An official website of the United States government
Heterogeneous ice nucleation in the atmosphere regulates cloud properties, such as phase (ice versus liquid) and lifetime. Aerosol particles of marine origin are relevant ice nucleating particle sources when marine aerosol layers are lifted over mountainous terrain and in higher latitude ocean boundary layers, distant from terrestrial aerosol sources. Among many particle compositions associated with ice nucleation by sea spray aerosols are highly saturated fatty acids. Previous studies have not demonstrated their ability to freeze dilute water droplets. This study investigates ice nucleation by monolayers at the surface of supercooled droplets and as crystalline particles at temperatures exceeding the threshold for homogeneous freezing. Results show the poor efficiency of long chain fatty acid (C16, C18) monolayers in templating freezing of pure water droplets and seawater subphase to temperatures of at least −30 °C, consistent with theory. This contrasts with freezing of fatty alcohols (C22 used here) at nearly 20 °C warmer. Evaporation of μL-sized droplets to promote structural compression of a C19 acid monolayer did not favor warmer ice formation of drops. Heterogeneous ice nucleation occurred for nL-sized droplets condensed on 5 to 100 μm crystalline particles of fatty acid (C12 to C20) at a range of temperatures below −28 °C. These experiments suggest that fatty acids nucleate ice at warmer than −36 °C only when the crystalline phase is present. Rough estimates of ice active site densities are consistent with those of marine aerosols, but require knowledge of the proportion of surface area comprised of fatty acids for application.
more »
« less
The freezing behavior of aqueous n -alcohol nanodroplets
We generate water-rich aerosols containing 1-propanol and 1-pentanol in a supersonic nozzle to study the effects of these solutes on the freezing behavior of water. Condensation and freezing are characterized by two complementary techniques, pressure trace measurements and Fourier Transform Infrared spectroscopy. When 1-pentanol and 1-propanol are present, condensation occurs at higher temperatures because particle formation from the vapor phase is enhanced by the decrease in interfacial free energy of mixed aqueous-alcohol critical clusters relative to those of pure water. FTIR results suggest that when ∼6 nm radius droplets freeze, the tetrahedral structure of the ice is well preserved up to an overall alcohol mole fraction of 0.031 for 1-propanol and 0.043 for 1-pentanol. In this concentration range, the ice nucleation temperature decreases continuously with increasing 1-propanol concentration, whereas the onset of freezing is not significantly perturbed by 1-pentanol up to a mole fraction of 0.03. Furthermore, once freezing starts the ice nucleation rates in the aqueous-alcohol droplets are very close to those for pure water. In contrast, at the highest mole fractions of either alcohol it is not clear whether droplets freeze to form crystalline ice since the final state of the particles cannot be adequately characterized with the available experimental techniques.
more »
« less
- PAR ID:
- 10276892
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 23
- Issue:
- 16
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 9991 to 10005
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract. Droplet freezing techniques (DFTs) have been used for half a century tomeasure the concentration of ice-nucleating particles (INPs) in the atmosphereand determine their freezing properties to understand the effects of INPs onmixed-phase clouds. The ice nucleation community has recently adopted dropletfreezing assays as a commonplace experimental approach. These dropletfreezing experiments are often limited by contamination that causesnonhomogeneous freezing of the “pure” water used to generate the dropletsin the heterogeneous freezing temperature regime that is being measured.Interference from the early freezing of water is often overlooked and notfully reported, or measurements are restricted to analyzing the moreice-active INPs that freeze well above the temperature of the backgroundwater. However, this avoidance is not viable for analyzing the freezingbehavior of less active INPs in the atmosphere that still have potentiallyimportant effects on cold-cloud microphysics. In this work we review a numberof recent droplet freezing techniques that show great promise in reducing theseinterferences, and we report our own extensive series of measurements usingsimilar methodologies. By characterizing the performance of differentsubstrates on which the droplets are placed and of different pure watergeneration techniques, we recommend best practices to reduce theseinterferences. We tested different substrates, water sources, dropletmatrixes, and droplet sizes to provide deeper insight into what methodologiesare best suited for DFTs. Approaches for analyzing droplet freezingtemperature spectra and accounting and correcting for the background “pure”water control spectrum are also presented. Finally, we propose experimentaland data analysis procedures for future homogeneous and heterogeneous icenucleation studies to promote a more uniform and reliable methodology thatfacilitates the ready intercomparison of ice-nucleating particles measured byDFTs.more » « less
-
Abstract. From extracellular freezing to cloud glaciation, the crystallization of water is ubiquitous and shapes life as we know it. Efficient biological ice nucleators (INs) are crucial for organism survival in cold environments and, when aerosolized, serve as a significant source of atmospheric ice nuclei. Several lichen species have been identified as potent INs capable of inducing freezing at high subzero temperatures. Despite their importance, the abundance and diversity of lichen INs are still not well understood. Here, we investigate ice nucleation activity in the cyanolichen-forming genus Peltigera from across a range of ecosystems in the Arctic, the northwestern United States, and Central and South America. We find strong IN activity in all tested Peltigera species, with ice nucleation temperatures above −12 °C and 35 % of the samples initiating freezing at temperatures at or above −6.2 °C. The Peltigera INs in aqueous extract appear to be resistant to freeze–thaw cycles, suggesting that they can survive dispersal through the atmosphere and thereby potentially influence precipitation patterns. An axenic fungal culture termed L01-tf-B03, from the lichen Peltigera britannica JNU22, displays an ice nucleation temperature of −5.6 °C at 1 mg mL−1 and retains remarkably high IN activity at concentrations as low as 0.1 ng mL−1. Our analysis suggests that the INs released from this fungus in culture are 1000 times more potent than the most active bacterial INs from Pseudomonas syringae. The global distribution of Peltigera lichens, in combination with the IN activity, emphasizes their potential to act as powerful ice-nucleating agents in the atmosphere.more » « less
-
Abstract All cloud and climate models assume ice crystals grow as if they were formed from pure water, even though cloud and haze droplets are solutions. The freezing process of a solution droplet is different than that of a pure water droplet, as shown in prior work. This difference can potentially affect the particle’s subsequent growth as an ice crystal. We present measurements of ice crystal growth from frozen sodium chloride (NaCl) solution droplets in the button electrode levitation diffusion chamber at temperatures between −61° and −40°C. Measured scattering patterns show that concentrated solution droplets remain unfrozen with classical scattering fringes until the droplets freeze. Upon freezing, the scattering patterns become complex within 0.1 s, which is in contrast with frozen pure water particles that retain liquid-like scattering patterns for about a minute. We show that after freezing, solution particles initially grow as spherical-like crystals and then transition to faster growth indicative of a morphological transformation. The measurements indicate that ice formed from solution droplets grows differently and has higher growth rates than ice formed from pure water droplets. We use these results to develop a power-law-based parameterization that captures the supersaturation and mass dependencies.more » « less
-
Abstract. Some biological particles, such as Snomax, are very active ice nucleating particles, inducing heterogeneous freezing in supercooled water at temperatures above −15 and up to −2 °C. Despite their exceptional freezing abilities, large uncertainties remain regarding the atmospheric abundance of biological ice nucleating particles, and their contribution to atmospheric ice nucleation. It has been suggested that small biological ice nucleating macromolecules or fragments can be carried on the surfaces of dust and other atmospheric particles. This could combine the atmospheric abundance of dust particles with the ice nucleating strength of biological material to create strongly enhanced and abundant ice nucleating surfaces in the atmosphere, with significant implications for the budget and distribution of atmospheric ice nucleating particles, and their consequent effects on cloud microphysics and mixed-phase clouds. The new critical surface area g framework that was developed by Beydoun et al. (2016) is extended to produce a heterogeneous ice nucleation mixing model that can predict the freezing behavior of multicomponent particle surfaces immersed in droplets. The model successfully predicts the immersion freezing properties of droplets containing Snomax bacterial particles across a mass concentration range of 7 orders of magnitude, by treating Snomax as comprised of two distinct distributions of heterogeneous ice nucleating activity. Furthermore, the model successfully predicts the immersion freezing behavior of a low-concentration mixture of Snomax and illite mineral particles, a proxy for the biological material–dust (bio-dust) mixtures observed in atmospheric aerosols. It is shown that even at very low Snomax concentrations in the mixture, droplet freezing at higher temperatures is still determined solely by the second less active and more abundant distribution of heterogeneous ice nucleating activity of Snomax, while freezing at lower temperatures is determined solely by the heterogeneous ice nucleating activity of pure illite. This demonstrates that in this proxy system, biological ice nucleating particles do not compromise their ice nucleating activity upon mixing with dust and no new range of intermediary freezing temperatures associated with the mixture of ice nucleating particles of differing activities is produced. The study is the first to directly examine the freezing behavior of a mixture of Snomax and illite and presents the first multicomponent ice nucleation model experimentally evaluated using a wide range of ice nucleating particle concentration mixtures in droplets.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0CP06131J
