Approach to evaluate the gas/aerosol partition coefficient of organic volatile compounds using DFT methods associated with polarizable continuum models

摘要

Gas/aerosol partition coefficients are important to the development of air quality models that include the physical chemistry of atmospheric aerosols because they are related to the solubility of gases in water. This paper presents an approach to calculate these coefficients using DFT methods associated with polarizable continuum models to include the solvent effect. The methodology was tested by comparing DFT-calculated Henry's law constants of 27 terpenes with experimental data available in the literature. Furthermore, Henry's law constants of the d-limonene and Delta(3)-carene oxidation products were evaluated. A correction to the ideal solution behavior was performed by evaluating the activity coefficient of each solute considering the aerosol's physical-chemical properties (size, pH and ionic strength), followed by the calculation of the gas/aerosol partition coefficient. The mean average deviations of the PBE1PBE and M06-2X functionals, excluding oxidized terpenes and considering the SMD model (Solvation Model Based on Solute Electron Density), were 2.26 x 10(-4) m(3) mol Pa-1 and 2.23 x 10(-4) m(3) mol Pa-1, respectively. As a general trend, the presence of inorganic compounds in aerosols lowers the solubility of oxidation products, which makes the transfer of organic species from the gas phase to the aerosol phase difficult. The proposed approach is simpler than methods involving explicit solvation models and does not require ensemble averages to be applied. The method returns reliable Henry's law constants of terpenes; however, experimental measurements are necessary to validate the constants computed for terpene oxidation products.