Uncertain Henry's Law Constants Compromise Equilibrium Partitioning Calculations of Atmospheric Oxidation Products
Chen Wang1, Tiange Yuan1, Stephen A. Wood1, Kai-Uwe Goss2,3, Jingyi Li4,a, Qi Ying4, and Frank Wania11Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON, M1C 1A4, Canada 2Department of Analytical Environmental Chemistry, Centre for Environmental Research UFZ Leipzig-Halle, Permoserstraße 15, Leipzig, D-04318, Germany 3Institute of Chemistry, University of Halle-Wittenberg, Kurt-Mothes-Straße 2, Halle, D-06120, Germany 4Environmental and Water Resources Division, Zachry Department of Civil Engineering, Texas A&M University College Station, TX 77843-3136 acurrent address: School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China
Received: 31 Jan 2017 – Accepted for review: 10 Feb 2017 – Discussion started: 14 Feb 2017
Abstract. Gas-particle partitioning governs the distribution, removal and transport of organic compounds in the atmosphere and the formation of secondary organic aerosol. 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 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 one 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 intramolecular 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.
Wang, C., Yuan, T., Wood, S. A., Goss, K.-U., Li, J., Ying, Q., and Wania, F.: Uncertain Henry's Law Constants Compromise Equilibrium Partitioning Calculations of Atmospheric Oxidation Products, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-92, in review, 2017.