Characterization of low-temperature vapour pressure estimates for secondary organic aerosol applications

Many models of secondary organic aerosol (SOA) formation, adhering to gas-particle equilibrium partitioning theory, require known vapour pressures for low volatility products of volatile organic compound (VOC) oxidation. Since the majority of such products have yet to be isolated and analysed, few experimental determinations of pertinent vapour pressures have been achieved, and models are forced to rely on vapour pressure estimates, such as those available through the common, computer-based SPARC and MPBPWIN property calculators. Thus, the accuracy of the respective estimation methods must be measured and evaluated, in order to determine the consequences in the resulting models. However, published evaluations, and most models themselves, typically focus on moderate to high ambient temperatures, which may not be applicable year-round in colder regions, where the rate of VOC oxidation slows, while the vapour pressures of the products decrease, indicating an increased tendency to condense into the aerosol phase. In this paper, the accuracy of the SPARC and MPBPWIN methods is evaluated over a broad temperature range, from 248.15 to 298.15 K, in five degree intervals using a test set of 45 compounds. The results are reported in terms of mean average error (MAE) and mean bias error (MBE), and given for alcohol, carboxylic acid, aldehyde, and ketone compound classes at each temperature. Specific trends in MAE and MBE with regard to compound class and changing temperature are discussed. More generally, the evaluation indicates that SPARC, with MAE decreasing from 0.288 at 248.15 K to 0.165 at 298.15 K and MBE increasing from −0.180 at 248.15 K to a peak of −0.081 at 293.15 K, is more accurate at low to moderate temperatures than MPBPWIN, with MAE decreasing from 0.436 at 248.15 K to 0.272 at 298.15 K and MBE increasing from −0.328 at 248.15 K to −0.213 at 298.15 K. Decreasing accuracy at lower temperatures emphasizes a need for focused experimental and model efforts in this temperature range.