Elucidating gas-surface interactions relevant to atmospheric particle growth using combined temperature programmed...

Understanding growth mechanisms for particles in air is fundamental to developing a predictive capability for their impacts on human health, visibility, and climate. In the case of highly viscous semi-solid or solid particles, the likelihood of impinging gases being taken up to grow the particle will be influenced by the initial uptake coefficient and by the residence time of the adsorbed gas on the surface. Here, a new approach that combines Knudsen cell capabilities for gas uptake measurements with temperature programmed desorption (TPD) for binding energy measurements of gases is described. The application of this unique capability to the uptake of organic gases on silica demonstrates its utility and the combination of thermodynamic and kinetic data that can be obtained. Lower limits to the initial net uptake coefficients at 170 K are (3.0 ± 0.6) × 10^-3, (4.9 ± 0.6) × 10^-3 and (4.3 ± 0.8) × 10^-3 for benzene, 1-chloropentane, and methanol, respectively, and are reported here for the first time. The uptake data demonstrated that the ideal gas lattice model was appropriate, which informed the analysis of the TPD data. From the thermal desorption measurements, desorption energies of 34.6 ± 2.5, 45.8 ± 5.5, and 40.0 ± 5.6 kJ mol^-1 (errors are 1σ) are obtained for benzene, 1-chloropentane, and methanol, respectively, and show good agreement with previously reported measurements. A multiphase kinetics model was applied to quantify uptake, desorption, and diffusion through the particle multilayers and hence extract desorption kinetics. Implications for uptake of organics on silica surfaces in the atmosphere and the utility of this system for determining relationships between residence times of organic gases and particle surfaces of varying composition are discussed in the context of developing quantitative predictions for growth of aerosol particles in air.