Many atmospheric aerosols contain both organic compounds and inorganic material, such as sulfate salts. In this work, we show that these sulfates could trigger some chemical transformations of the organic compounds by producing sulfate radicals, SO4−, when exposed to UV light (280–320 nm). In particular, we show by mass spectrometry (LC/ESI‐MSMS) that isoprene, methyl vinyl ketone, methacrolein, and α‐pinene in irradiated sulfate solutions (ammonium and sodium sulfate) produce the same organosulfates as previously identified in aerosols, and even some that had remained unidentified until now. With a typical time constant of 9 h instead of 4600 days for esterifications, these radical reactions would be a plausible origin for the atmospheric organosulfates. These reactions also produced efficient surfactants, possibly resembling the long‐chain organosulfates found in the experiments. Thus, photochemistry in mixed sulfate/organic aerosols could increase cloud condensation nuclei (CCN) numbers, which would be supported by previous atmospheric observations.

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.

Abstract. A significant fraction of the organic material in aerosols is made of highly soluble compounds such as sugars (mono-and polysaccharides) and polyols such as the 2-methyltetrols, methylerythritol and methyltreitol. Because of their high solubility these compounds are considered as potentially efficient CCN material. For the 2-methyltetrols, this would have important implications for cloud formation at global scale because they are thought to be produced by the atmospheric oxidation of isoprene. To investigate this question, the complete Köhler curves for C3-C6 polyols and the 2-methyltetrols have been determined experimentally from osmolality and surface tension measurements. Contrary to what was expected, none of these compounds displayed a higher CCN efficiency than organic acids. Their Raoult terms show that this limited CCN efficiency is due to their absence of dissociation in water, this in spite of slight surfacetension effects for the 2-methyltetrols. Thus, compounds such as saccharides and polyols would not contribute more to cloud formation than other organic compounds studied so far. In particular, the presence of 2-methyltetrols in aerosols would not particularly enhance cloud formation in the atmosphere, in contrary to recently suggested.

Abstract. A significant fraction of the organic material in aerosols is made of highly soluble compounds such as sugars (mono- and polysaccharides) and polyols, including the 2-methyltetrols, methylerythritol and methyltreitol. The high solubility of these compounds has brought the question of their potentially high CCN efficiency. For the 2-methyltetrols, this would have important implications for cloud formation at global scale because they are thought to be produced by the atmospheric oxidation of isoprene. To investigate this question, the complete Köhler curves for C3–C6 polyols and the 2-methyltetrols have been determined experimentally from osmolality and surface tension measurements. Contrary to what expected, none of these compounds displayed a critical supersaturation lower than those of inorganic salts or organic acids. Their Raoult terms show that this limited CCN efficiency is due to their absence of dissociation in water, this in spite of slight surface-tension effects for the 2-methyltetrols. Thus, compounds such as sugars and polyols would not contribute more to cloud formation in the atmosphere than any other organic compounds studied so far. In particular, the presence of 2-methyltetrols in aerosols would not particularly enhance cloud formation in the atmosphere, contrary to what has been suggested.

Abstract. The formation of clouds is an important process for the atmosphere, the hydrological cycle, and climate, but also a difficult one to predict because some aspects of the transformations of aerosol particles into cloud droplets are still not well 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 should significantly enhance their conversion into of cloud droplets. In the first part of this work, the cloud-nucleating efficiency (or "CCN" efficiency) of standard biosurfactants was characterized by osmolality and surface tension measurements and found to be better than for any aerosol material studied so far, including inorganic salts. These results identify molecular structures that give to organic compounds exceptional CCN properties. In the second part, atmospheric aerosols sampled at different locations (temperate & tropical, forested & marine ones) were found to all have a surface tension below 30 mN/m, which can only be accounted for by the presence of biosurfactants. The results also showed that these biosurfactants were concentrated enough to significantly affect the surface tension of these aerosols and enhance their CCN efficiency. The presence of such strong biosurfactants in aerosols would be consistent with the recent identification of organic fractions of higher CCN efficiency than ammonium sulfate in aerosols. And a role of microorganisms at the Earth's surface on clouds could also explain previously reported correlations between algae bloom and cloud cover. Our results also suggest that biosurfactants might be common in aerosols and thus of global relevance. If their impact on cloud formation is confirmed by future studies, this work would have identified a new role of microorganisms at the Earth's surface on the atmosphere, and a new component of the Earth's system and climate.