Aerosol particles are ubiquitous in the atmosphere. They exert significant effects on the Earth’s climate. First, they absorb and scatter incoming solar radiation (direct radiative forcing). Second, they act as cloud condensation nuclei (CCN), influencing the cloud microphysical and optical properties as well as their life-time and precipitation (indirect effect). The number size distribution of atmospheric aerosol particles is a basic, but essential parameter required in calculations of the effects of aerosols on climate, human health, and eco-systems. It is also an important parameter with regard to the description of aerosol dynamical processes as well as heterogeneous chemical reactions in the atmosphere.

In particular particle size distribution constrains:

i. aerosol radiative forcing, as they directly relate to the optical properties of an aerosol population,

ii. cloud forming capacity of an aerosol population, as they directly relate to the number of available cloud condensation nuclei (CCN),

iii. the health impact of aerosols, as they provide useful information on the lung-penetrating fraction of the aerosol population.

An accurate knowledge of long-term particle number and size variability is therefore clearly relevant to aerosol and climate sciences.

In particular, very little information exists to date from high altitude remote sites.

An important phenomenon associated with the atmospheric aerosol system is the formation of new atmospheric aerosol particles.

Once formed, aerosol particles need to grow further to sizes >50-100 nm in diameter until they are able to influence climate, even though smaller particles may have influences on human health and atmospheric chemistry.