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Discussion on Atmospheric Composition and IASI

Interviewer: Antoine Rondelet, Founder @ Ganddee

Guest: Camille Viatte, Research Scientist @ LATMOS


The LATMOS (Laboratoire Atmosphères, Observations Spatiales) (translated as Atmospheres, Space Observations Laboratory) is a French research laboratory that specializes in the study of fundamental physicochemical processes governing terrestrial and planetary atmospheres and their interfaces with the surface, the ocean, and the interplanetary environment. LATMOS builds innovative instruments (deployed from the ground and sometimes put into orbit) but also develops numerical atmospheric models that are used to interpret the various observations.

Camille Viatte is a physicist working, in the LATMOS team, on the atmospheric chemical composition monitored from remote sensing instruments. Her research focuses on trace-gases and aerosols observations with a special focus on atmospheric pollution and urban-climate issues.


Antoine: As a non-expert, I tend to see the planet as a sphere that is wrapped by a layer of gas called the atmosphere. In practice, I believe the atmosphere is decomposed into multiple layers. Could you tell us more about what the atmosphere is, what it looks like and what those layers are?

Camille (LATMOS): The atmosphere is just a mixture of gas surrounding the planet. That’s the simple way of putting it. The atmosphere makes life on Earth possible. It plays very important roles for human beings. For instance, the atmosphere protects us from harmful UV radiations from sunlight, and it also warms the surface of the planet via the greenhouse effect.

The atmosphere is mainly composed of gases such as Nitrogen and Oxygen, and all the other gases are called trace gases because they are in far smaller quantities. There are also small particles, liquid and solid, that scientists call aerosols.

Regarding its structure, the atmosphere grows thinner and less dense as we move upward from the surface. At about 100 km the air becomes so thin that we are considering this as the beginning of space. We distinguish 5 so-called layers. The first one is called the troposphere. It’s where we live, and it is located from the surface to a height of 10/15 km. “Tropo” in Greek means “rotating” so this layer is characterized by movements, winds, and turbulence. It is where most clouds are found and where the weather occurs. Above this troposphere, you have the stratosphere. The stratosphere is between 10 and 50 km above the ground and is where you find the ozone layer that absorbs the energy coming from the UV radiations from the sun. Ozone is a greenhouse gas, so it warms up this layer. The higher you go in the stratosphere, the warmer it gets. This layer is not moving, quite stable and stratified, hence its name. On top of this, you get the mesosphere, which is located from 50 km to 85 km above the surface. The mesosphere is where the sky changes from blue to black because there are so few molecules in this layer that the sunlight is not reflected up there. This is also where meteors burn up, giving us the shooting stars. Above the mesosphere, you have the thermosphere, this is where you find the aurora of the northern lights. And finally, on top of that, you get the exosphere which represents the transition between Earth and space.

I’d love to know more about the IASI instrument. I’ve read online that it is “a Fourier Transform Spectrometer based on a Michelson Interferometer…”. What does that mean in layman terms? How do you monitor the atmosphere with the IASI instrument?

IASI is an instrument looking at the Earth with, let’s say, an eye that has a special resolution of 12 km on the ground. It orbits around our planet at about 800 km to measure atmospheric composition and weather everywhere and every day since 2007. The IASI mission flies onboard 3 Metop satellites. You have 3 IASI instruments, 3 similar IASI instruments, that fly onboard 3 Metop satellites and that make measurements at the same locations twice a day: one at 9:30am and the other at 9:30pm. IASI is a Fourier transform spectrometer, so let me explain it in simple terms. The Earth and the atmosphere absorb solar radiation and re-emit it in the infrared. That’s the black body theory. Then, the gases and the particles present in the atmosphere absorb infrared radiations at a very very specific energy or wavelength. It’s like their fingerprint. So, let’s say, ozone, it will always interact with radiation at a specific energy level. And so, IASI, in space, captures the infrared radiation of the Earth minus the radiation that was absorbed by the gases and aerosols present in the atmosphere at the measurement location. That’s basically how we measure, and that’s what we call “absorption spectrum”. Up to now, we can retrieve 33 gases and aerosols in one spectrum and each IASI measures about a million spectra a day, so it’s a lot of measurements.

How do you get atmospheric chemistry data from the satellite observations? Are there specific processes you follow to transform raw observations into data points that can be shared with and used by the scientific community?

We have antennas in a few locations around the Earth. We have one on the roof at Sorbonne Université, where I work. This big antenna captures the raw data (the spectra). In these spectra, you have so much information. The position of the lines gives you the ID of the molecules you’re looking at, the length of these lines gives you, to put it simply, the concentration of these gases, and the shape/width of the lines gives you the altitude where a gas has been found. That’s the very simple way of putting it. After receiving these spectra, we use what we call retrieval algorithms, which are basically the transformation from the spectra to atmospheric composition.

Do you solely focus on Earth’s atmosphere or do you also study the atmosphere of other planets? (e.g. Mars)

At our Lab, yes, we look at other planets as well, but studying other planets is a separate field. With IASI, we only focus on Earth’s atmosphere.

Now that we have introduced the basic notions and have an idea of what the IASI instrument does and how it does it, let’s look into what has been observed with the instrument and what has been concluded from the collected data. What is the state of the atmosphere today? (healthy? unhealthy? recovering?)

It’s not an easy question. It’s almost impossible to reply as it is. It really depends on the timescale you’re considering. So comparing our atmosphere to like, long time ago, pre-industrial era, we can say that anthropogenic emissions have greatly affected our atmosphere. But comparing the state of the atmosphere to a few decades ago, I would say that for example in some areas like China and Europe it’s really less polluted now than 10 years ago. So, it really depends on the timescale you’re considering.

When we talk about global warming, do we specifically refer to the warming of the troposphere? If so, are other layers warming as well?

We are only referring to the warming of the troposphere, which is the layer where we’ve been emitting all these fossil fuel emissions. The stratosphere is actually cooling, and the cooling is due to the ozone depletion.

At the beginning I said that “tropo” means “rotation”, it’s because in the troposphere, the surface is warmer than the top of the layer, but we know that warm air doesn’t stay in place, it goes up. That’s why you have rotating movements in the troposphere. That’s the opposite in the stratosphere. In there, the warm air is on top of the cold air, so there is no movement.

In which layer do CO2 and other Greenhouse Gases (GHG) live?

They live everywhere, in all layers, but they are in higher amount close to the surface where we emit them. We also have to remember that the first natural greenhouse gas is water vapor, and water vapor is mainly located in the troposphere.

Has the atmosphere always looked like what is looks like now in terms of chemical composition? Like, how has it evolved over the last few centuries, more specifically since the industrial revolution?

The physical state of the atmosphere remains unchanged, it’s always and has always been troposphere, stratosphere etc., as I explained earlier. But the chemical composition has changed. At a very very large temporal scale the atmosphere has natural variability with temporal cycles which are very explainable, but after the industrial revolution the greenhouse gases emitted by humans have contributed to change the climate system, and now we have emitted so much greenhouse gases in such a short period of time that global temperature has increased very rapidly, and thus we have disturbed these natural temporal cycles.

How well are we able to model the atmosphere? Like, do we have models to help us estimate what was the atmosphere like in the past (when data is missing), and simulate what the atmosphere will look like in the future? (E.g. what the atmosphere will look like under different warming scenarios?)

First, to know what happened before, we have no satellite data. Satellites are pretty recent, so for past data, scientists mainly use ice cores. In ice cores, you get information about temperature and composition of the main greenhouse gases. You can go thousands of years back with the air trapped in these ice cores. In addition to that, scientists also have to be inventive, so they look at different things, what we call “proxies”. A funny one is the cherry blossom timing. In Japan, you have a dataset of the cherry blossom going back thousand years ago which was made from the diaries of emperors and monks for example. We now know that last year was actually the earliest bloom in 1200 years, to be blamed on global warming. So, that’s to go back in time.

Now, we do have a very good sense on how to model our atmosphere, we improve this modelling by coupling different climate models: ocean models, vegetation models, atmospheric models etc. to see how well they interact with each other. To evaluate our climate models, we try to reproduce the past climate, and we compare it to observations. This works pretty well. For now, we’re using these climate models to help us estimate what the future will look like based on statistical approaches. We run models under different scenarios that are possible stories on how quickly human population will grow, how land use will be affected, how mitigation policies will evolve etc. We have to keep in mind, however, that it’s like weather forecasts, uncertainty grows with time. For weather, for example, we can know pretty well what’s the weather going to look like tomorrow, but we’ll have a hard time knowing what the weather will look like next week. This is the same thing for climate at the longer timescale.

We’ve heard a lot about the ozone hole (in the news etc). Considering the different layers of the atmosphere we talked about earlier, in which layer is the ozone hole exactly? Also, is the ozone hole actually a hole?

The ozole hole is located in the stratosphere. Without this layer, life wouldn’t be possible, it has to be there. “Hole” is a misnomer, it’s not a hole, it’s a thinning of the ozone layer in the stratosphere at certain locations which are the poles.

I’ve read that the hole was above Antartica. Could you explain why the hole is above Antartica specifically?

The atmosphere is not static. Gases and polluting particles move around the Earth following the global scale dynamic. For example, pollution from one country doesn’t stay there, it can be transported far away from its source, which is why it’s a difficult issue to solve.

To create an ozone hole, you need two things. First, you need a chemical destroyer, so let’s say the CFCs, for example bromine and chlorine compounds. Second, you need cold. In Antarctica, you have both. The CFCs emitted at our latitudes are then transported with the global atmospheric circulation to the poles, and it stays there. Antarctica is surrounded by the ocean, so, in winter there is a very strong rotating wind that appears which isolates completely this land. The presence of this wind, that we call “polar vortex”, makes temperature drop very very low and allows the formation of polar stratospheric clouds in which the destruction of ozone occurs.

Is that due to the fact that Antarctica is a continent as opposed to Arctic which is an ocean?

Antarctica is very isolated and surrounded by oceans.

You have some land around the Arctic that makes it less isolated than Antarctica, and this is this isolation and the fact that it is largely surrounded by oceans that favor the formation of these strong winds.

I’ve read that ozone depletion was continuing to occur, especially above the Arctic, is that true?

We call it a hole, which is not a hole, as we discussed. It’s smaller in the Arctic, but can happen there too if it’s very cold. It’s a matter of timescale, for very long timescale, the ozone layer is recovering because CFCs emissions have dropped after the Montreal protocol ratification, but ozone depletion can occur on a yearly basis if the winter is very cold, and so we get some variability on top of the very long recovery process for the ozone layer.

Are there specific negative feedback loops you are closely monitoring? E.g. ozone gets depleted, so more UV-B hit the surface of the planet, contributing to global warming, which could release ozone-depleting substances and/or create more extreme atmospheric conditions that could deplete the ozone layer (and so on and so forth)

It’s a bit of a confusing question. Ozone in the stratosphere is thinner but only at the poles, not everywhere. Second, ozone is a greenhouse gas that has a positive effect. Ozone forms a layer that protects us from UV radiations, not infrared radiations. So, less ozone in the stratosphere will not affect the temperature at the surface, but will cool the stratosphere.

Would such instruments as IASI be affected by geoengineering techniques like atmospheric sulfur injection? Is your team generally engaged in discussions around geoengineering topics?

Let’s be positive and recall that there are no plans for geoengineering at the global scale yet. We haven’t talked about it yet, but the main limitation of IASI measurements are clouds. If there are too many clouds that block the infrared radiation from the Earth to the satellite, the signal is going to be too low, and then the spectra won’t be analyzable. So, for example, if you get sulfur injections somewhere, it’s going to affect the shape of the spectra, so it’s going to affect the IASI measurement. But it also greatly depends on the amount and the location. For example, if we do cloud seeding over specific regions, for sure the clouds will hinder the IASI measurement close to the surface.

We are not really engaged in this kind of geoengineering questions, and we are pretty skeptical because we don’t know yet how to quantify the advantages and the limitations of these approaches and, overall, it is a pity to focus on solving the consequences of climate change instead of solving the cause which is reducing fossil fuel emissions.

The floor is yours, anything you’d like to mention or share with the community?

Nothing I can think of now, thanks.


Additional reading


We originally published this interview on OCRA, a climate forum and community that we started. This interview has been moved to our blog following the closure of the OCRA forum.

Ganddee interviews offer an opportunity to put some guest community members at the center stage. During these interviews we ask questions to our expert guest to better understand their work, learn from them, and understand how they contribute to the fight against climate change. We hope these interviews will serve as an invitation to expend the discussion on the forum.

If you liked this interview, please be sure to hit the “like” button on this post to let us know. If there are specific topics you would like to see covered in an interview, please let us know by either sending an email at: or by dropping a comment below.

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