Relationship between the aerosol number distribution and the cloud condensation nuclei supersaturation spectrum

Thesis (Ph.D.) University of Alaska Fairbanks, 1999Though Cloud Condensation Nuclei (CCN) are a subset of atmospheric aerosol, relatively little is known of what links the two. A recently developed instrument called the CCN Remover, which directly relates the CCN supersaturation spectrum to the aerosol number distribution, is described. Instrumental errors are quantified and laboratory tests used to verify the instrument's accuracy are also presented. We made measurements with the CCN Remover in the Aerosol Characterization Experiment 2 (ACE 2) and the INdian Ocean EXperiment (INDOEX). These two multinational field campaigns shared the objective of investigating aerosol particles' ability to modulate cloud albedo by activating as CCN. In both instances, we found that aerosol particles were not activating with the characteristics of pure ammonium sulfate, which is generally regarded as the major component of the majority of aerosol particles which act as CCN. Either a substantial fraction of the aerosol was not participating in the activation process or the presence of a hydrophobic surface film inhibited water vapor transport. Measurements of the aerosol's chemical composition and hygroscopic growth factors are used to examine these possibilities. Anthropogenic activity is modifying the properties of natural aerosol particles in a way which could affect their ability to act as CCN. We discuss evidence for aerosol particles coated with sulfuric acid in an Arctic air mass in support of this claim. In some instances, the connection between aerosol and CCN can be inferred directly from the aerosol number distribution. Clouds segregate aerosol into two populations---those that act as CCN and those that do not, and when the cloud evaporates, the aerosol number distribution bears the signature of the cloud through which it has cycled---a minimum in the aerosol number distribution. The diameter at which this minimum occurs can be related to the maximum supersaturation in the cloud, and the number of particles larger than the minimum is the population of particles that acted as CCN. Over 1,000 bimodal aerosol number distributions from five widely separated locations have been analyzed for maximum supersaturation and cloud droplet (or CCN) et (or CCN) concentrations

Entropic aspects of supercooled droplet freezing

The freezing of supercooled water droplets in the atmosphere, with an emphasis on the entropic aspects of the problem, is examined. Supercooled water is a metastable state and, therefore, the associated phase transition must be irreversible. Temperature-dependent heat capacities of supercooled water and ice are used to calculate the entropy difference. That difference is then used to establish a lower bound on the amount of latent heat that can be liberated by the freezing droplets. The calculation is compared with tabulated values of the latent heat of fusion with surprising results. Based on a novel physical picture of the freezing process, the authors suggest a simple estimate for the effective latent heat that is suitable for heat budget calculations of glaciating clouds. In addition, the authors arrive at a quadratic dependence on supercooling, (ΔT)2, for the irreversible contribution to heat exchange during the freezing process. The proportionality factor is estimated as −0.3 J mol−1 K−2

Measurements of the vapor pressure of supercooled water using infrared spectroscopy

Measurements are presented of the vapor pressure of supercooled water utilizing infrared spectroscopy, which enables unambiguous verification that the authors’ data correspond to the vapor pressure of liquid water, not a mixture of liquid water and ice. Values of the vapor pressure are in agreement with previous work. Below −13°C, the water film that is monitored to determine coexistence of liquid water (at one temperature) and ice (at another, higher, temperature) de-wets from the hydrophilic silicon prism employed in the authors’ apparatus. The de-wetting transition indicates a quantitative change in the structure of the supercooled liquid

Laboratory measurements of contact freezing by dust and bacteria at temperatures of mixed-phase clouds

Laboratory measurements of freezing by aerosol particles in contact mode are presented. The fraction of particles catalyzing freezing is quantified for three mineral dusts and three strains of bacteria. This is the most comprehensive such dataset to date for temperatures greater than −20°C, relevant for warm, mixed-phase clouds. For Arizona Test Dust, feldspar, or rhyolitic ash, more than 103 particles are required to initiate a freezing event at −20°C in the contact mode. At −15°C, more than 105 particles are required. An ice-negative strain of Pseudomonas fluorescens is an order of magnitude more effective than the mineral dusts at every temperature tested. To the best of the authors’ knowledge, this is the first measurement of contact-mode freezing by an ice-negative bacterium. An ice-positive strain of Pseudomonas syringae reaches its maximum nucleating efficiency, E = 0.1, 12°C higher than does Pseudomonas fluorescens. This is consistent with the behavior of ice-negative and ice-positive bacteria in the immersion mode, as discovered 40 years ago. Surprisingly, cells of the ice-positive strain Pseudomonas syringae CC94 that do not express the ice nucleation active gene showed no contact-freezing activity, whereas the cells of the ice-negative strain of Pseudomonas fluorescens showed significant activity

Dispersion aerosol indirect effect in turbulent clouds: Laboratory measurements of effective radius

Cloud optical properties are determined not only by the number density nd and mean radius ṝ of cloud droplets but also by the shape of the droplet size distribution. The change in cloud optical depth with changing nd, due to the change in distribution shape, is known as the dispersion effect. Droplet relative dispersion is defined as d=σr / ṝ . For the first time, a commonly used effective radius parameterization is tested in a controlled laboratory environment by creating a turbulent cloud. Stochastic condensation growth suggests d independent of nd for a nonprecipitating cloud, hence nearly zero albedo susceptibility due to the dispersion effect. However, for size‐dependent removal, such as in a laboratory cloud or highly clean atmospheric conditions, stochastic condensation produces a weak dispersion effect. The albedo susceptibility due to turbulence broadening has the same sign as the Twomey effect and augments it by order 10%

Scaling of Turbulence and Microphysics in a Convection–Cloud Chamber of Varying Height

The convection–cloud chamber enables measurement of aerosol and cloud microphysics, as well as their interactions, within a turbulent environment under steady-state conditions. Increasing the size of a convection–cloud chamber, while holding the imposed temperature difference constant, leads to increased Rayleigh, Reynolds and Nusselt numbers. Large–eddy simulation coupled with a bin microphysics model allows the influence of increased velocity, time, and spatial scales on cloud microphysical properties to be explored. Simulations of a convection–cloud chamber, with fixed aspect ratio and increasing heights of H = 1, 2, 4, and (for dry conditions only) 8 m are performed. The key findings are: Velocity fluctuations scale as H1/3, consistent with the Deardorff expression for convective velocity, and implying that the turbulence correlation time scales as H2/3. Temperature and other scalar fluctuations scale as H−3/7. Droplet size distributions from chambers of different sizes can be matched by adjusting the total aerosol injection rate as the horizontal cross-sectional area (i.e., as H2 for constant aspect ratio). Injection of aerosols at a point versus distributed throughout the volume makes no difference for polluted conditions, but can lead to cloud droplet size distribution broadening in clean conditions. Cloud droplet growth by collision and coalescence leads to a broader right tail of the distribution compared to condensation growth alone, and this tail increases in magnitude and extent monotonically as the increase of chamber height. These results also have implications for scaling within turbulent, cloudy mixed-layers in the atmosphere, such as fog layers

Molecular simulations reveal that heterogeneous ice nucleation occurs at higher temperatures in water under capillary tension

Homogeneous ice nucleation rates occur at higher temperatures when water is under tension, otherwise referred to as negative pressure. If also true for heterogeneous ice nucleation rates, then this phenomenon can result in higher heterogeneous freezing temperatures in water capillary bridges, pores, and other geometries where water is subjected to negative Laplace pressure. Using a molecular model of water freezing on a hydrophilic substrate, it is found that heterogeneous ice nucleation rates exhibit a similar temperature increase at negative pressures as homogeneous ice nucleation. For pressures ranging from from 1 atm to &minus;1000 atm, the simulations reveal that the temperature corresponding to the heterogeneous nucleation rate coefficient jhet (m&minus;2 s&minus;1) increases linearly as a function of negative pressure, with a slope that can be approximately predicted by the water density anomaly and the latent heat of fusion at atmospheric pressure. Simulations of water in capillary bridges confirm that negative Laplace pressure within the water corresponds to an increase in heterogeneous freezing temperature. The freezing temperature in the water capillary bridges increases linearly with inverse capillary height (1/h). Varying the height and width of the capillary bridge reveals the role of geometric factors in heterogeneous ice nucleation. When substrate surfaces are separated by less than approximately h = 20 Angstroms the nucleation rate is enhanced and when the width of the capillary bridge is less than approximately 30 Angstroms the nucleation rate is suppressed. Ice nucleation does not occur in the region within 10 Angstroms of the air-water interface and shows a preference for nucleation in the region just beyond 10 Angstroms. These results help unify multiple lines of experimental evidence for enhanced nucleation rates due to reduced pressure, either resulting from surface geometry (Laplace pressure) or mechanical agitation of water droplets. This concept is relevant to the phenomenon of contact nucleation and could potentially play a role in a number of different heterogeneous nucleation or secondary ice mechanisms.</p

Is contact nucleation caused by pressure perturbation?

The reason why ice nucleation is more efficient by contact nucleation than by immersion nucleation has been elusive for over half a century. Six proposed mechanisms are summarized in this study. Among them, the pressure perturbation hypothesis, which arose from recent experiments, can qualitatively explain nearly all existing results relevant to contact nucleation. To explore the plausibility of this hypothesis in a more quantitative fashion and to guide future investigations, this study assessed the magnitude of pressure perturbation needed to cause contact nucleation and the associated spatial scales. The pressure perturbations needed were estimated using measured contact nucleation efficiencies for illite and kaolinite, obtained from previous experiments, and immersion freezing temperatures, obtained from well-established parameterizations. Pressure perturbations were obtained by assuming a constant pressure perturbation or a Gaussian distribution of the pressure perturbation. The magnitudes of the pressure perturbations needed were found to be physically reasonable, being achievable through possible mechanisms, including bubble formation and breakup, Laplace pressure arising from the distorted contact line, and shear. The pressure perturbation hypothesis provides a physically based and experimentally constrainable foundation for parameterizing contact nucleation that may be useful in future cloud-resolving models

Measurements of ice nucleation by mineral dusts in the contact mode

Formation of ice in Earth\u27s atmosphere at temperatures above approximately −20 °C is one of the outstanding problems in cloud physics. Contact nucleation has been suggested as a possible mechanism for freezing at relatively high temperatures; some laboratory experiments have shown contact freezing activity at temperatures as high as −4 °C. We have investigated Arizona Test Dust and kaolinite as contact nuclei as a function of size and temperature and find that the fraction of submicron particles that are active as contact ice nuclei is less than 10−3 for −18 °C and greater. We also find that the different dusts are quite distinct in their effectiveness as contact nuclei; Arizona Test Dust catalyzed freezing in the contact mode at all mobility diameters we tested at −18 °C whereas kaolinite triggered freezing only for mobility diameters of 1000 and 500 nm at that temperature