Descriptif

The atmosphere of the Earth is essentially composed of nitrogen (~78%) and oxygen (~20%), with smaller amounts of argon (~1%), carbon dioxide (~400 ppmv), methane (~1.8 ppmv), and variable amounts of water in solid, liquid, and gaseous phases. Apart from its major components, the atmosphere also contains many species of trace gases, with concentrations below 1 ppmv in the troposphere, but significant impacts on human health and/or the radiative balance of the atmosphere. These species include ozone, nitrogen oxides, sulphur dioxide and carbon monoxide, among many others, as well as particles with diameters up to several micrometers and concentrations generally below 1000 µg/m³.

These concentrations of trace gases are due to both natural and anthropogenic processes : emissions of Volatile Organic Compounds (VOCs) by the vegetation, emissions of sea-salt and mineral dust due to the action of wind on marine and continental surfaces respectively, emissions of trace gases and particles by wildfires and volcanic eruptions. In the stratosphere, trace gases such as ozone are produced by processes initiated by the photolysis of dioxygen under the action of UV radiation, which is the most important source of atmospheric ozone. Regarding anthropogenic sources, combustion of fossil fuel and biomass are major sources of trace gases such as carbon monoxide and nitrogen oxides, as well as particles such as soot and organic matter. Agricultural processes also contribute substantially to particle emissions, mainly due to species such as nitrates and ammonia that are emitted from fertilizing processes.

After the emission into the atmosphere of these primary pollutants, their concentrations and physico-chemical state evolve under the influence of many processes. Chemical reactions occur under the influence of sunlight, controlling the concentration of tropospheric ozone through a complex interaction between the concentrations of ozone, nitrogen oxides and VOCs, while other physico-chemical processes govern the conversion of gaseous species such as sulphur dioxide and VOCs into particulate species, and the particulate species themselves evolve due to processes such as coagulation, evaporation, sedimentation, and aqueous chemistry. Ultimately, trace gases and particles are removed from the atmosphere either by dry or wet deposition or by the termination of chemical cycles by production of one of the major atmospheric components. All these processes occur in the framework of an atmosphere that undergoes a dynamic motion, which mixes and transports the trace components of the atmosphere over short to long distances depending on their lifetime, while the atmospheric content of particles can also in turn have an impact on atmospheric motion through radiative and microphysical processes.

Due to the early-recognized adverse health effects of anthropogenic gases and particles in urbanized and industrial areas, smoke-abatement measures have been taken in parts of the US and Europe as soon as the end of the 19th century, evolving into the enforcement of systematic regulations on emissions and on observed pollutant concentrations in many countries during the second half of the 20th century, with also a significant effort on obtaining routine measurements of the particulate matter and trace gas concentrations in both urban and rural areas, at least in North-America and Europe. The emergence of these regulations and measurement networks in turn permitted a strong development of numerical tools for the modelling of atmospheric composition, an effort that led to the public availability of many modelling platforms that are able to take into account all or most of the above-mentioned processes.