Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Measured Saturation Vapor Pressures of Phenolic and Nitro-Aromatic Compounds” lease find our response to the comment by Wania et al. on our recent study on the saturation vapor pressures of phenolic and nitroaromatic compounds. First, we should note that we generally welcome predictions from tools such as COSMOtherm. In the review of Bilde et al.1 we specifically request further investigation into their role in supporting ongoing efforts in this area. The responses to each of the individual points raised are as follows: 1. We do not claim the values presented in this paper to act as a “gold standard”, although we are not fully convinced by Wania et al. on the arguments for the reported values being too low either (see also below for our discussion on the COSMOtherm predictions). We need comparative measurements from other instruments, noting that it is not as easy as prescribing one instrument to have a particular low or high bias (e.g., ref 2). We thank the authors for adding the existing data to ours to confirm some level of confidence in the values presented while highlighting potential problems. The authors present interesting arguments on why the data for dihydroxynaphthalenes might be too low, although the arguments seem to be mostly heuristic in nature. It is unclear what might cause this behavior in the KEMSparticularly as, for example, the KEMS data included in Bilde et al.1 tends to be rather on the high than the low side, over 4 orders of magnitude of VPs(Pa). Previous discussions on potential for sources of uncertainty in the KEMS include varying sample phase state, statistical noise, ionization cross sections or lack of appropriate calibration standard in multiple volatility ranges, all described in detail by Booth et al.3 However, we estimate that this results in a maximum uncertainty of 40% in experimentally determined solid state vapor pressures, a maximum uncertainty of 75% in the subcooled liquid state and will not result in a systematic error. These are exactly the type of reasons why more and reliable data on fundamental properties of, for example, atmospherically relevant chemicals is needed using various experimental setups. While the COSMOtherm calculations can indeed give some useful insights, without such hard empirical data it is impossible to conclude whether they values we present for dihydroxynaphtalines or dinitronaphtaline are biased low or not. 2. This is a useful addition. We would like to point out that albeit more fundamentally sound than the standard group contribution techniques, it is apparent that also COSMOtherm has been “calibrated” with a parametrization data set of known chemicals.4 This methodology is thus likely affected by the same lack of reliable empirical data for low-volatility species as the standard group contribution methods. While the authors do not advocate the use of COSMOtherm in predicting saturation vapor pressures despite comparisons made, a
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valuable initial contributory step might be additional peer reviewed comparisons with the carboxylic acids presented in Bilde et al.1 Initial comparisons made by Kurtén et al.5 suggest that COSMOtherm overpredicts the saturation vapor pressures of adipic and pimelic acids (which have subcooled liquid saturation vapor pressures of the order of 10−4 Pa) by about a factor of about 10− 40. This leads on to point “‘3”. 3. While the arguments presented in this paragraph and Wania et al. 6 are interesting, reverting back to partitioning coefficients would present problems for the atmospheric aerosol community in describing various atmospherically relevant processes - although using partitioning coefficients was indeed common practice in atmospheric calculations in the past (e.g., ref 7). One problem with using partitioning coefficients is related to the dependence of the partitioning on the condensed phase composition: the partitioning coefficients reported treat the partitioning molecules as dissolved to a solute, analogously to Henry’s law coefficients in describing nonideality of aqueous solutions. However, situations where this assumption breaks down can arise in, for example, the describing formation of new particles from vapor (e.g., ref 8), where a small number of vapors can participate in the early steps of particle formation and the mixture becomes more complex as these particles grow (e.g., refs 9, 10). Describing these processes accurately in atmospheric models are certainly not irrelevant for predicting, for example, the climate impacts of SOA, as a significant fraction of cloud condensation nuclei has likely originated from such secondary new particle formation (e.g., refs 11, 12, 13). Furthermore, compounds partitioning to the atmospheric aerosol population are not necessarily in thermodynamic equilibrium (e.g., refs 9, 14, 15), which questions the standard use of partitioning coefficients. The effective volatility of aerosol components is a net product of process and chemical complexity, the sensitivity to which we could not yet confidently quantify fully. This includes the influence of nonideality,16 phase state changes,17 bulk to surface transfer18 and a range of condensed phase reactions. This does not negate the usefulness of partitioning coefficients for some atmospheric applications−but shifting to the use of partitioning coefficients is not the answer. Finally, we would like to point out the general value of fundamental research on molecular properties of chemicals per se. Saturation vapor pressures are examples of such fundamental molecular properties, much more so than partitioning coefficients that depend, for example, on the matrix that the chemical is absorbed to and rely on the Published: June 14, 2017 7744
DOI: 10.1021/acs.est.7b02681 Environ. Sci. Technol. 2017, 51, 7744−7745
Environmental Science & Technology
Correspondence/Rebuttal
(15) Järvinen, E.; et al. Observation of viscosity transition in α-pinene secondary organic aerosol. Atmos. Chem. Phys. 2016, 16, 4423−4438. (16) Zuend, A.; Seinfeld, J. H. Modeling the gas-particle partitioning of secondary organic aerosol: the importance of liquid-liquid phase separation. Atmos. Chem. Phys. 2012, 12, 3857−3882. (17) Virtanen, A.; et al. An amorphous solid state of biogenic secondary organic aerosol particles. Nature 2010, 467 (7317), 824− 827. (18) Werner, J.; et al. Surface Partitioning in Organic−Inorganic Mixtures Contributes to the Size-Dependence of the Phase-State of Atmospheric Nanoparticles. Environ. Sci. Technol. 2016, 50, 7434− 7442.
assumption of the chemical not dominating the condensed phase composition. Importantly, we feel it is not “misguided” to continue community led efforts to resolve what level of accuracy might be attained to support predictions for systems of potentially millions of compounds when the alterative, including methods such as COSMOtherm, similarly remain largely invalidated for atmospheric systems. To reiterate, as we build additional series of saturation vapor pressure measurements, to which all contributors f rom multiple instruments can agree a level of confidence in their values, these comparisons should continue. .
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David Topping* Ilona Riipinen Carl Percival Thomas Bannan AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ilona Riipinen: 0000-0001-9085-2319 Thomas Bannan: 0000-0002-1760-6522 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Bilde, M.; et al. Saturation Vapor Pressures and Transition Enthalpies of Low-Volatility Organic Molecules of Atmospheric Relevance: From Dicarboxylic Acids to Complex Mixtures. Chem. Rev. 2015, 115 (10), 4115−4156. (2) Huisman, A. J.; et al. Vapor pressures of substituted polycarboxylic acids are much lower than previously reported. Atmos. Chem. Phys. 2013, 13, 6647−6662. (3) Booth, A. M.; et al. Design and construction of a simple Knudsen Effusion Mass Spectrometer (KEMS) system for vapour pressure measurements of low volatility organics. Atmos. Meas. Tech. 2009, 2 (2), 355−361. (4) Klamt, A.; et al. Refinement and Parametrization of COSMO-RS. J. Phys. Chem. A 1998, 102, 5074−5085. (5) Kurtén, T.; et al. α-Pinene Autoxidation Products May Not Have Extremely Low Saturation Vapor Pressures Despite High O:C Ratios. J. Phys. Chem. A 2016, 120, 2569−2582. (6) Wania, F.; et al. Novel methods for predicting gas−particle partitioning during the formation of secondary organic aerosol. Atmos. Chem. Phys. 2014, 14, 13189−13204. (7) Pankow, J. F. An absorption model of the gas/aerosol partitioning involved in the formation of secondary organic aerosol. Atmos. Environ. 1994, 28 (2), 189−193. (8) Kirkby, J.; et al. Ion-induced nucleation of pure biogenic particles. Nature 2016, 533 (7604), 521−526. (9) Riipinen, I.; et al. The contribution of organics to atmospheric nanoparticle growth. Nat. Geosci. 2012, 5, 453−458. (10) Tröstl, J.; et al. The role of low-volatility organic compounds in initial particle growth in the atmosphere. Nature 2016, 533, 527−53. (11) Merikanto, J.; et al. Impact of nucleation on global CCN. Atmos. Chem. Phys. 2009, 9, 8601−8616. (12) Kerminen, V.-M.; et al. Cloud condensation nuclei production associated with atmospheric nucleation: a synthesis based on existing literature and new results. Atmos. Chem. Phys. 2012, 12, 12037−12059. (13) Gordon, H.; et al. Reduced anthropogenic aerosol radiative forcing caused by biogenic new particle formation. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 432016. (14) Booth, A. M.; et al. The role of ortho, meta, para isomerism in measured solid state and derived sub-cooled liquid vapour pressures of substituted benzoic acids.″. RSC Adv. 2012, 2.10, 4430−4443. 7745
DOI: 10.1021/acs.est.7b02681 Environ. Sci. Technol. 2017, 51, 7744−7745
