Charging Processes of Atmospheric Particulates, Numerical Modeling and Field Observations

大氣顆粒的荷電機制、數值模擬及外場觀測研究

Student thesis: Doctoral Thesis

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Award date12 Dec 2019

Abstract

Our atmosphere consists not only of gases but also of particulate matter. The particulate matter can be either liquid droplets or solid particles, and both forms are called aerosol particles or atmospheric particles. This study focuses on atmospheric aerosols in the form of haze particles in the surface layer and cloud particles in the troposphere. The particles that are entrained in the airflow can become charged due to particle contact/collision or ion diffusion/adsorption. The particle charging would affect the particle movement behavior and particle size distribution. Severe haze weather and strong thunderstorms are two typical meteorological processes in nature. A deep understanding of their triggering mechanisms is of great significance for solving environmental problems. Both physical models are gas–particle two-phase flow. Severe haze events frequently occur in a static and stable weather; the atmospheric moisture and the interactions between aerosol particles might be the main influence factors for particle charging and particle agglomeration. Thunderstorms occur in a severe convective weather; the non-thermal equilibrium inside cloud particles caused by strong updraft-downdraft convection and the resulted thermoelectric effect might be the fundamental causes for thundercloud electrification.

The aims of this project are to study the forces acting on atmospheric particles and the dynamic evolution of particles. Through the field observations, the charging characteristics of aerosol particles on different days during different seasons are obtained, and the relationship between the charge amount of aerosol particles and the dynamic environmental factors (e.g., temperature, humidity) could be analyzed. Based on humidity parameter and charge measurement result, an extended discrete element method (DEM) is adopted to simulate the collision, agglomeration and fragmentation of aerosol particles; the effects of atmospheric humidity, particle charge on the interparticle interactions and particle agglomeration during haze weather could be analyzed. Besides, a three-stage model of thunderstorm development and the formation mechanism of electric field structure are proposed, which are based on the thermoelectric effect, the heat transfer characteristics of different forms of hydrometeors (cloud particles) with the ambient atmosphere during thunderstorm weather, and the non-thermal equilibrium electrification mechanism of different cloud particles. The main research contents of the project are as follows:

1.Investigation of the changing characteristics of the electric charges on the aerosol particles based on field experiments. The experimental campaigns were conducted on different days, during different seasons and in different places. The experimental findings from the particle charging measurements showed that most atmospheric particles would carry a net negative or positive charge. A strong relationship between the mass concentration of PM2.5 and the charge amount on the particles implied that the haze formation could be partly attributed to the variation of the particle charging state, which might be related to the meteorological conditions. Further experimental results showed diurnal and seasonal variations of ionic charges on atmospheric particles. The average electric charges on particles was higher during the winter than during the summer. The average electric charges were lower during the sand-dust day than during the severe haze day. Regional transport of total suspended particulates (TSPs) and enhanced particle contact/collision due to high wind velocity can also impact the charge characteristics of atmospheric particles. Through the comparison of the air quality index (AQI, PM2.5, PM10, SO2, NO2, O3), meteorological conditions and charging state of particles recorded in Xi’an City and Weinan City, the atmospheric humidity, Sulphur dioxide (SO2) and Ozone (O3) concentrations were identified to be associated with the strength of the charging property of the aerosol particles.

2.Study of the agglomeration of haze particles by an extended discrete element method (DEM). Through analyzing the scale of interactions of two particles of different sizes and different separation distances, I have illustrated the dominant interaction in a fine aerosol particle system, which are: the van der Waals (vdW) force, liquid bridge force, Brownian force and electrostatic force. I have successfully introduced interactive forces including van der Waal force, liquid bridge force and electrostatic force, through the Johnson–Kendall–Roberts (JKR) theory, into the DEM model. The Brownian force was also introduced as an additional force to the motion equation. The use of an extended soft-sphere DEM model would provide a more accurate description of the collision, agglomeration and fragmentation behaviors of particles in atmosphere. An experiment on the water-absorbing capacity of particles was conducted to obtain the correlation between the relative humidity (RH) and water content of particles. Based on the humidity ratio and particle charge obtained from experiments, a numerical simulation for the fine-particle evolution was conducted on OpenFOAM platform. The simulation results of the evolution of fine particles has illustrated that the agglomeration rate of particles would increase with a rise in the atmospheric humidity due to the increased liquid bridge forces and electrostatic interactions.

3.Development of a dynamic non-thermal equilibrium charging mechanism of cloud particles to elucidate the formation of thundercloud electrifications. I analyzed the process of ice crystal-graupel collision charging; from charge migration inside hydrometeors to charge separation between two hydrometeors. The influence factors (ambient temperature, humidity, particle size, particle shape and relative velocity), that lead to cloud particles entering the non-thermal equilibrium state and produce a temperature gradient inside particles, would affect the magnitude and sign of charge transfer. By comparing the response time and thermal relaxation time of evolving ice particles into a new environment within a strong convection of thunderstorms, almost all cloud particles were found in non-thermal equilibrium states in thunderstorms. On the basis of careful investigations of theoretical calculations and observational data, the consequential growth of the electrical potential difference between the surface and the inside of hydrometeors, i.e. the non-thermal equilibrium ionization of cloud particles, was introduced as a key process that enhances particle charging. The non-thermal equilibrium theory originating from the thermoelectric effect would provide a better explanation of the dynamics of thundercloud electrification.

4.Introduction of a three-stage evolution model for thundercloud electrification. Based on a multiphase flow and the non-thermal equilibrium theory, combined with the atmospheric convection and humidity distribution characteristics, the development of thundercloud can be divided into three stages: formation of a mixed-phase region, electric charging of particles and formation of an electric field. According to the dynamic three-stage evolution of thundercloud electrification, the updraft-downdraft transition of particles was elucidated. These layers are not separation zones of charged particles but are preferred locations for charge generation. The model was successfully applied to explain the tripole charge structure, four charged regions and more complex charge structure in thunderstorms. The modelling study has well clarified the relationship between the charge distribution of deep convective clouds and heavy precipitation such as hail.

    Research areas

  • Atmospheric particles, Haze, Thundercloud, Charge, Relative humidity, Non-thermal equilibrium