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The atmosphere contains gases, water vapours and tiny particles known as aerosols. On average several million tonnes of aerosols are emitted in the atmosphere every year. They originate from natural sources (volcanic ash, desert dust, sea spray) or from human activities (industrial smoke, soots from fossil fuel and biomass burning, gas-to-particle conversion, etc). The aerosol and gases effects on health, air quality and climate are affecting policy decisions and have direct impact on energy use and economic activities around the world.

Effect of aerosols on the climate system and the hydrologic cycle

- Interaction with solar and thermal radiation
By scattering sunlight and reflecting a fraction of it back to space aerosols cool the climate system (direct effect), which counteracts the warming of greenhouse gas (GHG); by absorbing sunlight in the atmosphere aerosols further cool the surface but warm the atmosphere, changing the temperature and humidity profiles and the conditions for cloud development and formation (semi-direct effect).
The direct effect is still not very accurately estimated over land where the single scattering albedo (the ratio between absorption and extinction properties of aerosol) is a key parameter difficult to apprehend at a global scale. For instance, the heating effect of absorbing aerosol is increased if the aerosol is located over a bright surface like a cloud when, over ocean, aerosols cool the atmosphere-ocean system by increasing the reflected solar flux.

- Interaction aerosol-clouds
Hygroscopic aerosol particles play a second role by impacting the hydrologic cycle: acting as cloud condensation nuclei aerosols control the cloud droplet concentration and size thus they can delay or prevent the development of precipitation. These processes (first and second aerosol indirect effects) may cause changes in precipitation patterns, changes in cloud cover, and possible changes in the severity of severe storms.
Accurate modeling of the indirect effects is very difficult, the observation of these aerosol-cloud interactions is also a great challenge. If cloud formation and its water content are determined predominantly by large scale processes, the cloud droplet or ice crystal concentration is driven primarily by the concentration and properties of aerosol particles that act as cloud condensation (CCN) and ice nuclei (IN).

Impact of aerosols on human health and air pollution

The tiniest the particles are, the deepest they can proceed in the respiratory tract and cross the different barrier. The small aerosol component, PM2.5 (mass of particles with an aerodynamic diameter lower than 2,5 μm) is of most concern to human health because the exposure to suspended particles can contribute in some lung and respiratory diseases and even premature death.

Aerosols have an impact on air pollution and thus on health and economy. In 2011, the Aphekom european study figured out that exceeding the annual average level of fine particles recommended by the World Health Organization (10 µg / m3) noticed in 25 European cities costs 31.5 billion euros each year in health spending, working days lost due to sickness and costs associated with the loss of welness, quality and life expectancy.