J. Phys. Chem. A 2010, 114, 2821–2829
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Aqueous Aerosol May Build Up an Elevated Upper Tropospheric Ice Supersaturation and Form Mixed-Phase Particles after Freezing A. Bogdan*,†,‡ and M. J. Molina§ Department of Physics, P.O. Box 48, and Laboratory of Polymer Chemistry, Department of Chemistry, P.O. Box 55, UniVersity of Helsinki, FI-00014 Helsinki, Finland, Institute of Physical Chemistry, UniVersity of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria, and Department of Chemistry and Biochemistry, UniVersity of California, San Diego, La Jolla, California 92093-0356 ReceiVed: September 8, 2009; ReVised Manuscript ReceiVed: January 19, 2010
Observations often reveal large clear-sky upper tropospheric ice supersaturation, Si, which sometimes reaches 100%. However, a water activity criterion (Nature 2000, 406, 611) does not allow the buildup of large Si by cooled aqueous aerosol. According to the criterion, Si produced by aqueous aerosol increases from ∼52% at 220 K to only ∼67% at 185 K. The nature of the formation of large upper tropospheric Si remains unclear. Here we present the results of the study of micrometer-scaled three-, four-, and five-component droplets containing different weight fractions of H2O, H2SO4, HNO3, (NH4)2SO4, (NH4)HSO4, NH4NO3, and (NH4)3H(SO4)2. The study was performed between 133 and 278 K at cooling rates of 3, 0.1, and 0.05 K/min using differential scanning calorimetery. We find that complex phase transformations, which include one, two, and three freezing and melting events, glass transition on cooling, and devitrification and crystallization-freezing on warming, can occur during the cooling and warming of droplets. Using the measured freezing temperature of ice, Ti, and the thermodynamic E-AIM model, we calculate the largest clear-sky Si which would be formed immediately prior to the formation of ice cirrus by homogeneous freezing of multicomponent aerosol. The calculations show that multicomponent aerosol of some compositions may produce Si >80% at temperatures higher than 185 K. We also find that similar to that of H2SO4/H2O and H2SO4/ HNO3/H2O aerosol the freezing of multicomponent aerosol can also produce mixed-phase cirrus particles: an ice core + a residual solution coating. 1. Introduction Observations often reveal enhanced and persistent moisture in the upper troposphere (UT) independently of whether cirrus ice clouds are present.1-3 Since water vapor is the dominant greenhouse gas, it is important to know the nature of the accumulation and persistence of water vapor in the UT. At the cold UT conditions, when the temperature drops below the homogeneous freezing temperature of water, Th ≈ 233 K,4,5 moisture is often expressed as relative humidity with respect to ice, RHi, or ice supersaturation, Si ) RHi - 100%. The use of RHi or Si instead of relative humidity with respect to liquid water, RH, is reasonable because the vapor pressure of ice is measured down to ∼173 K.6 In contrast, the vapor pressure of liquid water cannot be measured below the Th. The extrapolation of the temperature dependence of the vapor pressure of liquid water to the lowest atmospheric temperatures of ∼183-185 K may produce large errors in the calculations of RH.7 According to observations, clear-sky Si can reach a value as large as 100% (RHi ≈ 200%) and then persist even after the formation of cirrus ice clouds.1-3 However, a water activity criterion8 (below we will refer to it as WAC) does not allow the formation of Si >∼67% by the homogeneous freezing of aqueous droplets even at the lowest atmospheric temperature of ∼185 K. For aqueous aerosol the WAC predicts the existence of a so-called homogeneous ice nucleation threshold, which, being expressed as Si, is between ∼52 and 67% in the temperature region of †
University of Helsinki. University of Innsbruck. § University of California, San Diego. ‡
∼220-185 K.8 There are publications which support the WAC hypothesis9-12 and report about deviations from the predictions of the WAC.12-15 In ref 15, Swanson summarized literature data sets for ammonium sulfate and sulfuric acid aqueous solutions, showing quite large scatter in the estimated WA data, some of which are also well outside the uncertainty range of the WAC. The restriction imposed by the WAC gave a start to a search for mechanisms, other than the cooling/freezing of aqueous aerosol, which could account for the buildup of large Si in the UT. To account for the observed large clear-sky Si, several hypotheses have been put forward; they suggest that the formation of cubic ice,7,16,17 the existence of concentrated organic aerosol,9 and the presence of organic films on H2SO4/H2O aerosol3 may prolong the lifetime of water vapor in the UT. Cubic ice is a metastable form of ice, and therefore, its vapor pressure is thought to be larger than that of the stable hexagonal form of ice. Some authors7,16,17 believe that cubic ice may account for up to ∼3-10% of the enhanced Si. However, the most recent estimation of the enthalpy change of cubic-tohexagonal-ice transition suggests that the vapor pressure difference between cubic and hexagonal ices may be less than 1%.18 It has been proposed that organic aerosol with a concentration as large as 50-90 wt %, instead of freezing, may transform to a glassy state that could enhance UT moisture.9 However, the concentration of 50-90 wt % is similar to that of background stratospheric aerosol, 40-90 wt % H2SO4. In the stratosphere, a source of SO2, which transforms to H2SO4, is enormous volcanic eruptions, but the source of highly concentrated organic aerosol has not been reported. Furthermore, to account for the observed Si >100%, Jensen et al.3 proposed
10.1021/jp9086656 2010 American Chemical Society Published on Web 02/08/2010
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J. Phys. Chem. A, Vol. 114, No. 8, 2010
a hypothesis that organic films on H2SO4/H2O aerosol could hinder the uptake of H2O in moderately to rapidly cooling air that could postpone the formation of ice cirrus to colder temperatures and, consequently, produce Si >100%. The hypothesis is reminiscent of a work by Xiong et al.,19 who reported that organic films suppressed the growth rate of H2SO4/H2O aerosol over time scales of 6 and 10 s when the relative humidity changed from 20% to 85%. Using a one-dimensional microphysical model, Jensen et al.3 calculated that if organic films suppress the accommodation coefficient of H2O to values below ∼5 × 10-5 to 8 × 10-3, depending on the cooling rate, then H2SO4/H2O aerosol would remain more concentrated with lower freezing probability that would result in the buildup of Si >100%. Unfortunately, despite the fact that the authors kept track of the time-dependent sulfate mass fraction, they did not report the initial concentration of H2SO4/H2O aerosol and how it changed with temperature during ascension. The dependence of the freezing temperature on the composition of micrometerscaled H2SO4/H2O droplets placed on a hydrophobic organosilane surface20 and emulsified H2SO4/H2O droplets,21 which are surrounded by an organic lanolin surfactant, is well-established. For example, 10 and 25 wt % H2SO4 droplets freeze homogeneously at ∼222 and ∼188 K, respectively. Using the experimental freezing temperatures and the thermodynamic E-AIM model of the system H+-NH4+-SO42--NO3--H2O22 (or the expression which Jensen et al. used in their model), one can calculate the equilibrium water vapor pressures of H2SO4/H2O aerosol and ice and, consequently, the corresponding Si. Assuming that, in the Jensen et al. model, at 222 K the initial concentration of aerosol is 10 wt % H2SO4, one can calculate that, in the air parcel ascending to T ≈ 188 K, Si >100% can be reached only if the organic films ensure the increase of the concentration to ∼25 wt % H2SO4. Only in this case will the H2SO4/H2O aerosol remain liquid during the ascension and cooling from ∼222 to ∼188 K. The increase of the concentration can be either due to “pumping” H2SO4 inside the organic-coated H2SO4/H2O aerosol droplets or due to the depletion of H2O from them. The existence of organic films with such peculiar properties remains to be proved. The UT aerosol droplets, which are precursors of cold ice cirrus below ∼210 K, besides H2SO4, can contain HNO323 and ammonium salts (NH4)2SO4, (NH4)HSO4, NH4NO3, and (NH4)3H(SO4)2, which are formed by the reaction of H2SO4 and HNO3 with ammonia, NH3.24-28 In the UT, the acids and ammonia are of natural and anthropogenic origin: descending stratospheric H2SO4/H2O aerosol,29 volcanic eruptions,30 burning of sulfur-containing fossil fuels,31 nitrogen-based fertilizers, livestock, vegetation, and oceanic sources,32,33 etc. The measurements of the freezing behavior of binary and ternary aqueous systems (bulk34-37 and dispersed21,23,27,28,38 solutions) have been reported. To our best knowledge, the freezing behavior of fourand five-component aqueous droplets, which contain different weight fractions of H2O, H2SO4, HNO3, (NH4)2SO4, (NH4)HSO4, NH4NO3, and (NH4)3H(SO4)2, has never been studied before. In this paper, we present the experimental results which demonstrate that, similar to H2SO4/H2O21,39 and H2SO4/HNO3/ H2O23 aerosol, the homogeneous freezing of three-, four-, and five-component aerosol containing H+, NH4+, SO42-, and NO3can also produce mixed-phase particles: an ice core + a residual solution coating. Using the measured freezing temperature of ice, Ti, and the thermodynamic E-AIM model of the system H+-NH4+-SO42--NO3--H2O,22 we calculate that, in the UT, the cooling of multicomponent aerosol containing H+, NH4+, SO42-, and NO3- can build up a clear-sky Si slightly larger than
Bogdan and Molina 80% at temperatures higher than 185 K. Although our Si is smaller than the largest observed value of Si ≈ 100%,1-3 it is nevertheless larger than Si ≈ 67% predicted by the WAC8 at ∼185 K. Our results can give an impetus for the study of whether multicomponent aqueous aerosol droplets, which besides inorganic components also contain organics, may produce the observed Si >100%. 2. Experimental Section For the preparation of the populations of micrometer-scaled three-, four-, and five-component aqueous droplets, we used the corresponding bulk solutions and emulsion technique. The prepared droplets have been studied between 133 and 278 K by using differential scanning calorimetry (DSC). 2.1. Materials. For the preparation of bulk solutions we used two acids, 95-97 wt % H2SO4 and 65 wt % HNO3 (Riedel-de Hae¨n, Germany), and three salts, 99.999% (NH4)2SO4, 99.999% NH4NO3 (Sigma-Aldrich), and g99.5% (NH4)HSO4 (Fluka). The solutions were prepared by mixing the calculated amount of acids and salts with the corresponding amount of ultrapure deionized water. The solution of letovicite, (NH4)3H(SO4)2, was prepared by mixing solutions containing corresponding mole fractions of (NH4)2SO4 and (NH4)HSO4. The accuracy of the method of solution preparation was verified by the titration of H2SO4/H2O and HNO3/H2O against standard 2 N NaOH. The titration showed an accuracy of (0.1 wt %. We prepared several groups of solutions which contain about 200 multicomponent solutions with different weight fractions (wt %) of components. For example, one group of five-component solutions consists of 27 solutions with different weight fractions of (NH4)2SO4, H2SO4, NH4NO3, HNO3, and H2O, i.e., 7/17/5/4/67, 17/10/6/ 4/63, 5/15/5/4, etc. wt % (NH4)2SO4/H2SO4/NH4NO3/HNO3/ H2O. The following groups of solutions were prepared: (threecomponent)H2SO4/HNO3/H2O,(NH4)2SO4/H2SO4/H2O,(NH4)HSO4/ H2SO4/H2O, (NH4)2SO4/HNO3/H2O, (NH4)3H(SO4)2/H2SO4/ H2O, and (NH4)3H(SO4)2/NH4NO3/H2O; (four-component) (NH4)2SO4/H2SO4/HNO3/H2O, (NH4)HSO4/H2SO4/HNO3/H2O, (NH4)HSO4/H2SO4/NH4NO3/H2O, and (NH4)3H(SO4)2/H2SO4/ NH4NO3/H2O; (five-component) (NH4)3H(SO4)2/H2SO4/NH4NO3/ HNO3/H2O,(NH4)HSO4/H2SO4/NH4NO3/HNO3/H2O,and(NH4)2SO4/ H2SO4/NH4NO3/HNO3/H2O. The oil phase, which was used for the preparation of emulsions, was prepared by using halocarbon 0.8 oil (Halocarbon Products Corp.) and lanolin (Sigma-Aldrich). The halocarbon oil is a low molecular weight polymer of chlorotrifluoroethylene (PCTFE) and is chemically inert to practically all acids, alkalis, and oxidizing agents. Lanolin is a natural organic substance which consists of cholesterol, wool alcohols, and the esters derived from several fatty acids. Lanolin is insoluble in water but makes an emulsion. 2.2. Emulsification Procedure. The solution-in-oil emulsions were prepared according to an emulsion technique described elsewhere.38 In short, the oil phase was prepared by mixing 80 wt % halocarbon 0.8 oil and 20 wt % lanolin. The solution/oil mixtures of 1/10 ratio by volume were shaken for 5-12 min with a high-speed shaker (Thermolyne Maxi-Mix) at 1400-2000 rpm. The diameter of the droplets was measured with an optical microscope. In the emulsions of H2SO4/H2O, HNO3/H2O and H2SO4/HNO3/H2O the diameter of the droplets is
