Articles | Volume 17, issue 12
https://doi.org/10.5194/acp-17-7529-2017
https://doi.org/10.5194/acp-17-7529-2017
Research article
 | 
22 Jun 2017
Research article |  | 22 Jun 2017

Uncertain Henry's law constants compromise equilibrium partitioning calculations of atmospheric oxidation products

Chen Wang, Tiange Yuan, Stephen A. Wood, Kai-Uwe Goss, Jingyi Li, Qi Ying, and Frank Wania

Abstract. Gas–particle partitioning governs the distribution, removal, and transport of organic compounds in the atmosphere and the formation of secondary organic aerosol (SOA). The large variety of atmospheric species and their wide range of properties make predicting this partitioning equilibrium challenging. Here we expand on earlier work and predict gas–organic and gas–aqueous phase partitioning coefficients for 3414 atmospherically relevant molecules using COSMOtherm, SPARC Performs Automated Reasoning in Chemistry (SPARC), and poly-parameter linear free-energy relationships. The Master Chemical Mechanism generated the structures by oxidizing primary emitted volatile organic compounds. Predictions for gas–organic phase partitioning coefficients (KWIOM/G) by different methods are on average within 1 order of magnitude of each other, irrespective of the numbers of functional groups, except for predictions by COSMOtherm and SPARC for compounds with more than three functional groups, which have a slightly higher discrepancy. Discrepancies between predictions of gas–aqueous partitioning (KW/G) are much larger and increase with the number of functional groups in the molecule. In particular, COSMOtherm often predicts much lower KW/G for highly functionalized compounds than the other methods. While the quantum-chemistry-based COSMOtherm accounts for the influence of intra-molecular interactions on conformation, highly functionalized molecules likely fall outside of the applicability domain of the other techniques, which at least in part rely on empirical data for calibration. Further analysis suggests that atmospheric phase distribution calculations are sensitive to the partitioning coefficient estimation method, in particular to the estimated value of KW/G. The large uncertainty in KW/G predictions for highly functionalized organic compounds needs to be resolved to improve the quantitative treatment of SOA formation.

Download
Short summary
Three property prediction methods are used to predict equilibrium partitioning coefficients for a set of 3414 compounds implicated in secondary organic aerosol formation. Partitioning from the gas phase to water is found to be much more uncertain than estimates of partitioning into the organic matter of aerosol. This uncertainty matters, as phase distribution is very different depending on which prediction method is applied.
Altmetrics
Final-revised paper
Preprint