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J. Phys. Chem. A 2010, 114, 12682–12691
Direct Comparison of the Hygroscopic Properties of Ammonium Sulfate and Sodium Chloride Aerosol at Relative Humidities Approaching Saturation Jim S. Walker,† Jon B. Wills,† Jonathan P. Reid,*,† Liangyu Wang,‡ David O. Topping,§ Jason R. Butler,† and Yun-Hong Zhang‡ School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, U.K., School of Science, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing 100081, P. R. China, and School of Earth, Atmospheric and EnVironmental Sciences, Williamson Building, UniVersity of Manchester, Oxford Road, Manchester M13 9PL, U.K. ReceiVed: August 17, 2010; ReVised Manuscript ReceiVed: October 14, 2010
Holographic optical tweezers are used to make comparative measurements of the hygroscopic properties of single component aqueous aerosol containing sodium chloride and ammonium sulfate over a range of relative humidity from 84% to 96%. The change in RH over the course of the experiment is monitored precisely using a sodium chloride probe droplet with accuracy better than (0.09%. The measurements are used to assess the accuracy of thermodynamic treatments of the relationship between water activity and solute mass fraction with particular attention focused on the dilute solute limit approaching saturation vapor pressure. The consistency of the frequently used Clegg-Brimblecombe-Wexler (CBW) treatment for predicting the hygroscopic properties of sodium chloride and ammonium sulfate aerosol is confirmed. Measurements of the equilibrium size of ammonium sulfate aerosol are found to agree with predictions to within an uncertainty of (0.2%. Given the accuracy of treating equilibrium composition, the inconsistencies highlighted in recent calibration measurements of critical supersaturations of sodium chloride and ammonium sulfate aerosol cannot be attributed to uncertainties associated with the thermodynamic predictions and must have an alternative origin. It is concluded that the CBW treatment can allow the critical supersaturation to be estimated for sodium chloride and ammonium sulfate aerosol with an accuracy of better than (0.002% in RH. This corresponds to an uncertainty of e1% in the critical supersaturation for typical supersaturations of 0.2% and above. This supports the view that these systems can be used to accurately calibrate instruments that measure cloud condensation nuclei concentrations at selected supersaturations. These measurements represent the first study in which the equilibrium properties of two particles of chemically distinct composition have been compared simultaneously and directly alongside each other in the same environment. 1. Introduction The influence of aerosols on the radiative balance of the atmosphere through the direct and indirect effects remains one of the greatest sources of uncertainty in the current understanding of global climate change.1 In order to improve the assessment of the impact of aerosols on climate, it is essential that the thermodynamic and kinetic factors governing the size distributions and activation of atmospheric aerosol to cloud droplets be more fully understood. Uncertainties remain in understanding the thermodynamic equilibrium state at low and high values of relative humidity (RH). At low RH, aerosol particles may exist as supersaturated solutions that are metastable with respect to the crystallization of solutes.2,3 Indeed, aerosol particles may form amorphous glassy phases, gel-like structures, or highly viscous liquids.4-6 For multicomponent aerosol, in particular mixtures of inorganic and organic components, interactions between solute molecules must be considered and the solution may behave in a highly nonideal way.7,8 At high RH, the interplay of surface curvature and solute concentration determines the critical water vapor supersaturation above which aerosol particles are activated and act as cloud condensation * To whom correspondence should be addresed. E-mail:
[email protected]. † University of Bristol. ‡ Beijing Institute of Technology. § University of Manchester.
nuclei (CCN).9-14 Understanding the influence of surface composition is crucial for interpreting CCN measurements.14-17 Controlled laboratory measurements can provide important insights into the fundamental properties and processes that determine the state and impact of aerosol on the atmospheric radiative and chemical balance. In particular, laboratory studies can allow investigations of the equilibrium state of aerosol,18-20 their optical properties,21-24 pathways of chemical transformation and aging,25 and the dynamics of heat and mass transfer.26-30 Studies of hygroscopicity can provide a robust test of thermodynamic models, and recent work has focused on the behavior of mixed component aerosol at low relative humidity, the ice nucleation ability of aerosol, the fraction of aerosol activated as CCN with variation in water vapor supersaturation, and the sensitivity of this fraction to surface tension.3,5,9,12,14,20,31,32 In order to accurately assess these properties for complex multicomponent and even multiphase aerosols, it is crucial that our knowledge of the properties of single inorganic component aerosols be critically assessed.33,34 The accuracy with which the thermodynamic properties of single component sodium chloride (SC) or ammonium sulfate (AS) aerosol are known has been evaluated at both low and high RH.3,12,32 Even for these simple systems, it has been concluded that predictions from different models can vary significantly, and interpreting experimental data in the limits of low and high RH must be pursued with care.3,12
10.1021/jp107802y 2010 American Chemical Society Published on Web 11/10/2010
Ammonium Sulfate and Sodium Chloride Aerosol
J. Phys. Chem. A, Vol. 114, No. 48, 2010 12683
An improved characterization of the hygroscopic properties of single component aqueous aerosol at high RH remains highly desirable. Wex et al. have noted that an upper limit of 95% RH often associated with hygroscopicity measurements can lead to inaccurate assumptions about the degree of dissociation of solutes or the degree of nonideal behavior for more dilute solutions as saturation is approached, the RH regime relevant for CCN activation.14,15 The challenges associated with determining the RH close to saturation and the need to calibrate instruments against known aerosol standards are often limiting factor.12 There are few techniques for making reliable measurements at such high RH levels.35 Good et al. have recently illustrated the problems associated with characterizing the hygroscopicity of organic aerosol at high RH.36 Growth curves measured using three hygroscopicity tandem differential mobility analyzers under subsaturated conditions were found to show considerable variation, preventing the reconciliation of CCN and hygroscopicity measurements when using self-consistent assumptions. Aerosol optical tweezers have emerged as a new strategy for performing single particle characterization.37 A significant strength of the technique is the ability to capture two droplets in two parallel optical traps, separated by only a few tens of micrometers. This requires sequential capture of two droplets from two different nebulized samples. Comparative measurements can then be performed on aerosol particles that are different in chemical composition, and an aqueous sodium chloride droplet can be used as a highly sensitive and responsive probe of the gas phase conditions.19,38 We have shown that RH changes can be measured using such a probe droplet to an accuracy of better than (0.09% with immediate time response, both significant advances over the use of conventional probes.38,39 A limitation of this comparative approach is that the initially captured droplet may become contaminated with the second aerosol flow that is introduced to capture the probe droplet.40 In a recent publication41 we demonstrated a strategy to isolate a trapped droplet from a surrounding cloud of aerosol using an optical barrier formed using a spatial light modulator (SLM) and holographic optical tweezers (HOTs).39,42 In this publication, we will apply this technique to make comparative measurements of two very simple but distinct aqueous salt solution droplets containing SC and AS, and compare the measurements to standard thermodynamic models of hygroscopicity. In section 2 we describe the thermodynamic treatments available for predicting the hygroscopic growth of SC and AS aerosol and the previous measurements that have been made. In section 3 we describe the experimental technique used in this publication before describing and discussing the results of our measurements in section 4. 2. Previous Measurements and Models of SC and AS Aerosol Hygroscopicity At equilibrium, the vapor pressure of an aqueous solution droplet containing an involatile solute must be equal to the surrounding partial pressure of water in the gas phase, usually expressed as the RH or the activity of water in the gas phase. The vapor pressure of the droplet can be defined as a function of solute concentration and droplet radius by the Ko¨hler equation11,13
( )
RH ) abulk exp
2Vmσ RTr
(1)
which describes the interplay of the bulk solute and surface curvature effects in determining the vapor pressure. Vm is the
partial molar volume of water, σ is the surface tension of the solution, and r is the droplet radius. The activity of water in the bulk solution, abulk, corresponds to the activity of water in a solution at the same solute molality as the droplet when at a radius r. To estimate the equilibrium size of a solution droplet, it is essential to quantify the dependence of the water activity on the solute mass fraction, the dependence of surface tension on solution composition, and the density of the solution. We have discussed these in detail in a preceding publication for aqueous SC aerosol3 and will consider only the details relevant to the experiments presented here at high RH for both SC and AS aerosol. Growth factors (GFs) are a convenient way of representing experimentally determined trends in droplet hygroscopicity with variation in RH and are calculated from the ratio of wet (r) to dry (rdry) particle radii:10
GF(RH) )
r(RH) rdry
(2)
Expressing hygroscopicities in this form has the advantage of removing the dependence on dry particle size and automatically accounts for the variation in solution density with composition. This allows measurements from a range of droplets containing different amounts of solute to be meaningfully compared to a single prediction curve. A (1 nm accuracy in droplet size determination, typical of the experimental resolution achieved in our measurements, corresponds to a percentage error in GF of
