ON ICE FORMING NUCLEI

Cloud condensation nuclei(CCN)constitute a reservoir of latent ice-forming nuclei(IFN)active by condensation-followed-by-freezing and by sorption.Two classes of aerosol particles active as IFN by sorption werefound.The first produces IFN concentrations which are dependent on temperature only.The IFN concen-tration consecutively increases with lowering of temperature and does not respond to the increasing partialwater vapor pressure.The second produces IFN concentrations which are dependent on partial water vaporpressure only,The IFN concentration consecutively decreases with decreasing of the partial water vapor pres-sure that takes place with the lowering temperature.Sulfates(mostly ammonium sulfate)constitute an integral part of IFN active by condensation-followed-by-freezing and by sorption.Backward isentropic air parcel trajectories linked polluted air rich in sulfates andclean air with high and low concentrations of IFN active by condensation-followed-by-freezing at-19.7℃ andS_w=0.1%.The experiments were performed in a dynamic filter processing chamber with aerosol particles collectedon membrane filters.     

MEASUREMENTS OF ICE-FORMING RATES PRODUCED BY SUPERSONIC FLOW AND INVESTIGATION OF MECHANISMS OF ICE CRYSTAL GENERATION

The ice forming rates p_i produced by supersonic flow have been measured in supercooled fog suspended in a coldchamber at temperatures of 0 to-12℃.It has been shown that the ice-forming rates are 10~(11)to 10~(12)crystals per gramof air at Mach numbers of 1.1 to 1.84 and total pressures below 6 atm,and that the ice-forming rates slowly increasewith increasing Mach numbers and total pressures and with decreasing ambient temperatures.These results aretheoretically consistent with the rules for expansion in supersonic flow.In review of mechanisms of ice crystal generation,authors propose that the homogeneous nucleation-freezing(condensation-freezing)of water vapor in supersonic flow be the dominant mechanism.In our opinion,supersonic flow,for artificial precipitation,could be expected to provide an economical,non-polluting and convenient technology,andsuitable for the long-distance operation.The data obtained from our experiments indicate that it is feasible to develop anew technique for weather modification.     

MEASUREMENTS OF ICE-FORMING RATES PRODUCED BY SUPERSONIC FLOW AND INVESTIGATION OF MECHANISMS OF ICE CRYSTAL GENERATION*

The ice forming rates p_i produced by supersonic flow have been measured in supercooled fog suspended in a cold chamber at temperatures of 0 to -12℃.It has been shown that the ice-forming rates are 10¹¹ to 10¹² crystals per gram of air at Mach numbers of 1.1 to 1.84 and total pressures below 6 atm,and that the ice-forming rates slowly increase with increasing Mach numbers and total pressures and with decreasing ambient temperatures.These results are theoretically consistent with the rules for expansion in supersonic flow.In review of mechanisms of ice crystal generation,authors propose that the homogeneous nucleation-free zing(condensation-freezing)of water vapor in supersonic flow be the dominant mechanism.In our opinion,supersonic flow,for artificial precipitation,could be expected to provide an economical,non-polluting and convenient technology,and suitable for the long-distance operation.The data obtained from our experiments indicate that it is feasible to develop a new technique for weather modification.     

气溶胶影响混合相对流云降水的数值模拟研究

利用一种新的异质冰相核化参数化方案,研究了当气溶胶同时作为云凝结核和冰核时,在不同高度输送对混合相对流云和降水的影响。结果发现,对于本文研究的理想混合相对流云,气溶胶在边界层的输送导致液滴数浓度明显增加,有效半径减小,霰粒的生长受到抑制,引起霰粒质量浓度降低;而气溶胶在对流层中层4～6km输送时,导致冰晶和霰粒数浓度明显增加。由于较多的冰晶引起更加快速的贝吉隆过程,使霰粒的质量浓度增加;气溶胶在对流层中层2～4km高度输送时冰相形成作用相对较弱,并引起霰粒的数浓度略微增加,由于霰粒的有效半径减小导致其质量浓度下降。气溶胶在不同高度的输送都导致液态和固态降水率降低,随着背景气溶胶数浓度的增加,气溶胶在0～2km、2～4km以及4～6km的输送分别导致累积降水量减少28％～64％、4％～44％和3％～46％,并且对降水的抑制效应及所在高度不同引起的降水差异随着背景气溶胶数浓度的增加而减小。     

新疆天山山区大气冰核浓度的测量及分析

利用2001年6、7月份在新疆天山山区两个取样点取得的冰核资料，计算了山区中的冰核浓度，研究了冰核平均浓度日变化、日际变化的规律，并分析了天山山区冰核浓度与一些气象因子的关系及可能原因。得出以下结论：天山山区的大气冰核主要来源于地面土壤，比较缺乏；冰核浓度易受气象因子、地表状况的影响，日变化和日际变化明显．     

黄河上游云凝结核观测研究

分析了2001、2001年8月在黄河上游的玛曲－河南县利用美国Mee公司的130型云凝结核计数器观测的资料，讨论了凝结核的谱型，浓度随饱和度、天气状况及一些气象要素变化的关系，通过与其它地方观测结果的比较，可以看出在地表植被较好的黄河上游牧区，人类活动、污染较少，其自然凝结核少，浓度与青岛的测值相近，与海洋过饱和核谱型接近。     

贺兰山地区大气冰核浓度的测量及初步分析

根据 1 994年在银川和阿拉善左旗观测得到的冰核资料 ,给出了两地不同天气条件下的大气冰核浓度 ,分析并讨论了冰核浓度与风、降水、天空状况等的关系 ,给出了飞机在空中测量冰核的结果。     

Atmospheric nuclei in the remote free-troposphere

During May-June of 1990 an extensive flight series to survey aerosol present in the upper-troposphere was undertaken aboard the NASA DC-8 as part of the CLObal Backscatter Experiment (GLOBE). About 50,000 km were characterized between 8–12 km altitude and between 70°N and 58°S. Aerosol with diameters greater than 3nm were counted and sized with a combination of condensation nuclei counters and optical particle counters. Aerosol number and mass concentrations were separately identified with regard to both refractory and volatile components. Regions of the free-troposphere with the lowest mass concentrations were generally found to have the highest number concentrations and appeared to be effective regions for new particle production. These new particle concentrations appear inversely related to available aerosol surface area and their volatility suggests a sulfuric acid composition. The long lifetime of these new particles aloft can result in their growth to sizes effective as CN and CCN that can be mixed throughout the troposphere.     

Relationship of heterogeneous nucleation and condensational growth on aerosol nanoparticles

Heterogeneous nucleation and condensation of dibutylphthalate, octadecane, octadecanol, and octadecanoic acid vapors at various pressures on insoluble AgCl and Ag nanoparticles in a turbulent mixing condensation nuclei counter (TMCNC) have been studied theoretically. A method to interpret the particle size distributions measured with a DMA and estimate the parameters for nucleation on single particles is proposed. Based on this semi-empirical method, the Gibbs free energy is calculated and a rate of heterogeneous nucleation on single particles is estimated directly from the experimental “condensation spectra” of inactive and active CN using the DMA data. In some cases, the dependence of the Gibbs nucleation energy on the vapor supersaturation had two maximums and one minimum, instead of one maximum as described by Gibbs' classical thermodynamics of phase transitions. This phenomenon, called “double barrier nucleation” (DBN) is caused by the surface heterogeneity of nano-CN; this is first experimental verification of DBN that had been previously predicted theoretically. Two types of heterogeneity may be present: topographic or energetic. Focusing on energetic heterogeneity, a theoretical model of DBN for spherical geometry is developed. The surface heterogeneity for insoluble nano-sized CN is shown to be critical to explaining the unusual transformation of a monomodal size distribution of inactive CN into a bimodal distribution of activated CN when coagulation is excluded. Future studies will be directed toward more data for further refining the theory and developing a model that simultaneously accounts for both types of surface heterogeneity of nano-CN.     

黄河上游地区大气冰核浓度的观测研究

2000年8月在甘肃省玛曲县，利用Bigg型混合云室进行了四个温度下的大气冰核浓度的测量，分析了冰核浓度日变化的特点，以及随天气状况、气压等要素变化的关系。通过与北京、兰州、西宁等一些地区的观测结果的比较，可以看出在人为活动较少的玛曲，自然冰核和我国北方其他地区相比，接近于西宁、西安、北京（1963）和大连等地的浓度值，而比甸子山及北京90年代的观测值要少。     

沙尘气溶胶对云和降水影响的模拟研究

采用二维分档云模式,对比背景大气气溶胶分布,讨论了扬沙和沙尘暴天气条件下矿物气溶胶对云微物理结构、光学特性以及降水形成的影响.结果表明:扬沙和沙尘暴天气增加大气中大核和巨核的浓度,促进云中水汽的活化,使降水提前出现,暖云和冷云降水量均大幅增加,但可忽略巨核增加对云光学厚度和反照率的作用;当矿物沙尘粒子同时作为有效的云凝结核和冰核参与云的发展时,冰核浓度增加使水成物有效半径减小,抑制了暖云和冷云降水,云内存留的大量冰晶增强云的光学厚度和反照率.     

黄河上游河南县地面大气冰核观测分析

用Bigg型混合云室对2001年8月黄河上游青海省河南县的地面大气冰核浓度进行了观测,分析发现(1)青海省河南县冰核浓度低于北京20世纪90年代的测值,但高于大连、西安和兰州(20世纪60年代)等地的测值,同时也高于黄河上游的玛曲(2000年)、西宁(1989年)两地的测值,高温核尤其偏高.(2)降水时大气冰核浓度减少,降水的冲刷可能是其主要原因.(3)当地冰核浓度和气压成正相关.(4)大气冰核浓度受风向的影响,无风时测值较小,偏东风时测值较大,高温核浓度增加明显.     

巨核对暖云降水影响的模拟研究

利用耦合了新的暖云参数化方案的中尺度模式(MM5),研究了暖云降水中巨核的作用.在这个暖云方案里,先假定一个三模态的气溶胶正态对数分布,然后考虑对流、扩散、云滴和雨滴的核化(非核化)过程,再由气溶胶质量的预报量显式地计算出气溶胶的数浓度.选择了华北地区2005年6月25～26日的一次弱冷锋过程,并以此研究了巨核对云和降...     

Origin of Aitken particles in remote regions of the Southern Hemisphere

Seasonal variations in total particle concentrations at five remote Southern Hemisphere sites in Samoa, Tasmania and Antarctica are compared. The dominant particle production mechanism is deduced to be photochemical, with concentrations dictated by the availability of radiation in the eight months centred on the winter solstice. There is little variation with latitude in the mean summer concentrations. The middle troposphere is shown to act as an important reservoir of photochemically-produced particles in Tasmania and probably in the Antarctic. It is suggested that primary-particle production is mainly through the oxidation of dimethyl sulphide and that these particles then act as centres of condensation for sulphuric acid formed by the oxidation of sulphur dioxide.     

Isolating and identifying atmospheric ice-nucleating aerosols: a new technique

Laboratory studies examined two key aspects of the performance of a continuous-flow diffusion chamber (CFD) instrument that detects ice nuclei (IN) concentrations in air samples: separating IN from non-IN, and collecting IN aerosols to determine chemical composition. In the first study, submicron AgI IN particles were mixed in a sample stream with submicron non-IN salt particles, and the sample stream was processed in the CFD at −19°C and 23% supersaturation with respect to ice. Examination of the residual particles from crystals nucleated in the CFD confirmed that only AgI particles served as IN in the mixed stream. The second study applied this technique to separate and analyze IN and non-IN particles in a natural air sample. Energy-dispersive X-ray analyses (EDS) of the elemental composition of selected particles from the IN and non-IN fractions in ambient air showed chemical differences: Si and Ca were present in both, but S, Fe and K were also detected in the non-IN fraction.     

纳米碘化银在人工影响天气的应用研究 II: 室内实验

运用爆炸法,使用“37型”降雨实弹分别在室内20 L和1200 L云室对研制的人工影响天气纳米碘化银催化剂的成冰性能,包括成核率、成冰阈温、核化速率等,进行检测和研究.在相同实验条件下,在“37型”降雨实弹上将纳米碘化银催化剂与目前常用碘化银催化剂进行对比试验.结果表明,纳米碘化银催化剂成冰性能明显优于常规碘化银催化剂,其成核率比常规碘化银催化剂提高1～2个数量级;成冰阈温值增加;核化速率明显加快.     

大气冰核研究进展

大气冰核在冷云降水物理过程、人工影响天气、云和气溶胶的气候效应乃至全球变化等对大气和气候方面的研究中具有重要的意义。本文主要从大气冰核的观测方法、来源、化学组成、成冰活性、浓度时空分布及其与气象条件如天气系统、天气现象、气象因子等各方面综述了国内外近几十年大气冰核的研究进展,并在探讨国内外大气冰核研究现状的基础上,提出...     

用滤膜法观测大气冰核的静力扩散云室

介绍了经过改进的用滤膜法观测冰核的静力扩散云室及其取样装置。叙述了其各组成部分的原理、结构、达到的性能指标，分析了各系统的工作对冰核活化条件的影响。给出了对相对稳定的人工ＡｇＩ冰核源作不同体积的取样和不同温度下的处理结果，同时也给出了用该设备对北京市海淀区冬季冰核浓度的日变化所做的初步观测结果。     

Impacts of aerosol chemical composition on microphysics and precipitation in deep convection

Aerosols affect precipitation by modifying cloud properties such as cloud droplet number concentration (CDNC). Aerosol effects on CDNC depend on aerosol properties such as number concentration, size spectrum, and chemical composition. This study focuses on the effects of aerosol chemical composition on CDNC and, thereby, precipitation in a mesoscale cloud ensemble (MCE) driven by deep convective clouds. The MCE was observed during the 1997 department of energy's Atmospheric Radiation Measurement (ARM) summer experiment. Double-moment microphysics with explicit nucleation parameterization, able to take into account those three properties of aerosols, is used to investigate the effects of aerosol chemical composition on CDNC and precipitation. The effects of aerosol chemical compositions are investigated for both soluble and insoluble substances in aerosol particles. The effects of soluble substances are examined by varying mass fractions of two representative soluble components of aerosols in the continental air mass: sulfate and organics. The increase in organics with decreasing sulfate lowers critical supersaturation (S_c) and leads to higher CDNC. Higher CDNC results in smaller autoconversion of cloud liquid to rain. This provides more abundant cloud liquid as a source of evaporative cooling, leading to more intense downdrafts, low-level convergence, and updrafts. The resultant stronger updrafts produce more condensation and thus precipitation, as compared to the case of 100% sulfate aerosols. The conventional assumption of sulfate aerosol as a surrogate for the whole aerosol mass can be inapplicable for the case with the strong sources of organics. The less precipitation is simulated when an insoluble substance replaces organics as compared to when it replaces sulfate. When the effects of organics on the surface tension of droplet and solution term in the Köhler curve are deactivated by the insoluble substance, S_c is raised more than when the effects of sulfate on the solution term are deactivated by the insoluble substance. This leads to lower CDNC and, thus, larger autoconversion of cloud liquid to rain, providing less abundant cloud liquid as a source of evaporative cooling. The resultant less evaporative cooling produces less intense downdrafts, weaker low-level convergence, updrafts, condensation and, thereby, less precipitation in the case where organics is replaced by the insoluble substance than in the case where sulfate is replaced by the insoluble substance. The variation of precipitation caused by the change in the mass fraction between the soluble and insoluble substances is larger than that caused by the change in the mass fraction between the soluble substances.     

Coastal CCN measurements at Mace Head with enhanced concentrations in strong winds

Surface measurements of cloud condensation nuclei (CCN) number concentration (cm⁻³) are presented for unmodified marine air and for polluted air at Mace Head, for the years 1994 and 1995. The CCN number concentration active at 0.5% supersaturation is found to be approximately log-normal for marine and polluted air at the site. Values of geometric mean, median and arithmetic mean of CCN number concentration (cm⁻³) for marine air are in the range 124–135, 140–150 and 130–157 for the two years of data. Analysis of CCN number concentration for high wind speed, U, up to 20 m s⁻¹ show enhanced CCN production for U in excess of about 10–12 m s⁻¹. Approximately 7% increase in CCN per 1 m s⁻¹ increase in wind speed is found, up to 17 m s⁻¹. A relationship of the form log₁₀CCN=a+bU is obtained for the periods March 1994 and January, February 1995 for marine air yielding values a of 1.70; 1.90 and b of 0.035 for both periods.