Study on the role of Black Carbon aerosols in cloud droplets during the dissipation of a fog.

Cloud condensation nuclei (CCN) are the subset of aerosol particles able to form cloud droplets. CNN activation is influenced by the size distribution, chemical composition and number of particles. They consequently impact the cloud microstructure, which affects the radiative properties of clouds, atmospheric circulation and thermodynamics, as well as radiative budgets. By influencing the single scattering albedos of clouds, some particles lead to an increase of the solar irradiation absorption and solar heating in the cloud layers. A good example of these absorbing particles is those made of black carbon (BC), which is emitted during the combustion of various types of fuel and non-exhaust traffic-related processes. The present study deals with the role of BC in a solar radiative scheme and its interaction with clouds during a well-documented case of a fog that evolves into a low stratus cloud. To do so, the solar scheme of the computational fluid dynamic model Code Saturne is used for the estimation of fluxes and heating rates in the atmosphere. It is based on the two-stream parameterization with calculations done in the ultraviolet-visible (UV-Vis) and solar infrared (SIR) bands.

A special attention is given on the impact of BC on the dissipation of the fog. As expected, the introduction of BC in cloud droplets accentuates the heating in the layers at the top of the cloud where water liquid content is maximum. In the SIR band, there is an increase of approximatively 80 %. In the UV-Vis band, where absorption of solar irradiance by ozone is minor, the heating rate is now 10 times higher. The contribution of the UV-Vis band becomes more important. The augmentation of solar heating leads to a reduction of the liquid water content and, consecutively, to a faster dissipation of the fog and the stratus. Therefore, direct surface fluxes are also increased.

When increasing the volume fraction of black carbon in cloud droplets, the water liquid content is furthermore reduced leading to a faster dissipation of the fog. However, this impact is small, because the fog is formed in the morning. At this time, the cooling rate due to thermal radiation is higher than the solar heating at the top of the cloud. We expect the impact of black carbon in cloud droplets to be higher for more persistent clouds or for a fog in the boundary layer of the urban atmosphere, where the fraction of BC in particles is higher.