EGU General Assembly 2022
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Autocatalytic Uptake of Small Polar Molecules (H2O, HCl, NO2) on Processed Amorphous Carbon (Soot) and atmospheric Mineral Dust Materials

Michel J. Rossi1, Riccardo Iannarelli2, and Christian Ludwig1
Michel J. Rossi et al.
  • 1Ecole Polytechnique Fédérale de Lausanne (EPFL), CPM Chemical Processes and Materials GR-LUD, Lausanne Ecublens, Switzerland (
  • 2École Polytechnique Fédérale de Lausanne (EPFL), Safety Competence Center, EPFL RHO DSPS-SCC, Station 6, CH-1015 Lausanne, Switzerland

Ice Nucleation (Cirrus clouds) and aviation contrail formation in the wake of jet engine exhaust in the free troposphere and lower stratosphere as well as accelerated atmospheric cloud formation in the presence of atmospheric mineral dust particles are only partially understood phenomena that await a more profound fundamental knowledge base. In this presentation we report measurements from a flowing gas experiment in which probe molecules such as H2O, HCl and NO2 interact with a solid substrate such as Processed Amorphous Carbon (PAC) or mineral dust materials such as the clay mineral Bentonite or Arizona Test Dust (ATD. In these experiments that are performed under molecular flow conditions inside a Knudsen flow reactor both uptake and desorption experiments have been conducted that resulted in the measurement of the rate constants ka and kd for the reversible adsorption/desorption kinetics of the probe gas M in the presence of the solid substrate according to the Langmuir-type equilibrium M + SS = Mads wherein SS and Mads are the surface site density and the density of adsorbed probe gas molecules. Typical results for M = H2O adsorption on PAC are saturation at 0.3% of a monolayer, a surface residence time of the adsorbate Mads of 2500 s at ambient temperature and a rate constant ka that is accelerated by a factor of 75-125 when measured at desorption compared to adsorption. Initial adsorption of H2O on PAC is slow (“dry” case) with an uptake probability on the order of 10-4 to 10-3 per collision. In contrast, desorption from a H2O-saturated PAC surface is from large molecular clusters or nanodroplets adhering to the PAC surface (“wet” case) and is larger by the acceleration factor given above. The adsorption process is therefore autocatalytic in adsorbed H2O abundance which means that the more water that has been adsorbed the larger the adsorption rate constant ka is because the H2O molecules preferentially "choose" already adsorbed H2O for adsorption owing to a higher uptake probability. Bentonite clay and ATD are H2O or D2O saturated at a coverage of 10.6 and 11.7% of a formal molecular monolayer, respectively, with an associated surface residence time ts (= 1/kd) of 170 s for both substrates at ambient temperature. The corresponding acceleration factors for ka in going from the dry to the wet case are 34 and 80, respectively. We are aware that the transition from dry to wet or inversely is smooth, whereas in this work we have characterized merely the extremes, namely dry and wet. Future work will interpolate ka in parametrized form in order to encourage the use of numerical models describing the uptake and desorption of H2O and other small polar molecules by suitable atmospheric nuclei.

How to cite: Rossi, M. J., Iannarelli, R., and Ludwig, C.: Autocatalytic Uptake of Small Polar Molecules (H2O, HCl, NO2) on Processed Amorphous Carbon (Soot) and atmospheric Mineral Dust Materials, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1894,, 2022.