Water uptake on mineral dust and soot: A fundamental view of the hydrophilicity of atmospheric particles?

Abstract

The interaction of water vapour with mineral dust and soot surfaces has been studied in the temperature range 203 K < T < 298 K using a Knudsen cell reactor. For the uptake of water on mineral dust an initial uptake coefficient of γ_ini = (6.3 ± 0.7) × 10⁻² independent of temperature has been determined. In contrast the desorption rate has been found to be strongly temperature dependent with desorption rate constants decreasing from 1 × 10⁻³ at 265 K to 1 × 10⁻⁴ at 223 K. In addition, relatively high surface coverages have been determined from which an adsorption enthalpy of −40 kJ mol⁻¹ is inferred. For the uptake of water on soot the initial uptake coefficient has been found to be independent of temperature with a value of γ_ini = (4.7 ± 0.2) × 10⁻², similar to the case of mineral dust. However, the corresponding desorption rate constants have been found to be three orders of magnitude larger than for mineral dust. Consistent with this finding, low surface coverages with an adsorption enthalpy of −27 kJ mol⁻¹ have been derived. A comparison of the uptake kinetics and adsorption enthalpies of water on mineral dust and soot leads to the conclusion that water is much stronger interacting with mineral dust than with soot. In terms of a hydrophilicity concept the results suggest, that mineral dust may be regarded as hydrophilic whereas soot is hydrophobic and that fundamental kinetic and thermochemical quantities may be related to that concept.