Research On The Mechanism Of Charged Particles Promoting Droplet Growth

There are some problems of fresh water resources in China,such as low per capita,uneven spatial and temporal distribution,and prominent contradiction between supply and demand.Resource-based and development-oriented water shortage has seriously restricted the construction of ecological civilization and the sustainable development of economic society in China.Artificial rainfall technology is the most direct method to improve the utilization rate of atmospheric water resources,which can effectively alleviate the shortage of water resources.The traditional rainfall technology has strict requirements on natural conditions,and the required meteorological window such as temperature and humidity is narrow,resulting in its low efficiency.As a new type of artificial rainfall technology,charged particle catalysis technology has the advantages of wide application window,low cost,and no chemical working medium,and has attracted the attention of meteorological community in recent years.However,the mechanism of this technique and the microphysical process by which charged particles affect droplet growth are still unclear.Therefore,the effects of charged particles on droplet growth,including charging,nucleation and growth,are researched theoretically and experimentally,and the growth of charged droplets in supersaturated/subsaturated environment is observed and analyzed in this paper.Firstly,the diffusion charging and the field charging of aerosol particles are researched.The electric field distribution in the ionization region or drift region during corona discharge are investigated.Based on the traditional charging mechanism,an extended droplet charging model considering the collision effect between droplets is proposed.The parameter is used to predict the change of the charge rate of droplet in the weak electric field.By comparing the Rayleigh limit of charge,the maximum charge based on the corona discharge,and the saturated charge caused by electric field,a method for the maximum charge of droplets in atmospheric environment is proposed.Secondly,the nucleation process of droplets has been researched.Based on the modified Kelvin-equation and K?hler theory,the nucleation equation of charged droplets containing soluble particles in an electric field is proposed.The effects of electric charge cosidering electrostatic potential energy and ion-dipole interaction,soluble particles and electric field on nucleation of droplets are discussed.If charged particle is combined with soluble particle,the requirements of nucleation on environment will be reduced.And the nucleation can be achieved stably in subsaturated environment by combining electric field with charged or soluble particle.Based on the statistical law between droplet population and ambient supersaturation,equivalent surface tension can estimate the change of droplet population.Then,the droplet growth process has been researched.Based on water vapor diffusion and latent heat of condensation,the condensation growth equation of charged droplet containing soluble particles under the influence of electric field is derived.Since the condensation rate decreases rapidly with the increase of radius,nucleated charged droplets grow to micron size and then grow further by collisions.Based on the classical flux model,the collision growth equation of charged droplet was derived by considering the influences of Brownian diffusion,thermophoresis,diffusiophoeesis,electrostatic forces（Coulomb force,interaction between point charges and the dipole,interaction between induced imaginary charges）and droplets collide.Combined with the condensation growth equation and collision growth equation,the growth equation of droplet is proposed,which reveals that charged particles will greatly inhibit the evaporation of charged droplet,and even promote its condensation in the supersaturated/subsaturated environment.Finally,the condensation process of vapor in supersaturated and subsaturated environments was observed in the experiments.And the phenomena is analyzed by nucleation and growth theory.It is proved that charged particles can reverse the droplet from evaporation to condensation.Even if the ambient humidity is 80%,the concentration of droplets with a radius of 15.5μm can still be measured from 0 L^-1 to 10⁴ L^-1 in the cloud chamber.Based on the experimental phenomenon and theoretical analysis of charged droplet containing soluble salt in subsaturated environment,the effects of charged particle concentration,soluble salt concentration and soluble salt type on droplet growth is revealed.The difference of growth rate between charged droplets of common soluble salts with the same content and different types is small.