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Home > The ABC of air > The role of weather
The role of weather

For a given quantity of pollutant emissions, the quality of the air can be good or bad depending on weather conditions. Wind direction and speed, temperature and the evolution of temperature with altitude, sunshine and precipitation contribute to the dispersion of pollutants. Here are a few examples.
Bord de mer

Wind

Cerf_volant
A very low wind speed is an aggravating factor. Whereas a strong wind generally disperses pollutants, it can aggravate the situation locally by driving smoke onto a group of dwellings. But wind also carries air masses that can be laden with pollutants from sources located farther away geographically. These conditions can then lead to generalised pollution episodes.

Temperature gradient

In the troposphere (lower atmosphere), the temperature normally decreases with the altitude (in the neighbourhood of 0.65°C per 100 m at our latitudes). Due to this temperature gradient, smoke rises (on the hot-air balloon principle) and the pollutants disperse. On certain nights, the ground and the thin layer of air just above it cool more quickly than the layer of air above, which in such cases is called the inversion layer. It blocks the ascent of pollutants, preventing them from dispersing in the atmosphere. They are trapped by the layer of warmer air, creating a layer of pollution that is visible to the naked eye when this phenomenon occurs.
photo M Capot SMI PJ 2

photo : Maritxu Penez, Caux Vallée de Seine

Temperature

The temperature of the air has an influence on emissions of certain pollutants. So, for example, emissions related to heating (wood, coal, heating oil, etc.) will logically be more present during the winter. In summer, the formation of photochemical pollution, such as ozone, is encouraged by the higher rate of evaporation of the Volatile Organic Compounds (VOCs) that are the precursors of this type of pollution. Temperature and sunshine also play a role in the chemical reactions that are produced between different compounds.

Sea breeze

Brise de mer
The presence of the sea influences the movements of air masses. During the day, the air located above land heats more quickly than the air over the sea. It becomes lighter, gains altitude, and is replaced by the sea air, which is cooler: This is the sea breeze. It reaches maximum intensity in the afternoon, when the difference in temperature between the ground and the water is greatest. 
Brise de terre
After sunset, the ground cools more quickly than the sea. When the temperatures on land and at sea become equal, the wind quiets, and then phenomenon is reversed. The air above the sea rises and cools and is replaced by the air above the land: This is the land breeze. This phenomenon influences ozone concentrations, which are higher on average near the shore due to a recirculation of polluted air.

Precipitation

As it falls, the rain picks up certain gaseous or particulate pollutants present in the air, thus causing a “washing” effect and generally improving the quality of the air.

OTHER MAJOR POLLUTION PHENOMENA

Climate change

The sun’s radiation strikes the ground. The surface of the Earth returns that energy in the form of heat. Part of this radiated heat is dissipated in space, but another part is absorbed by certain gases present in the air. This is what is called the greenhouse effect, which exists naturally and without which the temperature on our planet would be an average of -18°C compared to +15°C currently.

Since the middle of the 19th century, the evolution of demographics and of industrial production have brought about a growth in emissions of so-called greenhouse-effect gases.

Certain greenhouse-effect gases:

  • carbon dioxide (CO2)
  • methane (CH4)
  • nitrous oxide (N2O)
  • chlorofluorocarbons (CFC)
  • ozone (O3)

These gases have the specific property of trapping the solar radiation received by the planet, and thus accentuating the natural greenhouse effect. A group of inter-governmental experts on climate change (IPCC) is studying this phenomenon and attempting to predict its consequences:

Among the predicted consequences of the increase in the greenhouse effect are:

  • climate disruption on a planetary scale
  • a rise in the sea level
  • an increase in diseases
  • displacement of populations

International agreements are being sought in order to halt the process.

The link between atmospheric pollution and climate change

Atmospheric pollution and climate change are indissociable because they have the same origin – anthropic and natural emissions of substances that contribute to the two phenomena in a concomitant or antagonistic way. Thus, the increase in temperatures has the effect of increasing emissions of biogenic Volatile Organic Compounds (VOCs) – that is, those emitted naturally by the forests. VOCs are precursors of ozone formation. The increase in the background level of ozone also has the direct consequence of amplifying the greenhouse effect. Another example is particles in suspension. Of natural (desert dust, agricultural soil, etc.) or anthropic (automobiles, industry, agriculture, etc.) origin, the size of these particles can vary from a nanometre (groups of molecules) to a few tens of micrometres (particles of dust and cloud droplets). These particles have a direct impact on the climate because they diffuse or absorb solar radiation, but they also have an indirect impact when they act as the condensation nuclei necessary for formation of clouds. The overall impact of the particles is a cooling of the atmosphere, but numerous uncertainties remain, in particular as regards interactions between gases, particles and clouds.

Certain initiatives intended to limit climate change can therefore be prejudicial to air quality, and conversely efforts to limit atmospheric pollution can contribute to climate change:

  • the use of biomass,
    as a replacement for fossil energy, to reduce emissions of greenhouse-effect gases, results in high emissions of atmospheric pollutants (VOCs, HAPs, particles and carbon monoxide) with negative consequences for human health.
 
  • diesel vehicles,
    while they consume less fuel and therefore emit less CO2, produce more polluting particles and nitrogen oxides (NOx) that are harmful to human health.
 
  • sulphate particles,
    which are hazardous to human health but also to vegetal and aquatic ecosystems, tend to cool the atmosphere. Efforts to limit their emissions will only protect health human and the environment to a certain extent.
 
  • measures intended to limit atmospheric pollution
    sometimes require energy and thus themselves result in emissions of greenhouse-effect gases.
 

Finally, the problem of the quality of the air in buildings can be cited. This sector consumes more than 40% of end-use energy and accounts for close to a fourth of all of greenhouse-effect gas emissions in France. To reduce this consumption, ambitious goals have been set in the context of the “Grenelle de l’environnement” process, to be attained through significant improvement of the energy efficiency of buildings, and in particular through improving their insulation. In this context, it is more than ever necessary to maintain adequate ventilation and limit emissions of polluting substances inside buildings.

Thus any energy policy, national, regional or local, can have an impact on the climate (via emissions of greenhouse-effect gases), but also on the quality of the air (with emissions of particles, ozone precursors, and other pollutants, etc.) These incidences are now better taken into account in seeking “win-win” actions.

The “hole” in the ozone layer

In the stratosphere (between 10 and 60 km altitude), ozone (O3) constitutes a natural filter that protects life on Earth from harmful ultraviolet (UV) radiation. The “hole” in the ozone layer corresponds to the partial disappearance of this filter due to the destructive effect of certain pollutants (CFCs, HCFCs, etc.) Since 1987, the Montreal protocol has strictly regulated the production and utilisation of the substances responsible for the destruction of the ozone layer. Nearly all countries have signed this protocol.