Atmospheric implications of fumaric acid - Water binary clusters

Abstract

Fumaric acid (FA) is amongst the most abundant low molecular-weight dicarboxylic acids in the atmosphere and is present in atmospheric aerosols. However, the climate implications of fumaric acid in the presence of water (W) are not yet known. In the present study, the structures and thermodynamics of the most stable isomers of FA(H₂O)_n (n = 1–10) are investigated using APF-D/6-311+G(d, p) and benchmarked with ωB97X-D/6–311++G(3df,3pd). The atmospheric abundance and IR spectra of these complexes are calculated alongside the use of the narrow-band model (NBM) to estimate the radiative forcing efficiencies (RE) of hydrates. Optical properties such as Rayleigh light scattering intensities for natural and polarized light were then calculated. The clusters FA(H₂O)_n are held together by strong hydrogen bonds formed between the water molecules and the carboxylic acid functional group. The binding free energies of the most stable clusters of n = 1–5 are negative and decrease with cluster size while for n = 6–10, they are positive. Cluster concentration decreases with relative humidity (RH) with the smaller clusters dominating at all investigated RH. These clusters show high evaporation rates at 298K and 1atm. Also, the radiative forcing efficiencies of FA(H₂O)_n = 1-10 are positive and increase with cluster size. This major part of the radiative forcing occurs within the atmospheric window region 250–1500 cm⁻¹. Analysis of the Rayleigh scattering properties showed that the Rayleigh scattering intensity is enhanced upon the increase of the number of water molecules in the cluster. Therefore in the atmosphere, FA(H₂O)_n = 1-10 induces a scattering which reduces its overall heating effect on the earth.