Thermodynamics of the Formaldehyde−Water and Formaldehyde−Ice Systems for Atmospheric Applications

Formaldehyde (HCHO) is a species involved in numerous key atmospheric chemistry processes that can significantly impact the oxidative capacity of the atmosphere. Since gaseous HCHO is soluble in water, the water droplets of clouds and the ice crystals of snow exchange HCHO with the gas phase and the partitioning of HCHO between the air, water, and ice phases must be known to understand its chemistry. This study proposes thermodynamic formulations for the partitioning of HCHO between the gas phase and the ice and liquid water phases. A reanalysis of existing data on the vapor−liquid equilibrium has shown the inadequacy of the Henry's law formulation, and we instead propose the following equation to predict the mole fraction of HCHO in liquid water at equilibrium, XHCHO,liq, as a function of the partial pressure PHCHO (Pa) and temperature T (K): XHCHO,liq = 1.700 × 10−15 e(8014/T)(PHCHO)1.105. Given the paucity of data on the gas−ice equilibrium, the solubility of HCHO and the diffusion coefficient (DHCHO) in ice were measured by exposing large single ice crystals to low PHCHO. Our recommended value for DHCHO over the temperature range 243−266 K is DHCHO = 6 × 10−12 cm2 s−1. The solubility of HCHO in ice follows the relationship XHCHO,ice = 9.898 × 10−13 e(4072/T)(PHCHO)0.803. Extrapolation of these data yields the PHCHO versus 1/T phase diagram for the H2O−HCHO system. The comparison of our results to existing data on the partitioning of HCHO between the snow and the atmosphere in the high arctic highlights the interplay between thermodynamic equilibrium and kinetics processes in natural systems.