Vapor Pressures of Substituted Polycarboxylic Acids Are Much Lower Open Access Than Previously Reported Biogeosciences Biogeosciences Discussions A
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EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmos. Chem. Phys., 13, 6647–6662, 2013 Atmospheric Atmospheric www.atmos-chem-phys.net/13/6647/2013/ doi:10.5194/acp-13-6647-2013 Chemistry Chemistry © Author(s) 2013. CC Attribution 3.0 License. and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques Discussions Open Access Vapor pressures of substituted polycarboxylic acids are much lower Open Access than previously reported Biogeosciences Biogeosciences Discussions A. J. Huisman1,*, U. K. Krieger1, A. Zuend2,**, C. Marcolli1, and T. Peter1 1Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland Open Access 2Department of Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA Open Access *now at: Chemistry Department, Union College, Schenectady, New York 12308, USA Climate **now at: Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland Climate of the Past of the Past Correspondence to: A. J. Huisman ([email protected]) and U. K. Krieger ([email protected]) Discussions Open Access Received: 30 November 2012 – Published in Atmos. Chem. Phys. Discuss.: 10 January 2013 Open Access Revised: 4 June 2013 – Accepted: 7 June 2013 – Published: 12 July 2013 Earth System Earth System Dynamics Dynamics Abstract. The partitioning of compounds between the erences therein). A key physical parameter which governs Discussions aerosol and gas phase is a primary focus in the study of the gas-particle partitioning is the pure component vapor pres- formation and fate of secondary organic aerosol. We present sure (p◦), which is difficult to accurately measure at atmo- Open Access Open Access measurements of the vapor pressure of 2-methylmalonic spheric temperatures for low-Geoscientific and semi-volatile organic com- Geoscientific (isosuccinic) acid, 2-hydroxymalonic (tartronic) acid, 2- pounds (L/SVOC) suchInstrumentation as polycarboxylic acids. Note that Instrumentation methylglutaric acid, 3-hydroxy-3-carboxy-glutaric (citric) throughout this article, theMethods term “vapor and pressure” is used Methods and acid and DL-2,3-dihydroxysuccinic (DL-tartaric) acid, as defined by chemical thermodynamicsData Systems to denote the par- which were obtained from the evaporation rate of supersatu- tial pressure of a compound in equilibrium with a liquid or Data Systems rated liquid particles levitated in an electrodynamic balance. solid phase, which is sometimes emphasized by the equiva- Discussions Open Access Open Access Our measurements indicate that the pure component liquid lent term “equilibrium vapor pressure”. Geoscientific vapor pressures at 298.15 K for tartronic, citric and tartaric Vapor pressure measurementsGeoscientific of L/SVOC may be made by Model Development acids are much lower than the same quantity that was de- following the volatilizationModel Development of bulk samples (e.g., Verevkin rived from solid state measurements in the only other room et al., 2000; Ribeiro da Silva et al., 2001; Booth et al., Discussions temperature measurement of these materials (made by Booth 2010; Bruns et al., 2012), ensembles of airborne or deposited Open Access et al., 2010). This strongly suggests that empirical correction droplets (e.g., Cappa et al., 2007; Salo et al., 2010Open Access ), and sin- terms in a recent vapor pressure estimation model to account gle trapped airborne dropletsHydrology (e.g., Zardini and et al. , 2006; Pope Hydrology and for the inexplicably high vapor pressures of these and similar et al., 2010; Soonsin etEarth al., 2010 ;SystemKrieger et al., 2012) or Earth System compounds should be revisited, and that due caution should by other techniques such as gas–liquid chromatography (see be used when the estimated vapor pressures of these and sim- review by Letcher and Naicker, Sciences2004). Generally, the agree- Sciences ilar compounds are used as inputs for other studies. ment of different techniques is reasonable for measurements Discussions Open Access in which the sample state is positively identifiedOpen Access and un- expectedly poor for measurements where the physical state Ocean Science is not well controlledOcean (Soonsin etScience al., 2010; M. Bilde, per- 1 Introduction sonal communication, 2012). Vapor pressures are generally Discussions reported for the liquid state p◦,L (not to be confused with The production of secondary organic aerosol (SOA) by gas- p◦, which we will use to denote the vapor pressure of a pure Open Access to-particle partitioning is generally represented by an equilib- compound in an unspecified physical state), whichOpen Access is the ref- rium partitioning model such as that of Pankow (1994, 2003). erence state for most models of atmospheric partitioning. Solid Earth This framework has recently been applied in box models However, vapor pressure measurementsSolid Earth of polyacids are gen- and large scale atmospheric models by means of a volatil- erally made in the solid state (p◦,S) as these compounds have Discussions ity basis set (VBS; see, e.g., Donahue et al., 2011, and ref- Open Access Open Access Published by Copernicus Publications on behalf of the European Geosciences Union. The Cryosphere The Cryosphere Discussions 6648 A. J. Huisman et al.: Vapor pressures of polyacids a melting point above room temperature. The measured solid additivity effects) (Compernolle et al., 2011) to estimate the state vapor pressure p◦,S can be converted to the liquid vapor aerosol mass generated by chemical processing of volatile pressure p◦,L by the enthalpy of fusion, the melting tempera- and semivolatile organic compounds in the gas and particle ture, and the change in heat capacity upon melting (Prausnitz phases. In this study, we focus on comparison of our mea- et al., 1999). While it is possible to measure either p◦,S or surements to the EVAPORATION model, which is intended p◦,L, it is preferable to measure directly in the liquid state to enable the prediction of p◦,L for polyfunctional molecules when reporting p◦,L to circumvent the phase conversion and that occur in SOA. The model has been optimized to account its associated uncertainty. As these compounds are solid at for the influence of hydrogen bonding and intramolecular room temperature, measurements of p◦,L can only be made group interactions on p◦,L and has been parameterized with directly in small droplets, in which strong supersaturations or the most recent experimental data for polyfunctional acids. subcoolings can be maintained without crystallization. Although Compernolle et al. (2011) endeavor to reproduce There are several cases (e.g., tartaric compared with suc- measurements of p◦,L for substituted polyacids reported by cinic acid) in which the p◦,L reported by Booth et al. (2010) Booth et al. (2010) by implementing an empirical correction increases upon functionalization of the carbon backbone term, they are not able to rationalize these effects. Because with an oxygen-bearing substituent, a counterintuitive result. only few measurements are available for such compounds, There have been a few specific cases reported in the litera- a danger of overfitting exists (see discussion in Barley and ture where addition of an oxygen-bearing functional group McFiggans, 2010), especially since some compounds have to a parent structure can lead to an increase of the solid- been measured only with a single technique. This highlights state vapor pressure. As discussed in Zuend and Seinfeld the need for accurate measurements of p◦ (and especially di- (2012), such effects have been reported, e.g., by Chattopad- rect measurements of p◦,L) to assure the accuracy of models hyay and Ziemann (2005) for substitutions where a ketone of partitioning-driven aerosol formation. In their manuscript, group is added in α-position to a carboxyl group. While keto Compernolle et al. (2011) noted that measurements using the substitutions in α-position of a carboxyl group can raise the methodology of Soonsin et al. (2010) were needed for these solid-state vapor pressure of a dicarboxylic acid with respect compounds. The measurements presented in this work begin to the unsubstituted parent structure, keto substitutions at a to address that need, but are not intended to be an exhaustive β-position or further away from the carboxyl group always set that will allow for a re-fitting of models such as EVAP- resulted in decreased solid-state vapor pressure. Chattopad- ORATION. Rather, we intend them to demonstrate the ex- hyay and Ziemann (2005) suggest that a ketone group in α- istence of substantial deviations from previous experimental position may form an internal hydrogen bond with the car- data and as an immediate caveat to anyone using EVAPORA- boxyl group. Such an internal hydrogen bond may reduce TION or similar models to predict partitioning of substituted the level of intermolecular hydrogen bonding and may lead polycarboxylic acids. Finally, we show that the EVAPORA- to an increase in vapor pressure relative to the unsubstituted TION model can be modified to yield predictions in better parent carboxylic acid. However, such effects on the pure agreement with our measurements. compound vapor pressure have been observed only for sam- ples measured in the solid