Application of physical adsorption thermodynamics to heterogeneous chemistry on polar stratospheric clouds

Abstract

Laboratory isotherms for the binding of several nonheterogeneously active atmospheric gases and for HCl to water ice are translated into adsorptive equilibrium constants and surface enthalpies. Extrapolation to polar conditions through the Clausius Clapeyron relation yields coverage estimates below the percent level for N,, Ar, CO,, and CO, suggesting that the crystal faces of type II stratospheric cloud particles may be regarded as clean with respect to these species. For HCl, and perhaps HF and HNO,, estimates rise to several percent, and the adsorbed layer may offer acid or proton sources alternate to the bulk solid for heterogeneous reactions with stratospheric nitrates. Measurements are lacking for many key atmospheric molecules on water ice, and almost entirely for nitric acid trihydrate as substrate. Adsorptive equilibria enter into gas to particle mass flux descriptions, and the binding energy determines rates for desorption of, and encounter between, potential surface reactants.

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume VI – heterogeneous reactions with liquid substrates

“…Moreover on the surface of bulk liquids, the second reactant molecule may originate in the gas phase or the bulk liquid phase. For parameterisation of the uptake coefficients we adopt the approach presented by Ammann et al (2003), which builds on earlier studies (Elliott et al, 1991;Mozurkewich, 1993;Tabazadeh and Turco, 1993;Carslaw and Peter, 1997).…”

Kinetic model framework for aerosol and cloud surface chemistry and gas-particle interactions – Part 1: General equations, parameters, and terminology

“…In the scientific literature similarly defined parameters have been called adsorption coefficient Turco et al, 1989;Worsnop et al, 2002), condensation coefficient (Pruppacher and Klett, 1997), sticking coefficient (Hanson, 1997), sticking probability (Clement et al, 1996;Garrett et al, 2006), trapping probability (Masel, 1996), adsorptive mass accommodation coefficient (Elliott et al, 1991), (mass) accommodation coefficient (Jayne et al, 1990;Tabazadeh and Turco, 1993;Pruppacher and Klett, 1997;Kulmala and Wagner, 2001;Huthwelker et al, 2006), and thermal accommodation coefficient (Li et al, 2001;Worsnop et al, 2002). For clarity and unambiguous distinction of the gas-to-surface mass transfer process specified above from mass transfer into the bulk, from chemical reactions, and from heat transfer (Sect.…”

“…This approach has already been introduced and applied for the description of selected adsorption and reaction processes in stratospheric aerosols at different temperatures (Elliott et al, 1991;Tabazadeh and Turco, 1993;Mozurkevich, 1993;Carslaw and Peter, unpublished manuscript, 1997), and for specific systems the required thermochemical data are available in the scientific literature of physical chemistry and chemical engineering (e.g. Masel, 1996; and references therein).…”

Experimental isotherms of HCl on H2O ice under stratospheric conditions: Connections between bulk and interfacial thermodynamics

“…18,20,21 A detailed physical-chemical mechanism explaining experimentally observed HCl coverage on ice remains elusive. Classic Langmuir modeling of uptake by Elliott et al 54 and Mozurkewich 55 represent early efforts. In a significant work Tabazadeh and Turco 56 used a Langmuir model to suggest that the independence of uptake on pressure observed by Hanson and Ravishankara 8 ͑which is representative of much of the other uptake work discussed above͒ required a minimum adsorption free energy of 56 kJ/mol, a number approaching the heat of hexahydrate sublimation or solution at infinite dilution and themselves proposed an adsorption enthalpy of 85 kJ/mol.…”