Impact of structure on the estimation of atmospherically relevant physicochemical parameters

Abstract. Many methods are currently available to estimate physicochemical properties of atmospherically relevant compounds. Though a substantial body of literature has focused on the development and intercomparison of methods based on molecular structure, there has been an increasing focus on methods based only on molecular formula. However, prior work has not quantified the extent to which isomers of the same formula may differ in their properties, or, relatedly, the extent to which lacking or ignoring molecular structure degrades estimates of parameters. Such an evaluation is complicated by the fact that structure-based methods bear significant uncertainty and are typically not well constrained for atmospherically-relevant molecules. Using species produced in the modeled atmospheric oxidation of three representative atmospheric hydrocarbons, we demonstrate here that differences between isomers are greater than differences between methods. Specifically, isomers tend to differ in their vapor pressures and Henry's Law Constants by a half to a full order of magnitude greater than differences between estimation methods, and differ in their kOH by a factor of two. Formula-based estimation of these parameters is shown to be possible with little bias and an approximately normally distributed error. Specifically vapor pressure can be estimated using a combination of two existing methods, Henry's Law Constants can be estimated based on vapor pressure, and kOH can be approximated as a constant for all formulas containing a given set of elements. Formula-based estimation is therefore reasonable when applied to a mixture of isomers, but creates uncertainty commensurate with the lack of structural information.