
Satellites, a remote comprehensive global picture of our visible and invisible atmosphere…
History contains many examples that, as humans, we have always wanted to explore, observe and understand the world and all its components. We have even built different types of instruments to “measure” our environment. Satellites are probably one of the most impressive built instruments. By taking some distance away from the surface and observing the whole Earth from space, they can measure the visible (what we can see) and the invisible (what we cannot see with our own eyes), what we can instantaneously feel (e.g. a smelly sulfur plume from volcanic eruptions) but also not directly feel (e.g. because of fast wind dispersion..), take a picture at a given time and also capture variability, over our region and the entire world! Therefore, satellite remote sensing can provide information on locations which are not (easily) accessible (e.g. at high altitudes, polar regions, areas subject to political restrictions).
Space-borne sensors have delivered, for the last 20 years, a large quantity of data with a completely new view on our atmosphere composition and its changes. Thanks to these data, we know the location of the main sources contributing to the atmosphere changes. We can monitor the amount of pollutants in the air at a given day, and some trends from year-to-year. We can study global phenomena like climate change, air quality, ozone hole, interactions within the Earth-system (e.g. emission sources, transport processes and chemical transformations). Space-borne sensors bring some solutions to observe the issues 2) and 3) mentioned above.
… but do we fully understand an atmospheric satellite measurement?
The physics describing how these atmospheric elements lead to the satellite measurements (i.e. spectra) is well-known and can be accurately described. However, the inverse step, i.e. going-back to the atmosphere constituents from one single satellite measurement, is not straightforward. This step is commonly named “retrieval”. It is very amazing to see that, after so many years and in spite of high quality and huge amount of works achieved by the satellite atmospheric retrieval community, we still have difficulties to accurately extract the atmosphere properties from satellite spectra. Large uncertainties remain in the atmospheric retrieval works. One example? From one single satellite spectrum, acquired in the visible wavelengths, we still cannot distinguish the presence of clouds from aerosols and/or bright surfaces (e.g. snow or desert). And this has consequences when retrieving trace gas concentrations…
Big challenge: to better and comprehensively exploit an atmospheric satellite measurement, today & tomorrow…
With the next generation of satellite instruments and unprecedented technology progress, our main challenge is to give, in the near-future, even more credibility to all satellite data. Therefore, innovative retrieval techniques need to be developed and evaluated to reduce these uncertainties. This is crucial in view of keeping ensuring the high-quality of information made available to the policy makers, citizens and in general everyone who is involved and/or concerned about industry, traffic, energy generation and food production.
But how to achieve these goals? Do we really fully understand all the physics information contained in satellite spectra measurements? What are the remaining and top-burning unknowns? What are the limits of current retrieval techniques? Is 1 single spectrum enough to describe the atmosphere composition at a given time? Shall we combine more measurements together? How to perform that? What should be the future retrieval developments? How to be prepared for the future satellite data? …
All these challenges and associated questions are the starting point of my interest, and therefore my research activities. They guide me every day through my diverse projects and scientific collaborations. And perhaps, this is or will be the case for you too. Hope then you will find (a few) answers to your questions, and enjoy this website.
Homepage Julien Chimot: a journey in Earth observation satellite science 