My motivations

In a nutchell

Extracted from Chimot, J., Global mapping of atmospheric composition from space – Retrieving aerosol height and tropospheric NO2 from OMI, PhD book, Delft University of Technology (TU Delft), The Royal Netherlands Meteorological Institute (KNMI), July 2018.

“Notre maison brûle et on regarde ailleurs (“Our house is burning and we are looking elsewhere”).” Jacques Chirac, French president 1995-2007, Earth Summit in Johannesburg, 2002

In the last decades, significant progress was achieved in global monitoring of the atmospheric composition from space, leading to key discoveries on the ozone layer, air quality and climate change. Recent research also shows that satellite observations can be used to monitor the effectiveness of policy measures on air quality and climate change. Therefore, it becomes more and more important to provide accurate data under any atmospheric condition. Furthermore, developing algorithms that can meet the requirements of an operational 24/7 data stream becomes challenging due to the tremendous growth of satellite measurements. Europe in general, but also other countries, have heavily invested in satellite instruments monitoring short-lived and greenhouse gases, and harmful fine particles, also named aerosols.

 

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.

04_julien-chimot
Designer team of DIG-it! Project, TU Delft / Valorisation center: Dorien van Alphen, Robbert van Leuven, Susanne Sleenhoff

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.

flowchart_mywork
From satellite spectral measurements to pollution maps: illustrative diagram of an atmospheric retrieval work

… 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.