Aerosols scatter and absorb solar radiation, with several consequences. By scattering, they reflect part of the sun's radiation back into space, causing local cooling of the atmosphere. Conversely, the absorption of solar radiation by aerosols leads to atmospheric warming. Aerosols thus impact both the water and carbon cycles.

Aerosols impact the radiation balance directly, but also indirectly by modifying the life cycle and properties of cloud systems, with consequences for the water cycle. The absorption properties of aerosols deposited on snow and ice increase the amount of radiation absorbed by these surfaces, accelerating their melting. Furthermore, depending on their absorption properties, aerosols affect the radiation available for photosynthesis, on which the primary productivity of ecosystems, and therefore the carbon cycle, depends.

The predominance of cooling or warming depends on the optical properties of aerosols, linked to their chemical composition. Absorption of solar radiation is primarily caused by sooty carbon aerosols, the second biggest contributor to global warming after greenhouse gases, but also by organic carbon and desert dust, which dominate absorption at short wavelengths (370-500 nm). The separation of total absorption by aerosols according to their chemical composition and vertical distribution, which induces a specific response from cloud cover (semi-direct effect), is therefore essential for reducing model and satellite uncertainties in estimating radiative forcing, as well as for establishing scientifically-based air quality control policies.

Thanks to funding from the CNRS-INSU medium-heavy program, the Laboratoire Inter-universitaire des Systèmes Atmosphériques (LISA), in collaboration with CNRM and Aerodyne, has developed a new airborne instrument based on the commercial CAPS-PMSSA sensor, to simultaneously measure the vertical profile of aerosol scattering and optical extinction at 450 and 630 nm, under ambient conditions. The instrument, named A2S2 (Aerosol Absorption Spectral Sizer), has been characterized in the laboratory to define airborne performance and operating conditions, in particular accuracy at low pressure and high measurement frequency, as well as particle size-dependent scattering corrective factors (truncation error). 

The A2S2 instrument in LISA's AVIRAD airborne platform on Safire's ATR-42 during the ACROSS campaign. © Référence