Freezing nuclei and condensation nuclei are collectively called ice nuclei, and ice crystals themselves are ice nuclei.Ice nuclei are not required to be soluble in water, but their molecular structures are required to be similar to ice crystals, so that water molecules can be arranged on the core surface according to certain rules to become ice crystals.The water vapor can directly condense on the surface of ice crystal, making the ice crystal continuously increase. The supercooled water drop will also freeze immediately when it contacts with the ice crystal, so the ice crystal itself is an ice core.[1]
Chinese name
Ice core
Foreign name
Ice nuclear
Meaning
General name of frozen nuclear and condensed nuclear
Ice nuclei are solid particles in the atmosphere that can cause condensation of water vapor or freezing of supercooled water droplets to form ice crystals.[2]At the beginning of the 20th century, some people believed that the appearance of ice crystals in clouds might require a nucleus that could lead to the freezing of small water droplets or the direct condensation of water vapor on it, which was the embryonic cluster of atmospheric ice nuclei.The actual extensive research on atmospheric ice nuclei began in the 1950s, and the research on atmospheric ice nuclei in China began in the 1960s.
Large ice nuclei form ice crystal streams in nature through four activation modes of sublimation, condensation freezing, immersion and contact freezing, which play a role in stimulating supercooled water to transform into ice crystals in cold cloud precipitation.In the mid latitude region, there are ice crystals in the negative temperature region of the cloud, and the conditional further growth of these ice crystals is an essential condition for the formation of precipitation.Therefore, the observation and analysis of atmospheric ice nuclei is the basic work to study natural cold cloud precipitation and artificial cold cloud precipitation.In 1991, at the International Conference on the Interaction of Aerosols and Clouds, experts again emphasized the role of ice phase processes in the atmosphere and the importance of ice nucleus observation, and believed that atmospheric ice nuclei could affect the atmospheric radiation process by affecting the cloud microphysical structure in addition to the precipitation process.In recent years, studies have pointed out that the increase of atmospheric ice nucleus concentration will lead to the increase of cold cloud albedo, which may lead to the cooling of the climate, indicating that the change of ice nucleus concentration may play an important role in global climate change.Therefore, scientists have always attached great importance to the observation and analysis of atmospheric ice nuclei.
Source of ice nuclei
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Ice nuclei mainly come fromAtmospheric aerosol particlesHowever, only a small part of aerosol particles can become ice nuclei, and the ratio of ice nuclei to aerosols is 10-3-10-6The nucleation efficiency varies with temperature and supersaturation state of ice.Both natural and human activities can produce ice nuclei.The sources of atmospheric ice nuclei include sand dust particles, mineral dust, industrial dust, volcanic ash streams and meteor dust from volcanic eruptions.
The observation of atmospheric ice nuclei in spring in Baicheng, Jilin Province, in the 1960s showed that the wind sand on the Loess Plateau was the main source of ice nuclei in Baicheng.Clay mineral(clay minerals) is also the most important ice core source.Kumai's research shows that about 70% of the cores of snow crystal centers in Japan, the United States, Greenland and southern Poland are ladybugs.The concentration and variation of ice nuclei in the atmosphere are the most important characteristics of atmospheric ice nuclei.
Although volcanic ash is considered as the source of ice nuclei, the concentration of ice nuclei in the atmosphere along the Pacific coast with more volcanoes is not very high.Meteor dust, as the source of ice core, its ice forming performance is not very good.[2]
Composition of ice nuclei
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In different regions, the composition of ice nuclei is different.For example, in the western United States, according to PALMS (NOAA's Particle Analysisby Laser Mass Spectrometer), the ice core is mainly composed of mineral dust/fly ash and metal particles, which account for 33% and 25% of the ice core respectively.However, the chemical composition of residual ice nuclei in cloud anvil cirrus clouds over Florida in the United States showed that the composition of salt was 30% for both small and large residual nuclei.In the small residual nucleus, the metal content is 1600, and the organic content is 1400;In the large residual nucleus, the metal content is 2900, and the crustal dust is 15%.MPACE (mixed phase Arctic cloud experiment) test in North Alaska shows that the main particle type of ice nucleus particles can be identified as 3900metallic oxide(dust), 35% carbon containing particles and 25% mixture composed of metal oxides (dust) and carbon compounds or salts (sulfate), indicating that the element composition of ice core particles has significant variability.[2]
Ice nucleation activity
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The activity of ice nuclei refers to the ability to cause the formation of ice crystals. Due to the different composition, size and shape of ice nuclei, their ability to form ice crystals is also different.The ice nuclei with higher activity can form ice crystals at not too low temperature, while the ice nuclei with lower activity can only play the role of ice nuclei at lower temperature.Therefore, to measure the concentration of atmospheric ice nuclei, it is necessary to know the temperature at that time.The research shows that under the same environmental conditions, the concentration of atmospheric ice nuclei increases with the decrease of temperature, and the observed concentration of ice nuclei increases by 10 for every 3-5 ℃ decrease.The observation of ice nuclei concentration in Shitashan, Fujian Province in the 1980s showed that the concentration of ice nuclei increased exponentially with the decrease of activation temperature, and the relationship can be used
Fit.The ice nucleation activity will increase with the increase of particle surface product, and the ice crystal growth rate will also increase with the decrease of temperature and the increase of supersaturation.[2]
Concentration of ice nuclei
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Temporal and Spatial Variation Characteristics of Ice Nucleus Concentration
The daily variation of ice core concentration is complex, and the research results vary greatly from place to place.Because the concentration of ice nuclei is related to the time, place and method of observation, the results of ice nuclei observation with different methods in different regions are less comparable.The results show that the average value of the condensation nuclei measured by diffusion cloud chamber is 0.880% of that measured by mixed cloud chamber.In addition to the poor comparability of atmospheric ice nuclei, the procedures for simulating the activation of ice nuclei in the observation are also more complex, which results in inconsistent changes in ice nuclei.
Relationship between Ice Nucleus Concentration and Meteorological Conditions
(1) Relationship with weather system
The weather system in the study area can affect the concentration of atmospheric ice nuclei.In Israel, the southern margins of the raincloud systems (mo, SMR) are located at high concentrations of ice nuclei. After a warm front becomes a cold front, the average concentration of ice nuclei decreases, and after a cold front, the concentration of ice nuclei approaches 0.2L-1At the last stage of the cyclone, the concentration of ice nuclei is 0.1L-1;The concentration of ice nuclei is relatively high in the Baicheng Hetao inverted channel in Jilin Province and the Inner Mongolia cyclone.Zhao Jianping et al. pointed out that the junction of two air masses is often a front active area, which will produce a large number of ice nuclei. When the front passes through, the concentration of ice nuclei will generally increase suddenly by 1-2 orders of magnitude (i.e., nuclear storms). The reason is that when the front passes through, it will cause strong local disturbance, and the ground particles will be brought to the near stratum to cause the enhancement mouth of ice nuclei.
(2) Relationship with weather phenomenon
Rainfall, sandstorm and fog can also affect the concentration of atmospheric ice nuclei.The research shows that after the rainfall, the concentration of ice nuclei decreases significantly. When the rainfall intensity is small, the impact of precipitation on the concentration of ice nuclei is not obvious, but the scouring effect of heavy rainfall on ice nuclei is obvious.Compared with cloudy days, there is no significant difference in ice nucleus concentration in sunny days].Compared with the weather with and without sand dust, the ice core concentration with sand dust is twice as high as that without sand dust.
(3) Relationship with meteorological factors
Meteorological factors such as wind direction, pressure and relative humidity can also affect the concentration of atmospheric ice nuclei.The relationship between the concentration of atmospheric ice nuclei and wind direction is related to the geographical location and surface environment of the study area.In Har Uilo, Israel, the concentration of ice nuclei in the southwest wind direction is three times higher than that in the northwest wind direction. The average concentration of high-temperature nuclei in the claw is basically higher than that in the calm wind when the wind is blowing in Henan County, Qinghai Province. The concentration of ice nuclei in the easterly wind is higher than that in the claw on the northern slope of the Central Tianshan Mountains, Xinjiang,The concentration of ice nuclei in the northerly wind is slightly higher than that in the southerly wind. The research in Baicheng, Jilin Province shows that the concentration of ice nuclei has little relationship with the wind direction and speed.
With the increase of relative humidity, the concentration of ice nuclei also has an increasing trend, but the concentration of ice nuclei does not change regularly with temperature.The concentration of ice nuclei in clear sky and in the sky with moderate and high clouds is higher than that in the sky with low clouds.From the perspective of the whole weather situation, in the low-pressure area, the air convergence is strong, which is conducive to the accumulation of ice nuclei.You Laiguang, Shi Aili and others pointed out that the low-pressure area often corresponds to a higher concentration of ice nuclei, but Li Shuri and other researchers showed that the concentration of ice nuclei is positively correlated with the pressure.It is worth mentioning that the research area and research method of Li Shuri and Shi Aili are the same, but the time is 2001 and 2003 respectively, but the conclusion is opposite, which shows that the relationship between pressure and ice nuclei is complex, and how the relationship needs further research.[2]
Ice Nucleus Observation Method
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Many factors such as observation time, location, cloud chamber temperature, supersaturation, ice crystal inspection method, ice crystal counting, frost formation on the inner wall of the cloud chamber, etc. of atmospheric ice nuclei will affect the observation results of atmospheric ice nuclei concentration. Therefore, the observation method of atmospheric ice nuclei is particularly important for the study of ice nuclei concentration.
In 1957, Bigg first proposed to measure the concentration of atmospheric ice nuclei using a rapid expansion cloud chamber (also known as a Bigger ice nucleus counter).The main body of the instrument is a mixed cold cloud chamber. The bottom of the cloud chamber is a supercooled sugar liquid tray. The sugar liquid tray is placed on a container full of antifreeze and can be pulled up and down. The mass ratio of sugar to water in the sugar liquid is 1:1.The outer wall of the cold chamber is an evaporation coil. The refrigerator cools down through the evaporation coil. The lower limit temperature of refrigeration is - 300 ℃.The inner wall of the cloud chamber is coated with glycerin to prevent frosting.After the extracted air sample reaches the predetermined temperature, saturated air is introduced to form supercooled fog, which can last for 2-3 minutes.The ice nuclei in the air are activated to form small ice crystals and fall on the sugar liquid disk. When they grow to a scale that can be counted visually, the sugar liquid disk is pulled up and read. After calculation, the concentration of ice nuclei can be obtained.The nucleation condition of the Bigger ice core counter is relatively close to the actual nucleation condition in the cloud, and it is easy to operate and relatively low in cost. However, it is difficult to achieve aerial sampling, and it is also not convenient to detect ice nuclei in different mechanismsPhysics of Cloud and PrecipitationIn depth research.
In 1963, Bigg et al proposed the membrane filters fol lowed by processing in a diffusion chamber (also known as membrane method).Since then, the filter membrane method has been widely used internationally to observe the concentration of ice nuclei, and it was once proposed as a standard method for determining ice nuclei.The filter membrane method is to filter the sample air through the filter membrane with an air pump, which contains ice nucleiAtmospheric aerosol particlesIt is retained on the filter membrane, and then the sampled filter membrane is sent to the cold cloud chamber for activation display processing.The filter membrane method has the advantages of separating sampling and activation display processing, continuous sampling, unlimited sampling locations, and high capture rate. However, unreasonable sampling volume will lead to errors caused by volume effect. When processing the sampling filter membrane, cooling time, infiltration method, cloud chamber height, etc. will all affect the amount of ice crystals displayed by the filter membrane.The filter membrane sampling diffusion cloud chamber method mainly measures the ice nuclei of condensation nuclei, while some smaller ice nuclei, such as frozen nodules, cannot be measured because they can penetrate the filter membrane or are embedded in the filter membrane, which will cause underestimate.
In 1973, Langer used the mixing chamber to measure ice nuclei;In 1988, Rogers introduced in detail the advantages, disadvantages and operation methods of continuous flow diffusion chamber (CFDC).The continuous flow diffusion cloud chamber (CFDC) can conduct aerial sampling, and has the advantages of aerosol particles sampling at normal atmospheric concentrations, temperature and supersaturation state controllable, and real-time observation. Therefore, internationally known field ice nuclei observation projects such as Winter Icing and Storms planThis method is applied to the oral study of INSPECT II (The Second Ice Nuclei Spetroscopy Campaign)/SUCCESS (Subsonic Aircraft: Contrail and Cloud Effects Special Study).However, this method also has some shortcomings such as poor sensitivity to some nucleation mechanisms, limited statistical samples, inability to operate automatically, and inability to sample particles>2 pm;In 1995, DeMot used a controlled expansion cloud chamber to observe ice nuclei;In 1996, Ropers et a11 '] proposed to use slow expansion cloud chamber ber to simulate realistic updrafts to observe atmospheric ice nuclei.
In 2006, Frankfurt Institute of Atmospheric Environment and Mainz Institute of Atmospheric Physics in Germany jointly developed the Fast Ice Nucleus Chamber, a continuous mixing chamber,Bundke et al introduced the design principle and operation method of FINCH in detail. For example, the design of FINCH can ensure that particles>3pm have 100% transmission efficiency at a flow rate of more than 50 min 1, and the newly developed optical detector can distinguish supercooled water droplets and ice particles, so as to obtain the quantitative concentration of ice nuclei and cloud condensation nuclei.FINCH has a sample flow rate of up to 5-10min1, and the number concentration of ice nuclei is 10IN. 1-1It only takes 1-2 min to get enough statistical data.In addition, the static vacuum water vapor diffusion cloud chamber FRIDUE (Frankfurt Ice Nuclei Deposition FreezingExperiment) was also mentioned.In 2010, Klein et al introduced the static vacuum water vapor diffusion cloud chamber FRIDUE in detail. This device can analyze ice nuclei collected on any type of substrate, even those with a diameter of 50 mm.
The concentration of ice nuclei sampled and measured in the above methods rarely takes into account the influence of aerosol size, shape and other factors. Berezinskiyt et al observed the relationship between the activity of ice nuclei and their size using the sedimentation diffusion cloud chamber method. The results show that the ice nuclei with larger size have higher activation temperature.With the development of science and technology, electron microscopy has been applied to the study of atmospheric ice nuclei.The formation process of ice and the growth process of ice crystals can be observed and imaged by using the electron microscope technology. Therefore, the electron microscope is suitable for studying the morphological characteristics of ice nuclei.At the same time, the chemical composition of atmospheric ice nuclei can also be analyzed by installing energy dispersive X-ray (EDX) on the electron microscope.[2]
influence
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Ice nuclei may have an important impact on the macro and micro structure, radiation characteristics and physical properties of clouds, but these have not been specially studied yet.When the temperature is above - 35 ℃, the formation of ice crystals mainly depends on the heteronucleation of ice nuclei. Therefore, the concentration of ice nuclei plays an important role in the number of primary ice crystals in most cold clouds, affecting the whole process of ice water conversion.Ice crystals play an important role in cold cloud precipitation, cloud radiation effect, water cycle and even stratospheric water vapor content.Therefore, ice nuclei play an important role in many physical processes, and are as important as cloud condensation nuclei.[3]
