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J. Phys. Chem. C 2009, 113, 2036–2040
Measurements of Depositional Ice Nucleation on Insoluble Substrates at Low Temperatures: Implications for Earth and Mars† Melissa G. Trainer,*,‡,§ Owen B. Toon,‡,| and Margaret A. Tolbert§,⊥ Laboratory for Atmospheric and Space Physics, UCB 392, CooperatiVe Institute for Research in EnVironmental Sciences, UCB 216, Department of Atmospheric and Oceanic Sciences, UCB 311, and Department of Chemistry and Biochemistry, UCB 215, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed: June 11, 2008; ReVised Manuscript ReceiVed: October 13, 2008
Heterogeneous depositional nucleation of ice particles may dominate in atmospheric regions with very low temperatures, where low vapor pressures and insoluble nuclei would be expected. However, depositional ice nucleation has not been studied extensively at temperatures below 200 K and not at all below 170 K. Here, we show results of a study of heterogeneous depositional nucleation of ice on a solid silicon substrate at temperatures from 150 to 180 K. The results presented herein demonstrate an unexpected temperature dependence in which very high critical supersaturations are required to nucleate ice at T e 180 K. The vapor pressures necessary for nucleation below 175 K are greater than the vapor saturation for supercooled liquid water. The temperature dependence observed is not predicted by classical heterogeneous nucleation theory, when assuming a temperature-independent contact parameter, m. Here, we use our new measurements in combination with previously published data to derive a temperature-dependent expression for the contact parameter of a nonsoluble substrate: m(150 - 240 K) ) 0.94 - 6005 exp(-0.065T). Our findings have significant implications for ice cloud nucleation in the terrestrial mesosphere as well as the Martian atmosphere, which are at temperatures below those previously studied in the laboratory. Introduction Ice clouds on Earth play an important role in influencing the Earth’s radiation budget;1-3 yet, they represent one of the most uncertain elements in current climate assessments.4 Modeling ice cloud formation is challenging, as the nucleation mechanisms and ice nuclei properties are not well-constrained. Particularly for regions of the atmosphere where ice clouds form at cold temperatures, there is little laboratory data to coincide with observations. In addition, in situ measurements of water vapor and relative humidity in the upper troposphere have been difficult due to the low temperature and pressure conditions, and a large deviation between instruments exists in the literature. For example, recent in situ measurements have indicated that water vapor supersaturations can exceed 100% without ice particles present5 and have even been observed exceeding 200% with respect to ice at 185 K in the upper troposphere.6 These saturations are greater than expected based on measurements of ice nucleation on hygroscopic aerosols. Models predict that ice nucleation occurs through homogeneous freezing of deliquesced aerosols with critical saturation ratios of approximately 1.4-1.7 in the region of 240-170 K.7 However, even within the cited work, large variations can be seen in measurements of relative humidity, and there is much disagreement regarding the existence of high supersaturations. Independent of the controversial relative humidity measurements, recent observations of ice crystals in the tropical tropopause layer (TTL) have * To whom correspondence should be addressed. Tel: 303-492-1433. Fax: 303-492-1149. E-mail:
[email protected]. † Part of the special section “Physical Chemistry of Environmental Interfaces”. ‡ Laboratory for Atmospheric and Space Physics. § Cooperative Institute for Research in Environmental Sciences. | Department of Atmospheric and Oceanic Sciences. ⊥ Department of Chemistry and Biochemistry.
indicated the presence of large ice crystals for which nucleation and growth scenarios require that high supersaturations (Sice ≈ 2) be sustained.6 The authors of this study note that the observed particles are difficult to form using typical assumptions of ice nucleation in liquid aerosols in the troposphere. Rather, ice clouds may nucleate via heterogeneous depositional nucleation at very low temperatures, where the ice nuclei might be insoluble materials8 or nondeliquesced solid particles.9,10 Understanding the dominant mechanisms for ice nucleation has important implications for the predicted frequency and thus the impacts of these clouds. Particularly for regions of the atmosphere at very low temperatures, such as the polar mesosphere, there is concern that there is not a complete understanding of microphysical processes of nucleation at low water vapor pressure conditions.8 The composition and physical properties of nucleating particles at T e 190 K may be altered sufficiently to preclude homogeneous freezing. Ice cloud formation likely occurs via depositional nucleation. Observations suggest that heterogeneous nucleation preferentially occurs on minerals or other insoluble nuclei; yet, such particles have not been studied in the laboratory at temperatures below 195 K.11,12 Ice nucleation at low temperatures also has relevance for Mars, where water ice clouds are observed regularly from 3 to 20 km above the surface.13-15 These ice clouds have an important effect on the temperature16,17 and water cycle18 in the Martian atmosphere. Temperature profiles correlated with ice cloud appearances show that they are formed within the temperature range from 150 to 200 K.19 Models of cloud formation on Mars typically assume that there is little or no barrier to heterogeneous ice nucleation on airborne dust particles (critical saturation of unity). However, it is unlikely that the dust particles in the Mars atmosphere are such efficient ice nuclei. Current studies of nucleation on mineral or other
10.1021/jp805140p CCC: $40.75 2009 American Chemical Society Published on Web 12/19/2008
Depositional Ice Nucleation on Insoluble Substrates
J. Phys. Chem. C, Vol. 113, No. 6, 2009 2037
substrates do not exist below 195 K;11,20 yet, models use these data to simulate ice cloud formation at the lower temperatures on Mars. Here, we present new results from a study in which the saturations for heterogeneous ice nucleation from vapor deposition were measured from 150 to 180 K. Coupled with previous studies of depositional ice nucleation performed by our group to 240 K,10,21 we are able to demonstrate a significant temperature dependence of critical saturation ratios (Scrit). Results presented herein will show that in this temperature range unexpectedly high saturations may be required to nucleate ice. Experimental Methods Laboratory ice nucleation experiments were performed using a high-vacuum chamber, which has been described in detail in previous publications.22 To monitor the water vapor pressure in several pressure regimes, the chamber was equipped with an MKS Baratron capacitance manometer, a Bayard-Alpert ionization gauge, and an MKS Residual Gas Analyzer. Ice nucleation and growth were monitored using transmission Fourier-transform infrared (FTIR) spectroscopy, with a resolution of 2 cm-1. The substrate used was a monocrystalline silicon (Si) wafer maintained at a low temperature through a combination of compressed helium cooling (APD Cryogenics) and resistive heating, as monitored by thermocouples attached to the cryostat arm and sample mount. The cryostat arm was enclosed in a separately pumped vacuum jacket isolated with a Teflon seal. This allowed only the sample mount and substrate to be exposed within the chamber and thus minimized any temperature gradients. The range of temperatures studied was approximately 150-180 K. The experimental procedure used to measure Scrit values as a function of temperature has been described previously.10,21 Scrit is defined as
Scrit(T) ) Pnuc(T) ⁄ Pice(T)
Figure 1. Typical nucleation experiment. In both sections of the figure, the saturation ratio (Sice, black line, left axis) is calculated at each point in time as the ratio of the pressure of H2O in the chamber to the equilibrium vapor pressure measured in the frost point calibration. The integrated absorbance (gray line, right axis) of the -OH stretch (2700-3800 cm-1) is plotted as a function of time. (a) The nucleation event, identified at approximately 90 min by the rise in absorbance and decrease in saturation ratio. (b) The frost point calibration, showing the equilibration of the ice film as denoted by neither growth nor evaporation observed in the IR absorbance. The vapor pressure here is used to define Sice ) 1. The gap in time between the onset of nucleation in panel a and the frostpoint determination in panel b is due to the time needed for the film to grow to an appropriate thickness for the frost point to be performed. The calibrated temperature for this experiment was 161 K, and the measured Scrit was 5.1.
(1)
where Pnuc(T) is the experimental vapor pressure at which the onset of nucleation occurred and Pice(T) is the equilibrium vapor pressure of ice at the experimental temperature. In this study, the chamber was first evacuated to
