This page lists the main variables related to the atmosphere composition, that we try to observe from satellite measurements. This list is likely not exhaustive, and primarily (but not only) focuses of variables of interest associated with my past and current research activities.


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


What is the “atmosphere composition”?

The atmosphere is a 100 km layer composed of gases (i.e. air molecules with a size of about 1 nm or less), clouds (or ice crystals), water droplets and particles (larger than 1 nm) between the Earth surface and the vacuum of outer space (Burrows et al., 2011). This layer is retained by the Earth gravity. Together with the Sun and the surface, they form a complex but essential system for maintaining our environment and life. Historically, the young atmosphere, at the formation of the Earth approximately 4.54 billion years ago, was produced by out gassing from volcanic eruptions and surface. The big rise of oxygen was brought by the biosphere when life appeared about 3.8 billion years ago.

The term of “atmosphere composition” here mainly refers to the gases, named here-after “trace gases”, and aerosols (or particles) that are present in our atmosphere (and thus in the air that we breathe).

What are the “trace gases”?

Dry air = 99% ([di]Nitrogen + [di]Oxygen) + 0.04 % (so many other gases…)

Relative proportion (%) of the main gases present in the dry atmosphere

Nowadays, more than 99% of the dry air molecules only include 2 gases: [di]Nitrogen N2 (78 %) and [di]Oxygen O2 (21 %). Some rare gases such as Argon (0.93 %) are also present. The rest of the gaseous part of the dry atmosphere is called “trace gases”. Their small relative amount (less than 0.04 %) is one of the main challenges of scientists working on the atmospheric composition measurements. Among these elements, one can name the green-house gases (e.g. CO2 – Carbon dioxide, CH4 – Methane, N2O, O3 – Ozone), and the pollutant gaseous with a relative short lifetime (e.g. NO2 – Nitrogen dioxide, SO2, CO – Carbon monoxide, etc…). Their very low amount does not mean they are inert in the atmosphere: most of them directly affect the atmospheric radiation and thus our climate, air quality and the chemistry processes in our atmosphere.

So, why do we care about the trace gases?

So little amounts, and yet so major impacts…

Some of these trace gases are naturally present. But, since the industrial revolution in the middle of the 19th century, it is clearly demonstrated that concentration of the trace gases have been largely increasing. In spite of their very low relative concentrations, trace gases (and aerosols) lead to major changes on the atmosphere chemistry processes. Therefore, depending on the characteristics of each of them, they either contribute to increase pollution toxicity (and thus adverse health impacts), affect our Earth climate on a very short time period (i.e. close to the human time scale, a couple of years), and sometimes even both simultaneously.

As you can read on the next pages, or by clicking on the associated links, most of these components are either directly, or indirectly, emitted by anthropogenic (i.e. human) activities in the troposhere.

Air pollution, climate change, and depletion of the O3 – Ozone layer are related because they are caused by changes in atmospheric composition. Sources and sinks, chemical conversion, and transport of gases and aerosols determine the composition of the atmosphere, as shown in the figure below.

Composition diagram showing the evolution/cycles of various elements in Earth’s atmosphere (Source: Strategic Plan for the U.S. Climate Change Science Program).

A list of the trace gases?

Here below, a distinction is made between pollutants and greenhouse gases which are both parts of the trace gases: pollutants are gases that are directly harmful for human health; greenhouse gases are gases that directly contribute to the on-going climate change (through the well known green-house process).

Most of the green-house gases are primarily, below a reasonable level, not directly harmful for our health (e.g. CO2). Most of the pollutants do not have a DIRECT effect on our climate (e.g. NO2, SO2 etc…).

But there are some noticeable exceptions. For example:

  • O3 – Ozone is an important contributor of Earth’s radiative balance and air quality. Depending on its location, ozone can be a life saving or a toxic gas. At high altitude (i.e. the stratosphere), its high abundance is vital for living species in the lower layers as it absorbs harmful ultraviolet (UV) radiation coming from the Sun leading then to stratospheric heating. Its concentration decrease, detected during the 1980’s, has led to major concerns. But its presence close to the surface is harmful to animals and humans and damages natural systems and agricultural crops. Ozone is also an important green-house gas in the whole atmosphere due to its thermal infrared absorption and emission properties. It makes a significant contribution to the radiative balance of the upper troposphere and lower stratosphere. Its main uncertainty is related to the changes with respect to altitude that depend on the regions and other chemistry processes (Lacis et al., 1990; Worden et al., 2008).
  • Some gases, such as NO2 or SO2, are primarily pollutants, but because they modify the relative proportions of other gases in the atmosphere, they INDIRECTLY perturb the climate.
  • Water liquid and H2O – Water vapour play a central part in atmospheric and hydrology processes. Water vapour abundance is very variable at the surface from almost null over desert regions to about 4% over oceans. It is the main component of weather and climate, accounting for about 90% of the Earth’s natural greenhouse effect. When air temperature falls below the “condensation temperature”, the air becomes saturated and the water vapour condenses then into cloud droplets. This usually occurs during adiabatic cooling, when moist air is lifted.

Finally, note that although aerosols are not part of trace gases (since they are particles), they impact both air quality and climate…

Pollutant gases

NO2 – Nitrogen Dioxide, SO2 – Sulfur Dioxide, (tropospheric) O3 – Ozone, HCHO – Formaldehyde, NH3 – Ammonia, CO – Carbon Monoxide, …

Green-house gases

H2O – Water Vapor, CO2 – Carbon dioxide, CH4 – Methane, O3 – Ozone, …

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