The time dependence of immersion freezing was studied for temperatures between 236 K and 243 K. Droplets with single immersed, size-selected 400 nm and 800 nm kaolinite particles were produced at 300 ...K, cooled down to supercooled temperatures, and the fraction of frozen droplets with increasing residence time was detected. To simulate the conditions of immersion freezing in mixed-phase clouds we used the Zurich Ice Nucleation Chamber (ZINC) and its vertical extension, the Immersion Mode Cooling chAmber (IMCA). We observed that the frozen fraction of droplets increased with increasing residence time in the chamber. This suggests that there is a time dependence of immersion freezing and supports the importance of a stochastic component in the ice nucleation process. The rate at which droplets freeze was observed to decrease towards higher temperatures and smaller particle sizes. Comparison of the laboratory data with four different ice nucleation models, three based on classical nucleation theory with different representations of the particle surface properties and one singular, suggest that the classical, stochastic approach combined with a distribution of contact angles is able to reproduce the ice nucleation observed in these experiments most accurately. Using the models to calculate the increase in frozen fraction at typical mixed-phase cloud temperatures over an extended period of time, yields an equivalent effect of -1 K temperature shift for an increase in times scale by one order of magnitude. This suggests that temperature is more important than time.
The new portable ice nucleation chamber (PINC) developed by the Institute for Atmospheric and Climate Sciences of ETH Zurich was operated during two measurement campaigns at the high alpine research ...station Jungfraujoch situated at 3580 m a.s.l, in March and June 2009. During this time of the year, a high probability of Saharan dust events (SDE) at the Jungfraujoch has been observed. We used an impactor with a cutoff size of 1 μm aerodynamic diameter and operated the system at −31 °C and relative humidities of 127 % and 91 % with respect to ice and water, respectively. Investigation of the ambient number concentration of ice nuclei (IN) in the deposition nucleation mode and during a SDE in the free troposphere is reported. The results discussed in this paper are the first continuous IN measurements over a period of several days at the Jungfraujoch. The average IN concentration found during the campaign in March was 8 particles per liter whereas during the campaign in June, the average number concentration was higher up to 14 particles per liter. Two SDEs were detected on 15 and 16 June 2009. Our measurements show that the SDEs had IN number concentration up to several hundreds per liter. We found the best correlation between the number concentration of the larger particle fraction measured by an optical particle counter and the IN number concentration during a Saharan dust event. This correlation factor is higher for particles larger than 0.5 μm meaning that a higher concentration of larger particles induced higher IN number concentration. No correlation could be found between the black carbon mass concentration and the variations in IN number concentration.
A measurement campaign (IMBALANCE) conducted in 2009 was aimed at characterizing the physical and chemical properties of freshly emitted and photochemically aged combustion particles emitted from a ...log wood burner and diesel vehicles: a EURO3 Opel Astra with a diesel oxidation catalyst (DOC) but no particle filter and a EURO2 Volkswagen Transporter TDI Syncro without emission aftertreatment. Ice nucleation experiments in the deposition and condensation freezing modes were conducted with the Portable Ice Nucleation Chamber (PINC) at three nominal temperatures, −30 °C, −35 °C and −40 °C. Freshly emitted diesel particles showed ice formation only at −40 °C in the deposition mode at 137% relative humidity with respect to ice (RHi) and 92% relative humidity with respect to water (RHw), and photochemical ageing did not play a role in modifying their ice nucleation behaviour. Only one diesel experiment where α-pinene was added for the ageing process, showed an ice nucleation enhancement at −35 °C. Wood burning particles also act as ice nuclei (IN) at −40 °C in the deposition mode at the same conditions as for diesel particles and photochemical ageing also did not alter the ice formation properties of the wood burning particles. Unlike diesel particles, wood burning particles form ice via condensation freezing at −35 °C whereas no ice nucleation was observed at −30 °C. Photochemical ageing did not affect the ice nucleation ability of the diesel and wood burning particles at the three different temperatures investigated but a broader range of temperatures below −40 °C need to be investigated in order to draw an overall conclusion on the effect of photochemical ageing on deposition/condensation ice nucleation across the entire temperature range relevant to cold clouds.
The Portable Ice Nucleation Chamber 2 (PINCii) is a newly developed continuous flow diffusion chamber (CFDC) for measuring ice nucleating particles (INPs). PINCii is a vertically oriented ...parallel-plate CFDC that has been engineered to improve upon the limitations of previous generations of CFDCs. This work presents a detailed description of the PINCii instrument and the upgrades that make it unique compared with other operational CFDCs.
The PINCii design offers several possibilities for improved INP measurements. Notably, a specific icing procedure results in low background particle counts, which demonstrates the potential for PINCii to measure INPs at low concentrations (<10 L−1). High-spatial-resolution wall-temperature mapping enables the identification of temperature inhomogeneities on the chamber walls. This feature is used to introduce and discuss a new method for analyzing CFDC data based on the most extreme lamina conditions present within the chamber, which represent conditions most likely to trigger ice nucleation. A temperature gradient can be maintained throughout the evaporation section in addition to the main chamber, which enables PINCii to be used to study droplet activation processes or to extend ice crystal growth.
A series of both liquid droplet activation and ice nucleation experiments were conducted at temperature and saturation conditions that span the spectrum of PINCii's operational conditions (-50≤ temperature ≤-15 ∘C and 100 ≤ relative humidity with respect to ice ≤160 %) to demonstrate the instrument's capabilities.
In addition, typical sources of uncertainty in CFDCs, including particle background, particle loss, and variations in aerosol lamina temperature and relative humidity, are quantified and discussed for PINCii.
Immersion freezing is the most relevant heterogeneous ice nucleation mechanism through which ice crystals are formed in mixed-phase clouds. In recent years, an increasing number of laboratory ...experiments utilizing a variety of instruments have examined immersion freezing activity of atmospherically relevant ice-nucleating particles. However, an intercomparison of these laboratory results is a difficult task because investigators have used different ice nucleation (IN) measurement methods to produce these results. A remaining challenge is to explore the sensitivity and accuracy of these techniques and to understand how the IN results are potentially influenced or biased by experimental parameters associated with these techniques. Within the framework of INUIT (Ice Nuclei Research Unit), we distributed an illite-rich sample (illite NX) as a representative surrogate for atmospheric mineral dust particles to investigators to perform immersion freezing experiments using different IN measurement methods and to obtain IN data as a function of particle concentration, temperature (T), cooling rate and nucleation time. A total of 17 measurement methods were involved in the data intercomparison. Experiments with seven instruments started with the test sample pre-suspended in water before cooling, while 10 other instruments employed water vapor condensation onto dry-dispersed particles followed by immersion freezing. The resulting comprehensive immersion freezing data set was evaluated using the ice nucleation active surface-site density, ns, to develop a representative ns(T) spectrum that spans a wide temperature range (-37 degree C < T < -11 degree C) and covers 9 orders of magnitude in ns. In general, the 17 immersion freezing measurement techniques deviate, within a range of about 8 degree C in terms of temperature, by 3 orders of magnitude with respect to ns. In addition, we show evidence that the immersion freezing efficiency expressed in ns of illite NX particles is relatively independent of droplet size, particle mass in suspension, particle size and cooling rate during freezing. A strong temperature dependence and weak time and size dependence of the immersion freezing efficiency of illite-rich clay mineral particles enabled the ns parameterization solely as a function of temperature. We also characterized the ns(T) spectra and identified a section with a steep slope between -20 and -27 degree C, where a large fraction of active sites of our test dust may trigger immersion freezing. This slope was followed by a region with a gentler slope at temperatures below -27 degree C. While the agreement between different instruments was reasonable below ~ -27 degree C, there seemed to be a different trend in the temperature-dependent ice nucleation activity from the suspension and dry-dispersed particle measurements for this mineral dust, in particular at higher temperatures. For instance, the ice nucleation activity expressed in ns was smaller for the average of the wet suspended samples and higher for the average of the dry-dispersed aerosol samples between about -27 and -18 degree C. Only instruments making measurements with wet suspended samples were able to measure ice nucleation above -18 degree C. A possible explanation for the deviation between -27 and -18 degree C is discussed. Multiple exponential distribution fits in both linear and log space for both specific surface area-based ns(T) and geometric surface area-based ns(T) are provided. These new fits, constrained by using identical reference samples, will help to compare IN measurement methods that are not included in the present study and IN data from future IN instruments.
Persistence of orographic mixed‐phase clouds Lohmann, U.; Henneberger, J.; Henneberg, O. ...
Geophysical research letters,
16 October 2016, Letnik:
43, Številka:
19
Journal Article
Recenzirano
Odprti dostop
Mixed‐phase clouds (MPCs) consist of ice crystals and supercooled water droplets at temperatures between 0 and approximately −38°C. They are thermodynamically unstable because the saturation vapor ...pressure over ice is lower than that over supercooled liquid water. Nevertheless, long‐lived MPCs are ubiquitous in the Arctic. Here we show that persistent MPCs are also frequently found in orographic terrain, especially in the Swiss Alps, when the updraft velocities are high enough to exceed saturation with respect to liquid water allowing simultaneous growth of supercooled liquid droplets and ice crystals. Their existence is characterized by holographic measurements of cloud particles obtained at the high‐altitude research station Jungfraujoch during spring 2012 and winter 2013 and simulations with the regional climate model COSMO (Consortium of Small‐Scale Modeling).
Key Points
Mixed‐phase clouds occur more commonly than previously believed
Mixed‐phase clouds can be persistent in orographic terrain, like the Alps, under strong orographic forcing
Regional model simulations are able to simulate mixed‐phase clouds and support our observations
The University of Toronto Continuous Flow Diffusion Chamber (UT-CFDC) was used to study ice formation onto monodisperse Arizona Test Dust (ATD) particles. The onset relative humidity with respect to ...ice (RHi) was measured as a function of temperature in the range 251−223 K for 100 nm ATD particles. It was found that for 0.1% of the particles to freeze, water saturation was required at all temperatures except 223 K where particles activated at RHi below water saturation. At this temperature, where deposition mode freezing is occurring, we find that the larger the particle size, the lower the onset RHi. We also demonstrate that the total number of particles present may influence the onset RHi observed. The surface area for ice activation, aerosol size, and temperature must all be considered when reporting onset values of ice formation onto ATD mineral dust particles. In addition, we calculate nucleation rates and contact angles of ice germs with ATD aerosols which indicate that there exists a range of active sites on the surface with different efficiencies for activating ice formation.
Seven different instruments and measurement methods were used to examine the immersion freezing of bacterial ice nuclei from Snomax® (hereafter Snomax), a product containing ice-active protein ...complexes from non-viable Pseudomonas syringae bacteria. The experimental conditions were kept as similar as possible for the different measurements. Of the participating instruments, some examined droplets which had been made from suspensions directly, and the others examined droplets activated on previously generated Snomax particles, with particle diameters of mostly a few hundred nanometers and up to a few micrometers in some cases. Data were obtained in the temperature range from −2 to −38 °C, and it was found that all ice-active protein complexes were already activated above −12 °C. Droplets with different Snomax mass concentrations covering 10 orders of magnitude were examined. Some instruments had very short ice nucleation times down to below 1 s, while others had comparably slow cooling rates around 1 K min−1. Displaying data from the different instruments in terms of numbers of ice-active protein complexes per dry mass of Snomax, nm, showed that within their uncertainty, the data agree well with each other as well as to previously reported literature results. Two parameterizations were taken from literature for a direct comparison to our results, and these were a time-dependent approach based on a contact angle distribution (Niedermeier et al., 2014) and a modification of the parameterization presented in Hartmann et al. (2013) representing a time-independent approach. The agreement between these and the measured data were good; i.e., they agreed within a temperature range of 0.6 K or equivalently a range in nm of a factor of 2. From the results presented herein, we propose that Snomax, at least when carefully shared and prepared, is a suitable material to test and compare different instruments for their accuracy of measuring immersion freezing.
It has recently been shown that pore condensation and
freezing (PCF) is a mechanism responsible for ice formation under cirrus
cloud conditions. PCF is defined as the condensation of liquid water in
...narrow capillaries below water saturation due to the inverse Kelvin effect,
followed by either heterogeneous or homogeneous nucleation depending on the
temperature regime and presence of an ice-nucleating active site. By using
sol–gel synthesized silica with well-defined pore diameters, morphology and
distinct chemical surface-functionalization, the role of the water–silica
contact angle and pore width on PCF is investigated. We find that for the
pore diameters (2.2–9.2 nm) and water contact angles (15–78∘) covered in this study, our results reveal that the water contact angle
plays an important role in predicting the humidity required for pore filling,
while the pore diameter determines the ability of pore water to freeze. For
T>235 K and below water saturation, pore diameters and water
contact angles were not able to predict the freezing ability of the
particles, suggesting an absence of active sites; thus ice nucleation did not
proceed via a PCF mechanism. Rather, the ice-nucleating ability of the
particles depended solely on chemical functionalization. Therefore,
parameterizations for the ice-nucleating abilities of particles in cirrus
conditions should differ from parameterizations at mixed-phase clouds
conditions. Our results support PCF as the atmospherically relevant ice
nucleation mechanism below water saturation when porous surfaces are
encountered in the troposphere.
The University of Toronto continuous flow diffusion chamber (UT-CFDC) was used to study heterogeneous ice nucleation at the International Workshop on Comparing Ice Nucleation Measuring Systems (ICIS ...2007) which also represented the 4-th ice nucleation workshop, on 14–28 September 2007. One goal of the workshop was to inter-compare different ice nucleation measurement techniques using the same aerosol sample source and preparation method. The aerosol samples included four types of desert mineral dust, graphite soot particles, and live and dead bacterial cells (Snomax®). This paper focuses on the UT-CFDC results, with a comparison to techniques of established heritage including the Colorado State CFDC and the AIDA expansion chamber. Good agreement was found between the different instruments with a few specific differences, especially at low temperatures, perhaps due to the variation in how onset of ice formation is defined between the instruments and the different inherent residence times. It was found that when efficiency of ice formation is based on the lowest onset relative humidity, Snomax® particles were most efficient followed by the desert dusts and then soot. For all aerosols, deposition mode freezing was only observed for T<45 K except for the dead bacteria where freezing occurred below water saturation as warm as 263 K.
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