High above the ocean: unexplored molecular processes

Oceans cover more than 70% of the Earth's surface and generate particles in the atmosphere that are viewed as a core component of the climate system, especially due to their role in marine cloud formation and the strong control they have on their ability to reflect sunlight to space.

With our recent findings, we have made great progress in documenting the formation of new particles by nucleation of low volatile gas-phase species in the lowest altitudes of the open ocean atmosphere and its relation to water biogeochemistry.

Nucleation is however expected to be promoted at higher altitudes, in the marine free troposphere (MFT), and the entrainment of growing particles formed in the MFT is further suspected to be a major source of low-level marine cloud forming particles.

Yet, because of limited observations, very little is known about MFT nucleation, which contributes to major uncertainties both in the assessment of present-day radiative forcing associated to aerosol-cloud interactions and in future climate projections.The main purpose of this project is to identify the species driving MFT nucleation and evaluate the impact of the process on cloud condensation nuclei (CCN) number concentration, and ultimately on marine cloud formation and properties.

To achieve this goal, we will first use an innovative experimental setup consisting of a nucleation chamber filled with natural free tropospheric air coupled to state of the art instruments to identify the nature of the nucleating species.

Experiments will be performed on Reunion Island, in the southern Indian Ocean, which is particularly sensitive to natural aerosol sources.

These measurements will in a second step be used to initiate a modelling study that will further complement the observations, allowing first to evaluate the impact of MFT nucleation on the CCN population at the regional scale, and then to study in detail the impact on marine cloud formation and properties over the open ocean.