1Laboratoire de Meteorologie Physique, CNRS, Université Blaise Pascal, 24, av. des landais, 63177 Aubiere cedex, France
2Laboratoire de Glaciologie et Géophysique de l’Environnement, 54, rue Molière, 38402 St. Martin d’H`eres cedex, France
3now at MPI Heidelberg, Germany
Abstract. Chemical reactions of dissolved gases in the liquid phase play a key role in atmospheric processes both in the formation of secondary atmospheric compounds and their wet removal rate but also in the regulation of the oxidizing capacity of the troposphere (Lelieveld and Crutzen, 1991). The behaviour of gaseous species and their chemical transformation in clouds are difficult to observe experimentally given the complex nature of clouds.
In this study, we have deployed an experimental set-up to provide an in-situ quantification of phase partitioning and chemical transformation of both organic (CH3COOH, HCOOH, H2C2O4) and inorganic (NH3, HNO3, SO2, HCl) species in clouds.
We found that, carboxylic acids, nitrate, and chloride can be considered close to Henry's law equilibrium, within analytical uncertainty and instrumental errors. On another hand, for reduced nitrogen species, dissolution of material from the gas phase is kinetically limited and never reaches the equilibrium predicted by thermodynamics, resulting in significant sub-saturation of the liquid phase. On the contrary, sulfate is supersaturated in the liquid phase, indicating the presence of significant aerosol-derived material transferred through nucleation scavenging.
Upon droplet evaporation, most species, including SO2, tend to efficiently return back into the gas phase. In that sense, these species contribute to acidification (for carboxylic acids) or neutralization (for NH3) of the liquid-phase but not totally of the processed aerosols. The only species that appears to be modified in the multiphase system is nitrate. A fraction of at least 10 to 40% of the liquid phase NO3 originates from dissolved HNO3 of which only a fraction evaporates back to the gas phase upon evaporation, resulting in an NO3 enrichment of the aerosol phase. In-cloud gas-to-particle transfer of HNO3 possibly plays a key role in aerosol acidification and in the modification of their hygroscopic properties.
Our study emphasizes the need to account for the in-cloud interaction between particles and gases to provide an adequate modeling of multiphase chemistry systems and its impact on the atmospheric aerosol and gas phases.