ROLE OF ATMOSPHERIC IONS ON CONDENSATION AND CLOUD FORMATION PROCESSES

Abstract

In the moist atmosphere, under suitable conditions of temperature and pressure, thy; condensation and deposition of water vapour cause the formation of minute water droplets and ice crystals.Theseparticles form aggregates which are termed as rfogt and tcloudf depending upon their occurrence directly above the ground or at a certain altitude. The fogs and clouds are the colloidal systems consisting of water droplets, ice crystals and sometimes their mixture. The possibilities of condensation and deposition of water vapour are determined by the general thermodynamic conditions in the atmosphere. Formation or dissipation and the the physical nature of fogs and clouds are largely dependent on these thermodynamic conditions. The microprocesses inside the cloud due to special characteristics of the cloud particles play an important and decisive role. In the development of clouds through the formation of minute particles the nucleation phenomenon is the funda- mental process. Under various atmospheric conditions, the nucleation phenomenon has been divided into two groups: homogeneous, and heterogeneous. Each group is further sub, classified as (i) homomolecular, and (ii) heteromolecular, when the air-atmosphere system contains only one component or more than one component of the condensing vapours, respectively. We have examined in this thesis the role of ions in the nucleation phenomenon and in the further development of cloud. The ions are believed to enhance the nucleation. In so much as the ions in the atmosphere are very much governed by the solar activity, this may lead to a better understand-ing of sun-weather relationship. The entire work has been compiled in following six chapters: 1. An introduction to nucleation phenomenon. 2. The effect of ions on homogeneous cloud condensat-ion and ice nucleation. On the enhancement of probability of ion induced nucleation on partially wettable, water insoluble planar substrates. 4. The effect of external electric field on nucleation process of water vapour condensation and ice deposition. 5. Influence of ions on the theoretical estimate of H2SO4-H20 aerosol particles in ambient atmosphere. 6. Role of ionic condensation in solar terrestrial relationship. In Chapter 1 we briefly survey the information available concerning the nucleation phenomenon pertaining to cloud forming processes. The purpose of the survey is to give a general picture of the overall nucleation phenomenon. We review the development in nucleation phenomenon and its role -x- in atmospheric processes. Effect of ions on nucleation have also been discussed. Solar-terrestrial relationship and the effect of ions have been discussed in general. The previous work on nucleation phenomenon in micro-physical processes of clouds has been reviewed. The modifica-tion in this phenomenon with respect to the changes in the system has also been incorporated. It is found that although the classical drop model is based on a number of unrealistic assumptions, specially the macroscopic surface tension and bulk density, it can be applied to nearly all the problems of cloud condensation and proper explanation can be offered to the observed behaviour. Chapter II presents the comparative study of homogene-ous homomolecular (water molecules; nucleation between ion free and ion induced cases. The Helmholtz free energy of a thermodynamic system is the fundamental parameter. In ThoMsonls classical model, the Helmholtz free energy consists of two terms: volume free energy and surface free energy terms. In ion induced case, the Helmholtz free energy is modified due to the addition of the electrostatic energy term. It has been found that for ion induced nucleation at a given superseturation, the Helmholtz free energy of formation of critical water nucleus, its radius and number of water molecules in it, are comparatively less than those in ion free case. As a result, equilibrium concentration and the rate of nucleation are found to increase. Similar results have been obtained in the case of vapour-ice phase change. The ion induced nucleation has been found to be more effective at low supersaturation ratios (even at Sv.w< 1) and the effect decreases for larger nuclei. It has been shown that in ion induced nucleation, even the smaller nuclei (radius < 10 8 cm) are stable at S v.w< 1 while, in ion free case, they can not exist. For a given small radius, the supersaturation ratio required is much less in ion induced nucleation than that in the ion free case, but for larger nuclei (radius> 10-7 cm) there is no effect of ions. The observation that the phase change of water vapour is more fav-oured by negative ions than that for positive ions has been explained. In addition, in Chapter III the heterogeneous homo-molecular nucleating behaviour of partially wettable, water insoluble planar substrates has been discussed. The rate of nucleation and the probability of nucleation were found to be enhanced in ion induced case. The effect is more pronounced at low supersaturation ratios and low temperatures for the material with low angle of contact. The effect of angle of contact on the nucleating behaviour of the insoluble planar substrate is studied. In low level clouds e.g. cumulus, these particles play a significant role. These particles act as cloud condensation centres. The nucleability of such particles has been found to be enhanced, when an ion gets attached to it and it becomes electrically charged. However, nucleability decreases with increasing angle of contact.