Study Of Aerosol Charging And Its Applications In Submicron Aerosol Measurement

Aerosol charging exists in the ambient atmosphere and is also widely applied to the air purification,aerosol measurement,etc.Aerosol chargers are necessary to the particle size distribution measurement and size classification of submicron particles.Particles collide with ions in the aerosol charger,and different fractions of particles carry zero,one,two or more elementary charges,which is called charge distribution.Charge distribution is the key factor that influences the accuracy of submicron particle measurement.The bipolar charger,where positive and negative ions coexist,is often applied to measure electrical mobility size distributions of particles.The variation of charge distribution in the bipolar charger will lead to the inaccurate measurement of particle size distribution.Particles carrying more than one charge can have the same electrical mobility as smaller particles with one charge,which will interfere with the size classification based on electrical mobility.This multiple charge effect is more severe in the unipolar charger,where only positive or negative ions exist.Therefore,it is necessary to investigate how to calculate charge distributions accurately and regulate charge distributions effectively in applications of aerosol charging.To improve the accuracy of particle size distribution measurement and data inversion,this study proposed a new method to calculate bipolar charge distributions and a new method to do particle number size distribution(PNSD)inversion,based on which a new approach to measure PNSDs was developed.According to bipolar charging theory,both positively and negatively charged particles should be considered to obtain accurate PNSDs,rather than only positively or negatively charged particles are measured conventionally.The ion mobility ratio could be calculated based on the measured raw PNSDs of positively and negatively charged particles,and then a modified formula could be applied to calculate charge distributions.In typical conditions,it was found that the summation of positive and negative charge distributions was relatively stable even with the variation of ion properties for particles in the size range of 5-500 nm,which could be utilized to invert the particle size distribution more accurately.The new method to calculate charge distributions and the new method to do PNSD inversion both could avoid the influence of uncertain charge distributions on PNSD inversion results,and they could control the relative error in the concentration integrated from PNSDs within 4%.Based on these new methods,a bipolar electrical mobility sizer was developed and applied to the PNSD measurement of atmospheric particles.The change law of singly charged fractions was investigated by a unipolar charging model,and methods to tune the singly charged fraction were proposed,which helped to develop two new unipolar chargers effectively.The singly charged fraction was determined by the ion concentration and residence time of particles in the charger.The ion concentration could be tuned to change the singly charged fraction.Aimed at the miniature electrical mobility sizer,a plate and tube unipolar charger operated at a low corona current was designed,evaluated and compared.Low corona current can guarantee low ion concentration to improve singly charged fractions.The plate charger has a comparable extrinsic charging efficiency as the tube charger but has a smaller multiple charge effect,which makes it more suitable for the accurate size classification.Based on the plate charger,an electrical ultrafine particle sizer(e UPS)was developed,which could measure the size distribution of particles effectively.For the tube charger,a curved electrical aerosol analyzer(curved EAA)was designed and evaluated to extend the measured size range of the sizer.Aimed at the requirement of size-dependent chemical composition measurement,a soft X-ray unipolar charger was designed to substitute the conventional charger with radioactive materials as the ion sources.At the flow rate of 5 lpm,the current charger could achieve a similar extrinsic charging efficiency to that in previous studies.