Response to Comment on “More of EPA’s SPARC Online CalculatorsThe Need for High Quality Predictions of Chemical Properties” We agree with many of the sentiments expressed by Boethling et al. (1) and thank them for their comments and continuing efforts in developing the freely accessible EPI Suite software. EPI Suite is a highly regarded package for screening a wide variety of environmentally relevant end points, and is more transparent, user-friendly, and amenable to batch processing than the current version of SPARC. Boethling et al. (1) noted our Viewpoint (2) neglected to consider the needs of the pollution prevention community for which EPI Suite currently serves as a screening tool. However, besides being transparent, user-friendly, and convenient, such screening tools must also be as accurate and as widely applicable as possible. Compared to EPI Suite, it is our view that SPARC has the potential to be more accurate as a screening tool and to cover a broader set of environmentally relevant applications because its underlying architecture is more mechanistic. Our concern is that the apparent, current marginalization of SPARC prevents it from developing its considerable potential. The architecture of EPI Suite primarily utilizes molecular fragment approaches to estimate partitioning parameters, and thus depends on the availability of large, high-quality experimental data sets to account for as many of the limitless “non-additive” correction factors as possible. This approach has two major disadvantages: (i) unless all possible combinations of molecular fragments are accounted for in the calibration domain, there will always be outliers due to nonadditive effects (e.g., internal H-bonding, resonance structures, nearest
neighbor effects, etc.) (3); (ii) the number of partitioning processes specifically calibrated by EPI Suite is quite small, which limits the environmental partitioning processes with which they can be correlated, especially for broad chemical sets. SPARC, on the other hand, treats all partitioning processes in a unified approach, using SARs, LFERs, and PMO theory to estimate underlying solute-solvent interactions (e.g., dispersion, induction, H-bond, etc.) (4). As a result, much larger data sets can be used to calibrate its underlying model, allowing SPARC to be highly robust, accurate, and flexible. Two recent examples illustrate this: (i) air-water partitioning for various bifunctional compounds (i.e., molecules with 2 polar moieties) was predicted much better by SPARC than by EPI Suite (3); (ii) explicit modeling of sorbing phases (and not just chemicals) allowed SPARC to better predict humic acid-air partitioning coefficients than PcKocWIN within the EPI Suite package (5). Currently, SPARC and EPI Suite offer their individual advantages to different end-users from environmental, industrial, scientific, regulatory, and public backgrounds, and we would like to see both approaches continue to evolve, thrive, innovate, and become more open to end-user feedback and contributions. We were pleased to learn that a new version of EPI Suite will contain external links to web-based programs and data sources like SPARC (1). Potential additional integration between SPARC architecture and the EPI Suite interface could be fortuitous in utilizing the unique advantages of each. Whatever path is chosen by funding entities, we hope that the finite resources available for freely accessible property prediction tools are allocated to efficiently developing their openness to users, general utility, and quality.
7746 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / October 15, 2010
HANS PETER H. ARP* Norwegian Geotechnical Institute, Oslo, Norway
[email protected] STEVEN T. J. DROGE,* SATOSHI ENDO,* AND KAI-UWE GOSS UFZ Helmholtz Centre for Environmental Research, Leipzig, Germany
[email protected] [email protected] WALTER GIGER Giger Research Consulting (GRC), Zurich, Switzerland and Swiss Federal Institute of Aquatic Science and Technology (Eawag), Du ¨ bendorf, Switzerland STEVEN B. HAWTHORNE Energy and Environmental Research Center, University of North Dakota, Grand Forks, North Dakota SCOTT A. MABURY University of Toronto, Ontario, Canada PHILIPP MAYER Aarhus University, Roskilde, Denmark MICHAEL S. MCLACHLAN Stockholm University, Sweden JAMES F. PANKOW Portland State University, Portland, Oregon ´ P. SCHWARZENBACH RENE Swiss Federal Institute of Technology Zurich, Switzerland FRANK WANIA University of Toronto Scarborough, Ontario, Canada
10.1021/es102925s
BAOSHAN XING University of Massachusetts, Amherst, Massachusetts 2010 American Chemical Society
Published on Web 09/10/2010
(1) Boethling, R.; Davies, C.; Fehrenbacher, C.; Henry, T.; Howard, P.; Lavoie, E.; Libelo, L.; Mayo, K.; Meylan, W.; Morlacci, L.; Sommer, E.; Tunkel, J.; Waugh, W.; Webb, T.; Zeeman, M. Comment on “More of EPA’s SPARC Online CalculatorsThe Need for HighQuality Predictions of Chemical Properties”. Environ. Sci. Technol. 2010, 44, DOI 10.1021/es102727q. (2) Arp, H. P. H.; Droge, S. T. J.; Endo, S.; Giger, W.; Goss, K.-U.; Hawthorne, S. B.; Mabury, S. A.; Mayer,
P.; McLachlan, M. S.; Pankow, J. F.; Schwarzenbach, R. P.; Wania, F.; Xing, B. More of EPA’s SPARC Online CalculatorsThe Need for High-Quality Predictions of Chemical Properties. Environ. Sci. Technol. 2010, 44 (12), 44004401. (3) Goss, K.-U.; Arp, H. P. H.; Bronner, G.; Niederer, C. Nonadditive Effects in the Partitioning Behavior of Various Aliphatic and Aromatic Molecules. Environ. Toxicol. Chem. 2009, 28 (1), 52–60.
(4) Hilal, S. H.; Karickhoff, S. W.; Carreira, L. A. Prediction of the solubility, activity coefficient and liquid/ liquid partition coefficient of organic compounds. QSAR Comb. Sci. 2004, 23 (9), 709–720. (5) Niederer, C.; Goss, K.-U. Quantum chemical modeling of humic acid/ air equilibrium partitioning of organic vapors. Environ. Sci. Technol. 2007, 41 (10), 3646–3652.
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October 15, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 7747
