Modeling of Gas-Wall Partitioning of Organic Compounds Using a Quantitative Structure–Activity Relationship

Abstract. This study streamlines modeling of the gas–wall process (GWP) of semivolatile organic compounds (SVOC) by predicting gas–wall equilibrium partitioning constant (K_w,i) and accommodation coefficient (α_w,i) of SVOC(i) using a quantitative structure–activity relationship. PaDEL-Descriptor, software that calculates molecular descriptors, is employed to obtain physicochemical parameters (i.e., hydrogen bond acidity (H_d,i), hydrogen bond basicity (H_a,i), dipolarity/polarizability (S_i), and polarizability (α_i)) of SVOC(i). For the prediction of K_w,i, activity coefficients (γ_w,i) of SVOC(i) to the chamber wall are semiempirically predicted using chamber data in the form of a polynomial equation coupled with the physicochemical parameters. γ_w,i of various SVOCs differ in functionalities and molecular sizes ranging from 100 to 104. We conclude that the estimation of γ_w,i is essential to improve the prediction of K_w,i. To predict the impact of relative humidity (RH) on GWP, each coefficient in the polynomial equation for ln(K_w,i) was correlated to RH. Increasing RH enhanced GWP significantly for all polar SVOCs. For example, the predicted K_w,i of 1-heptanoic acid increased more than three times (from 0.58 to 1.96) by increasing RH from 0.4 to 0.75 due to the reduction in γ_w,i. The characteristic time for GWP are estimated using K_w,i and α_w,i to evaluate the effect of GWP on secondary organic aerosol (SOA) mass. It might be significant in the absence of inorganic aerosol, but is insignificant in the presence of electrolytic salts, where aqueous reactions dominate SOA growth.