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

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 k_a and k_d 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 = M_ads wherein SS and M_ads 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 M_ads of 2500 s at ambient temperature and a rate constant k_a 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 k_a 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 t_s (= 1/k_d) of 170 s for both substrates at ambient temperature. The corresponding acceleration factors for k_a 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 k_a 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.