Introduction

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.

What is the “atmosphere composition”?

The atmosphere is a ~500 km layer of gases (so-called “air”), clouds, water droplets and particles. This layer is retained by the Earth’s gravity.

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”?

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

trace-gases
Relative proportion (%) of the main gases present in the atmosphere (Source: )

More than 99% of the air only include 2 gases: [di]Nitrogen (N2) and [di]Oxygen (O2). Some rare gases such as Argon (0.93% of the air) are also present. But, the challenge of scientists working on the atmospheric composition measurements is the quantification and monitoring of the concentrations of the remaining gases,  that are then present in our atmosphere in a very very low proportion (less than 0.04% of the total atmospheric gases). This is why these gases are named “trace gases”.

Gases in the atmosphere may be classified in two categories:

  • 1) the gases with a (relative) constant concentration in the atmosphere (at least in the lower part): N2, O2, and the rare  gases. They are part of dry air and their behavior can follow the (scientific) law of a perfect gas.
  • 2) the gases with a variable concentration in the atmosphere: the trace gases.

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.

Note that H2O (water vapor) is one of these trace gases, and is mostly present in the troposphere.  It contributes to the humidity variation in the air over the regions, to the precipitation regime (depending on temperature conditions) and also to the climate.

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.

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:

  • While O3 – Ozone is a green-house gas in the troposphere and stratosphere, it is also a toxic pollutant gas when present close to the surface (opposite to the stratosphere). You have probably already heard some warnings on TV, during summer time in big cities, about peak of O3 pollution…
  • 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.

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 dioxode, CH4 – Methane, O3 – Ozone, …