J. Phys. Chem. A 2008, 112, 9413–9422
9413
Comparative Thermodynamic Studies of Aqueous Glutaric Acid, Ammonium Sulfate and Sodium Chloride Aerosol at High Humidity⊥ Kate L. Hanford,† Laura Mitchem,† Jonathan P. Reid,*,† Simon L. Clegg,‡ David O. Topping,§ and Gordon B. McFiggans§ School of Chemistry, UniVersity of Bristol, Bristol, BS8 1TS, U.K., School of EnVironmental Sciences, UniVersity of East Anglia, Norwich NR4 7TJ, U.K., and School of Earth, Atmospheric and EnVironmental Sciences, UniVersity of Manchester, Williamson Building, Oxford Road, Manchester, M13 9PL, U.K. ReceiVed: March 23, 2008; ReVised Manuscript ReceiVed: May 14, 2008
Aerosol optical tweezers are used to simultaneously characterize and compare the hygroscopic properties of two aerosol droplets, one containing inorganic and organic solutes and the second, referred to as the control droplet, containing a single inorganic salt. The inorganic solute is either sodium chloride or ammonium sulfate and the organic component is glutaric acid. The time variation in the size of each droplet (3-7 µm in radius) is recorded with 1 s time resolution and with nanometre accuracy. The size of the control droplet is used to estimate the relative humidity with an accuracy of better than (0.09%. Thus, the Ko¨hler curve of the multicomponent inorganic/organic droplet, which characterizes the variation in equilibrium droplet size with relative humidity, can be determined directly. The measurements presented here focus on high relative humidities, above 97%, in the limit of dilute solutes. The experimental data are compared with theoretical treatments that, while ignoring the interactions between the inorganic and organic components, are based upon accurate representations of the activity-concentration relationships of aqueous solutions of the individual salts. The organic component is treated by a parametrized fit to experimental data or by the UNIFAC model and the water activity of the equilibrium solution droplet is calculated using the approach suggested by Clegg, Seinfeld and Brimblecombe or the Zdanovskii-Stokes-Robinson approximation. It is shown that such an experimental strategy, comparing directly droplets of different composition, enables highly accurate measurements of the hygroscopic properties, allowing the theoretical treatments to be rigorously tested. Typical deviations of the experimental measurements from theoretical predictions are shown to be around 1% in equilibrium size, comparable to the variation between the theoretical frameworks considered. I. Introduction Knowledge of the solution thermodynamics governing aerosol hygroscopicity is of considerable importance for understanding the partitioning of water between the gas and condensed phases in atmospheric aerosol. A change in the activity of water in the gas phase must be matched by a corresponding change in the condensed phase for the gas-liquid equilibrium to be maintained.1,2 As a consequence, solution thermodynamics are key to interpreting the change in cloud droplet size distributions with change in relative humidity (RH) and the hygroscopicity of aerosol is central to understanding the activation of cloud condensation nuclei.3 Thus, predicting the gas-liquid partitioning of water is crucial for interpreting the influence of aerosols on the properties of clouds, and this is central to resolving the uncertainties surrounding the impact of aerosol on climate. 4 While the hygroscopic properties of typical inorganic solutes, such as sodium chloride (SC) and ammonium sulfate (AS), are well understood through bulk solution and aerosol measurements, many uncertainties remain in predicting the influence of organic components on aerosol hygroscopicity.3,5,6 This is partly due to the complexity of the organic mass fraction and ⊥
Part of the “Stephen R. Leone Festschrift”. * To whom correspondence should be addressed. E-mail: j.p.reid@ bristol.ac.uk. † University of Bristol. ‡ University of East Anglia. § University of Manchester.
the challenges associated with quantifying even the dominant organic components.7,8 Although it is now recognized that 20-70% of the total particulate carbon in the atmosphere is water soluble,7 incorporating organic components in thermodynamic models remains challenging.9 A thermodynamic treatment should include the nonideality of aqueous solutions, incorporating the interactions between the organic and inorganic solutes and their influence on activities where known.9,10 This becomes particularly important at the high solute concentrations/ low RHs that are characteristic of supersaturated solutions.6,9 The solubility of the organic component in the aqueous phase may be such that immiscible hydrophobic and hydrophilic phases form. As a consequence, it is necessary to consider phase partitioning between the gas phase and discrete aqueous and organic phases.5,11 Further, the impact of the dissociation equilibria of organic components should be included in the thermodynamic treatment if the mass fraction of the organic components are high with respect to inorganic components, and if the dissociation constants are large enough that the mole fractions of the dissociated species are significant.12,13 For example, the acid-base equilibria of dicarboxylic acids can affect the pH of aerosols, altering the partitioning of volatile components, such as ammonia and nitric acid, between the gas and condensed phases and thus influencing the partitioning of water.12 The vapor pressure and equilibrium size of a solution droplet are influenced by surface curvature as well as solution thermo-
10.1021/jp802520d CCC: $40.75 2008 American Chemical Society Published on Web 07/02/2008
9414 J. Phys. Chem. A, Vol. 112, No. 39, 2008 dynamics. Ko¨hler theory incorporates both of these effects, with the role of solution thermodynamics governed by a solute term and the surface curvature effect governed by a Kelvin term.2,3 Calculation of the Kelvin term, which reflects the fractional increase in vapor pressure of a curved surface over that of a flat surface, requires knowledge of the surface composition and this is dependent on the surface activity of the organic components, which can be related to surface tension.14 Further, the surface composition is expected to have an influence on the kinetics of mass transfer between the gas and liquid phases.15 Both water soluble and insoluble organic compounds may have an impact on the evaporation or condensation rates of water,16,17 and it has been suggested that this could complicate the measurement of aerosol hygroscopicity.18 To fully interpret the influence of organic components on the hygroscopic properties of aerosol, it is necessary to examine the variation in wet particle size over a wide range of RH, to characterize the partitioning of components between immiscible organic and aqueous phases, to probe surface composition and the influence of surface activity on wet particle size, and to ensure that the recorded size is not determined by kinetic limitations to mass transfer. We have recently reported a direct strategy for comparing in situ the variation in wet particle size of two droplets (2-8 µm radius) of differing composition using aerosol optical tweezers.19 The size of each droplet can be determined with nanometre accuracy and with high timeresolution,
