Competition between water uptake and ice nucleation by glassy organic aerosol particles (original) (raw)
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2021
The time dependence of ice-nucleating particle (INP) activity is known to exist, yet for simplicity it is often omitted in atmospheric models as an approximation. Hitherto, only limited experimental work has been done to quantify this time dependency, for which published data are especially scarce regarding ambient aerosol samples and longer timescales. In this study, the time dependence of INP activity is quantified experimentally for six ambient environmental samples. The experimental approach includes a series of hybrid experiments with alternating constant cooling and isothermal experiments using a recently developed cold-stage setup called the Lund University Cold-Stage (LUCS). This approach of observing ambient aerosol samples provides the optimum realism for representing their time dependence in any model. Six ambient aerosol samples were collected at a station in rural Sweden representing aerosol conditions likely influenced by various types of INPs: marine, mineral dust, continental pristine, continental-polluted, combustion-related and rural continental aerosol. Active INP concentrations were seen to be augmented by about 40 % to 100 % (or 70 % to 200 %), depending on the sample, over 2 h (or 10 h). Mineral dust and rural continental samples displayed the most time dependence. This degree of time dependence observed was comparable to, but weaker than, that seen in previous published works. A general tendency was observed for the natural timescale of the freezing to dilate increasingly with time. The fractional freezing rate was observed to decline steadily with the time since the start of isothermal conditions following a power law. A representation of time dependence for incorporation into schemes of heterogeneous ice nucleation that currently omit it is proposed. Our measurements are inconsistent with the simplest purely stochastic model of INP activity, which assumes that the fractional freezing rate of all unfrozen drops is somehow constant and would eventually overpredict active INPs. In reality, the variability of efficiencies among INPs must be treated with any stochastic theory.
State transformations and ice nucleation in amorphous (semi-)solid organic aerosol
Atmospheric Chemistry and Physics, 2013
Amorphous (semi-)solid organic aerosol particles have the potential to serve as surfaces for heterogeneous ice nucleation in cirrus clouds. Raman spectroscopy and optical microscopy have been used in conjunction with a cold stage to examine water uptake and ice nucleation on individual amorphous (semi-)solid particles at atmospherically relevant temperatures (200-273 K). Three organic compounds considered proxies for atmospheric secondary organic aerosol (SOA) were used in this investigation: sucrose, citric acid and glucose. Internally mixed particles consisting of each organic and ammonium sulfate were also investigated. Results from water uptake experiments followed the shape of a humidity-induced glass transition (T g (RH)) curve and were used to construct state diagrams for each organic and corresponding mixture. Experimentally derived T g (RH) curves are in good agreement with theoretical predictions of T g (RH) following the approach of Koop et al. (2011). A unique humidity-induced glass transition point on each state diagram, T g (RH), was used to quantify and compare results from this study to previous works. Values of T g (RH) determined for sucrose, glucose and citric acid glasses were 236, 230 and 220 K, respectively. Values of T g (RH) for internally mixed organic/sulfate particles were always significantly lower; 210, 207 and 215 K for sucrose/sulfate, glucose/sulfate and citric acid/sulfate, respectively. All investigated SOA proxies were observed to act as heterogeneous ice nuclei at tropospheric temperatures. Heterogeneous ice nucleation on pure organic particles occurred at S ice = 1.1-1.4 for temperatures below 235 K. Particles consisting of 1:1 organic-sulfate mixtures took up water over a greater range of conditions but were in some cases also observed to heterogeneously nucleate ice at temperatures below 202 K (S ice = 1.25-1.38). Polynomial curves were fitted to experimental water uptake data and then incorporated into the Community Aerosol Radiation Model for Atmospheres (CARMA) along with the predicted range of humidity-induced glass transition temperatures for atmospheric SOA from Koop et al. (2011). Model results suggest that organic and organic/sulfate aerosol could be glassy more than 60 % of the time in the midlatitude upper troposphere and more than 40 % of the time in the tropical tropopause region (TTL). At conditions favorable for ice formation (S ice > 1), particles in the TTL are expected to be glassy more than 50 % of the time for temperatures below 200 K. Results from this study suggests that amorphous (semi-)solid organic particles are often present in the upper troposphere and that heterogeneous ice formation on this type of particle may play an important role in cirrus cloud formation.
Including Surface Kinetic Effects in Simple Models of Ice Vapor Diffusion
Journal of the Atmospheric Sciences, 2014
A model for kinetically limited vapor growth and aspect ratio evolution of atmospheric single ice crystals is presented. The method is based on the adaptive habit model of J. Chen and D. Lamb but is modified to include the deposition coefficients through a theory that accounts for axis-dependent growth. Deposition coefficients are predicted for each axis direction based on laboratory-determined critical supersaturations and therefore extends the adaptive habit approach and the capacitance model to low ice supersaturations. The new model is used to simulate changes in single-crystal primary habit in comparison to a hexagonal growth model. Results show that the new model captures the first-order features of axis-dependent, kinetically limited growth. The model reproduces not only the strong reductions in growth as supersaturations decrease but is also able to reproduce the near cessation of minor axis growth as saturations decline. While the new model reproduces the qualitative features of kinetically limited growth, relative errors are generally between 5% and 20% but can become larger than 50%. Parcel model simulation comparisons show that the new growth method reproduces the general features of axis-dependent growth in a changing temperature environment. The method also produces relatively accurate estimates of mass evolution with spherical particles, indicating that it may have broad applicability. Although the model compares well to a detailed method, uncertainties remain in the knowledge of surface kinetics that future studies need to unravel.
Heterogeneous nucleation of ice in the atmosphere
Journal of Physics: Conference Series, 2017
The occurrence of ice-nucleating aerosols in the atmosphere has a profound impact on the properties of clouds, and in turn, influences our understanding on weather and climate. Research on this topic has grown constantly over the last decades, driven by advances in online and offline instruments capable of measuring the characteristics of these cloud-modifying aerosol particles. This article presents different aspects to the understanding of how aerosol particles can trigger the nucleation of ice in clouds. In addition, we present some experimental results obtained with the Dynamic Filter Processing Chamber, an off-line instrument that has been applied extensively in the last years and that circumvents some of the problems related to the measurement of Ice Nucleating Particles properties.
A Review of Ice Particle Formation Models
Modelling of ice clouds in the atmosphere is in general a more complex task than modelling their liquid water counterparts, owing to the plethora of ice crystal nucleation processes and their non-linear dependence on surrounding conditions. Accurate modelling of ice clouds plays an important role in weather prediction and climatology, particularly in their contribution to greenhouse effect, global warming and precipitation processes, and the impact of aviation on the environment. In this paper, we review different ice particle formation models, focusing on the underlying assumptions, advantages and limitations of each scheme.
Geoscientific Model Development
In the Arctic, during polar night and early spring, ice clouds are separated into two leading types of ice clouds (TICs): (1) TIC1 clouds characterized by a large concentration of very small crystals and TIC2 clouds characterized by a low concentration of large ice crystals. Using a suitable parameterization of heterogeneous ice nucleation is essential for properly representing ice clouds in meteorological and climate models and subsequently understanding their interactions with aerosols and radiation. Here, we describe a new parameterization for ice crystal formation by heterogeneous nucleation in water-subsaturated conditions coupled to aerosol chemistry in the Weather Research and Forecasting model coupled with chemistry (WRF-Chem). The parameterization is implemented in the Milbrandt and Yau (2005a, b) two-moment cloud microphysics scheme, and we assess how the WRF-Chem model responds to the run-time interaction between chemistry and the new parameterization. Well-documented reference cases provided us with in situ data from the spring 2008 Indirect and Semi-Direct Aerosol Campaign (ISDAC) over Alaska. Our analysis reveals that the new parameterization clearly improves the representation of the ice water content (IWC) in polluted or unpolluted air masses and shows the poor performance of the reference parameterization in representing ice clouds with low IWC. The new parameterization is able to represent TIC1 and TIC2 mi-crophysical characteristics at the top of the clouds, where heterogenous ice nucleation is most likely occurring, even with the known bias of simulated aerosols by WRF-Chem over the Arctic.
2012
Abstract. A new analytical parameterization of homoge-neous ice nucleation is developed based on extended classi-cal nucleation theory including new equations for the critical radii of the ice germs, free energies and nucleation rates as si-multaneous functions of temperature and water saturation ra-tio. By representing these quantities as separable products of the analytical functions of temperature and supersaturation, analytical solutions are found for the integral-differential su-persaturation equation and concentration of nucleated crys-tals. Parcel model simulations are used to illustrate the gen-eral behavior of various nucleation properties under various conditions, for justifications of the further key analytical sim-plifications, and for verification of the resulting parameteri-zation. The final parameterization is based upon the values of the supersaturation that determines the current or maximum con-centrations of the nucleated ice crystals. The crystal concen-tration is...
Atmospheric Chemistry and Physics, 2018
Secondary organic aerosol (SOA) particles have been found to be efficient ice-nucleating particles under the cold conditions of (tropical) upper-tropospheric cirrus clouds. Whether they also are efficient at initiating freezing under slightly warmer conditions as found in mixedphase clouds remains undetermined. Here, we study the icenucleating ability of photochemically produced SOA particles with the combination of the Manchester Aerosol Chamber and Manchester Ice Cloud Chamber. Three SOA systems were tested resembling biogenic and anthropogenic particles as well as particles of different phase state. These are namely α-pinene, heptadecane, and 1,3,5-trimethylbenzene. After the aerosol particles were formed, they were transferred into the cloud chamber, where subsequent quasi-adiabatic cloud activation experiments were performed. Additionally, the ice-forming abilities of ammonium sulfate and kaolinite were investigated as a reference to test the experimental setup. Clouds were formed in the temperature range of −20 to −28.6 • C. Only the reference experiment using dust particles showed evidence of ice nucleation. No ice particles were observed in any other experiment. Thus, we conclude that SOA particles produced under the conditions of the reported experiments are not efficient ice-nucleating particles starting at liquid saturation under mixed-phase cloud conditions.