Significance of water complexes in the atmosphere

Abstract

Using standard statistical and thermodynamic procedures, we calculate equilibrium constants for the formation of select, hydrogen-bonded water complexes, namely the water dimer and the cyclic trimer and tetramer, and use them to estimate the atmospheric abundances of these species. We generate water complex altitude profiles (0–30 km) for both a saturated and an unsaturated atmosphere and discuss the dominant factors influencing our results. In our analysis, particular emphasis is given to the significance that water monomer concentrations, complex binding energies, hydrogen-bond energies, and entropy have on the calculated abundance profiles. We examine the importance of enthalpy and entropy at atmospheric temperatures and show how each contributes to our calculated equilibrium constants. By applying a universal 2 °C temperature increase throughout the troposphere and lower stratosphere, we are able to model the effect that global warming would have on (H₂O)_n abundances in a saturated atmosphere. We also illustrate the effect that this thermal variation would have on entropy, enthalpy, and K_p(T) values. Based on our results, we assess the atmospheric significance of water dimers and cyclic water complexes.