Environ. Sci. Technol. 2010, 44, 388–393 their role in cloud formation (1, 9). However, there is still Vapor Pressure Measurements on substantial uncertainty associated with the treatment of SOAs Low-Volatility Terpenoid Compounds in global climate models. For example, SOA formation depends directly on the vapor pressures of the constituent by the Concatenated Gas Saturation compounds (1, 9, 10), but only a few vapor pressure measurements have been made on these compounds Method (1, 8, 11-14). Consequently, models of aerosol formation have to rely on estimated values of vapor pressure for aerosol JASON A. WIDEGREN AND precursors (9). For this reason, a recent review of organic THOMAS J. BRUNO* aerosols and global climate modeling concluded that accurate measurements of vapor pressure (and enthalpy of vaporiza- Thermophysical Properties Division, National Institute of tion/sublimation) are needed in order to decrease the Standards and Technology, 325 Broadway, Boulder, Colorado 80305 uncertainty in global climate models (1). The lack of known vapor pressures for monoterpenoid compounds illustrates a broader need for vapor pressure Received August 28, 2009. Revised manuscript received measurements on low-volatility compounds for scientific and November 20, 2009. Accepted November 23, 2009. environmental applications (15-17). Typically, for organic compounds with high molar mass, the best one can hope to find in the literature is a single measurement at high The atmospheric oxidation of monoterpenes plays a central temperature (such as a boiling temperature). Even when a role in the formation of secondary organic aerosols (SOAs), high temperature measurement has been reported, it often has a large uncertainty because of thermal decomposition which have important effects on the weather and climate. or other reasons, which can force a reliance on group However, models of SOA formation have large uncertainties. contribution estimates (18). One reason for this is that SOA formation depends directly on The vapor pressures of compounds of high molar mass the vapor pressures of the monoterpene oxidation products, are typically less than 1 kPa at temperatures below 323 K (the but few vapor pressures have been reported for these compounds. temperature range of greatest concern for environmental As a result, models of SOA formation have had to rely on and climate studies), which limits the number of useful vapor estimated values of vapor pressure. To alleviate this problem, pressure measurement methods (16, 19). Methods that we have developed the concatenated gas saturation method, directly measure the pressure exerted by the vapor phase which is a simple, reliable, high-throughput method for measuring (e.g., static gauged bombs and ebulliometry) are often not the vapor pressures of low-volatility compounds. The suited to measurements on low-volatility samples for multiple reasons. First, the contributions of volatile impurities to concatenated gas saturation method represents a significant measured pressures can be significant at extremely low advance over traditional gas saturation methods. Instead of a impurity mole fractions (19, 20). The reason, of course, is single saturator and trap, the concatenated method uses that impurities can have vapor pressures that are orders of several pairs of saturators and traps linked in series. Consequently, magnitude greater than that of the sample compound. several measurements of vapor pressure can be made Second, these methods typically require sample masses of simultaneously, which greatly increases the rate of data at least a few grams. Obviously, large amounts of highly pure collection. It also allows for the simultaneous measurement of material are difficult to obtain for many compounds of a control compound, which is important for ensuring data environmental interest. quality. In this paper we demonstrate the use of the concatenated A variety of “indirect” methods are capable of measuring the vapor pressures of low-volatility compounds (11-14, 16, gas saturation method by determination of the vapor pressures - of five monoterpene oxidation products and n-tetradecane 21 27). However, the gas saturation method and the effusion - method (when high-purity samples are available) are gener- (the control compound) over the temperature range 283.15 313.15 ally considered to be the most accurate of these methods for K. Over this temperature range, the vapor pressures ranged low vapor pressures (16). The gas saturation method (16, from about 0.5 Pa to about 70 Pa. The standard molar enthalpies 19-21, 28-32) is a simple technique that involves the of vaporization or sublimation were determined by use of the saturation of a carrier gas stream with the vapor of a Clausius-Clapeyron equation. condensed phase of the compound of interest. The most common approach is to strip the vapor from a measured volume of the saturated carrier gas using an adsorber or cold Introduction trap, and then measure the recovered mass with an ap- Globally, vegetation emits a tremendous quantity of volatile propriate analytical method. The vapor pressure is then organic compounds (VOCs) (1-5). Annually, plants and trees calculated with the ideal gas equation, eq 1, emit more than 1012 kg of carbon as VOCs, about 40% of ) · · · which is isoprene (2-5). The balance of these emissions is psat (m R T)/(V M) (1) mostly monoterpenes such as R- and -pinene. In the atmosphere, monoterpenes are readily oxidized to less where psat is the vapor pressure, m is the recovered mass of volatile “monoterpenoid” compounds, which then form the vapor, R is the gas constant, T is the temperature of the aerosols (1, 5-8). Such secondary organic aerosols (SOAs) saturator, V is the volume of carrier gas at the temperature have an important impact on climate because of the way and pressure of the saturator, and M is the molar mass of the that they scatter and absorb solar radiation, and because of compound. The gas saturation method has several key advantages * Corresponding author phone: (303) 497-5158; fax: (303) 497- (16, 21, 27, 28). Calibration is not required. Impurities have 5927; e-mail: [email protected]. a relatively small effect on the measured vapor pressures, 388 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 1, 2010 10.1021/es9026216 2010 U.S. Government Published on Web 12/10/2009 SCHEME 1. Chemical Structures and Abbreviations for the Monoterpenoids Studied FIGURE 1. A concatenated gas saturation apparatus featuring several saturator-adsorber pairs linked in series. The apparatus assuming that a technique such as gas chromatography is used for this study has 18 saturator-adsorber pairs (X ) 14). used to determine the amount of solute vapor, so samples of limited purity can be used. Little sample is needed for a measurement (typically tens of milligrams or less), again cylinder to facilitate mass determinations. SF6 was chosen assuming that a sensitive analytical method is used to as the carrier gas for a variety of reasons: it is inert, it is determine the amount of vapor. Finally, such an apparatus less prone to leak than gases like nitrogen or argon, is simple and inexpensive to build and operate. On the other diffusion in SF6 is relatively slow (mass transfer by diffusion hand, traditional gas saturation methods have two significant is undesirable in this system), and its high density facilitates drawbacks. First, measurement periods can be quite long if mass determinations (see below). a large volume of carrier gas is needed in order to collect a Concatenated Gas Saturation Apparatus. The apparatus sufficient amount of vapor for analysis. Second, the method used for these measurements was designed and constructed is susceptible to certain types of systematic errors (e.g., leaks) at NIST. A detailed description of the apparatus is published that can be difficult to detect. in a recent paper (34), so only an overview of the principal The “concatenated” gas saturation method (21) was components is given here. It is also worth noting that this developed in order to compensate for the drawbacks apparatus is similar to an earlier apparatus for which a mentioned above. In this type of apparatus, several saturator- detailed description has also been published (21). The adsorber pairs are linked in series, so that multiple measure- principal components of the apparatus are illustrated ments can be made simultaneously with the same carrier schematically in Figure 1. The carrier gas supply includes an gas stream. This approach greatly speeds data collection. It aluminum gas cylinder, pressure regulator, and flow con- also allows for strategies that ensure data quality. For example, troller. The cylinder-regulator assembly must be removed a control compound with a well-known vapor pressure can between measurements to determine the mass of carrier gas be measured simultaneously with the sample compounds. that was used. To facilitate this procedure, it is connected If the measurement yields the expected vapor pressure for to the flow controller by a short stainless steel capillary with the control compound, one has a high level of confidence a valve at each end. Closing these two valves allows the in the other measurements that were made simultaneously. cylinder-regulator assembly to be removed with the loss of The concatenated gas saturation method can make a only the capillary’s volume of carrier gas (approximately 1 valuable contribution to vapor pressure measurements on mg of SF6). Carrier gas from the flow controller first passes environmentally important compounds. By use of a recently through an adsorbent column packed with the porous constructed apparatus with 18 saturator-adsorber pairs, we polymer adsorbent poly(2,6-diphenyl-1,4-phenylene oxide) demonstrate the utility of the method with a series of (33, 35-37). The gas then flows through the 18 saturator- monoterpenoid compounds. The monoterpenoids were (+)- adsorber pairs. The saturators are located inside a forced- carvone (CAR), (1S,2S,5S)-(-)-2-hydroxy-3-pinanone (HP), air, temperature-controlled chamber and consist of PTFE (1R,2R,3R,5S)-(-)-isopinocampheol (IPC), (+)-trans-myr- tubes (1 m in length with an inside diameter of 0.48 cm) tanol (TM), and (-)-trans-pinocarveol (TPC).