StrOng surfactaNts in ATmospheric Aerosols and their role on cloud formation – SONATA

The formation of cloud droplets from aerosol particles in the atmosphere is still not well understood and a main source of uncertainties in the climate budget today. One of the principal parameters in these processes is the surface tension of atmospheric particles, which can be strongly affected by trace compounds called surfactants. But the knowledge of the surfactants present in atmospheric aerosols and of their effects on cloud formation is still very limited. The presence of strong surfactants in atmospheric aerosols was revealed only recently by the development of a specific extraction method by the group of B. Nozière. Their exceptional tension-active properties suggested that they could be of microbial origin (“biosurfactants”), which would imply that microorganisms at the Earth’s surface could affect cloud formation. Since this would have important implications for the Earth’s climate and hydrological cycle the structure and origin of the atmospheric surfactants need now to be further investigated. In addition, their effects on cloud formation are currently largely underestimated, both experimentally and in models, because most instruments measuring these processes operate with inadequate residence times. Current “partitioning” models predicting cloud formation in the atmosphere are thus missing dynamic components of the surface tension that are essential to predict the formation of cloud droplets.
The objective of this project is to bring unprecedented information on atmospheric surfactants and their effects on cloud droplet formation by combining unique expertise from the groups of B. Nozière and A.-M. Delort, in France, and the group of R.C. Cohen at UC Berkeley, USA. Innovative approaches will be developed to obtain information that was out of reach until now:
- the molecular structure of the surfactants in atmospheric samples will be investigated by using the new extraction method and state-of-the-art Liquid Chromatography/Mass Spectrometry (LC/MS) and NMR techniques, in collaboration between the groups of B. Nozière and A.M. Delort. The compounds identified will be compared with model surfactants, in particular with surfactants produced by microorganisms present in atmospheric waters, to try to identify their origin,
- the effects of model and atmospheric surfactants on the activation of sub-micron particles will be investigated in laboratory as a collaboration between the groups of B. Nozière and R.C. Cohen. The dynamic surface tension and molecular interactions missing in current models will be investigated with the hanging droplet technique in Lyon, and a new Aerosol Optical Trap/Raman spectroscopy set-up will be built at UC Berkeley to measure, for the first time, the Raoult’s term and surface tension of micron-size particles, which will represent a breakthrough in this discipline.
Most of the techniques and methods to be used in this project have been or will be developed by the PIs, making this project unique and innovative. In addition to improving the prediction of cloud droplet formation, these investigations will resolve long-standing instrumental and theoretical biases in the study of these processes, thus benefiting a wide community of modelers, instrument designers and instrument users.