Abstract
Abstract. The formation of clouds is an important process for the atmosphere, the hydrological cycle, and climate, but some aspects of it are not completely understood. In this work, we show that microorganisms might affect cloud formation without leaving the Earth's surface by releasing biological surfactants (or biosurfactants) in the environment, that make their way into atmospheric aerosols and could significantly enhance their activation into cloud droplets. In the first part of this work, the cloud-nucleating efficiency of standard biosurfactants was characterized and found to be better than that of any aerosol material studied so far, including inorganic salts. These results identify molecular structures that give organic compounds exceptional cloud-nucleating properties. In the second part, atmospheric aerosols were sampled at different locations: a temperate coastal site, a marine site, a temperate forest, and a tropical forest. Their surface tension was measured and found to be below 30 mN/m, the lowest reported for aerosols, to our knowledge. This very low surface tension was attributed to the presence of biosurfactants, the only natural substances able to reach to such low values. The presence of strong microbial surfactants in aerosols would be consistent with the organic fractions of exceptional cloud-nucleating efficiency recently found in aerosols, and with the correlations between algae bloom and cloud cover reported in the Southern Ocean. The results of this work also suggest that biosurfactants might be common in aerosols and thus of global relevance. If this is confirmed, a new role for microorganisms on the atmosphere and climate could be identified.
Highlights
Clouds play important roles in the atmosphere, the hydrological cycle, and climate
Unlike the microorganisms these substances could make their way into sub-micron aerosol particles, which are important for cloud droplet nucleation
To our knowledge it is the first time that specific organic molecules are shown to have better cloud-forming efficiencies than inorganic salts (McFiggans et al, 2006). These results are important because they identify the molecular structures that give to organic compounds exceptional CCN properties, and which should be investigated in atmospheric aerosols
Summary
Clouds play important roles in the atmosphere, the hydrological cycle, and climate. They constitute the largest cooling contribution to climate, and the one containing the largest uncertainties (Foster et al, 2007a) because the nucleation of atmospheric aerosols into cloud droplets is still not completely understood. A possible contribution of the biosphere in these processes has been investigated for decades One of these contributions was proposed as the CLAW hypothesis (Charlson et al, 1987, 2001), where the phytoplankton of the oceans emits dimethylsulfide (DMS), which is converted into sulfate salts in the atmosphere (Shaw, 1983), an efficient cloud-nucleating material (Charlson et al, 1987, 2001). Rather than studying the role of airborne microorganisms in the formation of cloud droplets we study the role of substances that are produced by some microorganisms at the Earth’s surface and released in the environment, biosurfactants Unlike the microorganisms these substances could make their way into sub-micron aerosol particles, which are important for cloud droplet nucleation. The presence of biosurfactants in real aerosol samples and their effect on their surface tension is investigated
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