Surface charges on aerosol particles – Accelerating particle growth rate and atmospheric pollution

Abstract

Many Chinese mega-cities frequently experienced severe and persistent haze pollution in recent years. Especially, the so-called ‘explosive growth’ of PM2.5 concentration has often been observed at the beginning of a severe pollution event in winter. As shown in Figure 1(a), the explosive growth of PM2.5 concentration could be increased from below 50 mg/m up to 250 mg/m in only 5–10 h. The explosive growth of PM2.5 concentration in cities can have a strong impact on people’s life and health. In contrast, in Figure 1(b), during 1–5 November 2013 in winter, the PM2.5 mass concentration grew accumulatively in Xi’an, increasing the concentration from 23 to 137 mg/m. Meanwhile, the speed of coagulation of new particles in these PM2.5 concentrations during the explosive growth event is evidently far more than the speed of normal accumulation of these PM2.5 concentrations as shown in Figure 1(b) and with long duration. Although many scholars have put forward ideas to analyse the pollution of high PM2.5 concentration from various directions, the details and theory of particulate explosive growth process are still not clearly defined and elucidated. Secondary reactions may have contributed to the increment growth of particle number concentration, but the high speed of chemical reaction is confusing the possible theory. Actually, at the beginning of a severe haze, the process of explosive growth of PM2.5 concentration could be undergoing a series of physical–chemical processes, in which the charging particles have a significant influence. The charges and ions on particle surfaces could affect the collision and coagulation behaviour of particles, and also the subsequent chemical reactions occurring on the particle porous surfaces. Aerosols in any form are usually electrically charged, no matter in solid, air or liquid (including cloud aerosol, fog aerosol and smog aerosol), either in natural or artificial form. There are many natural or artificial factors that could charge the aerosol particles, such as cosmic rays in space, radiation from radioactive material in air and on earth, atmospheric lightning, electromagnetic radiation, high temperature discharge and static electricity caused by particles collision. On the other hand, because fine particles are porous media with rough surfaces, changeable shapes and uneven energy distribution on surfaces, the distribution of ions and electrical charges on particle surfaces must be uneven, and the energy would gather at the location with small curvature. Thus, the high potential and electrical charges could easily aggregate at the cusp. When a fine particle is separated from a larger particle, a static charge transfer would arise giving two charged particles. Particles with different diameters would become charged after their collision and then separate from each other. When aerosol particles are carrying electrical charges, particle interactions and surface chemical reactions could be affected by these surface charges, thus changing some physical characteristics of particles, such as cohesion, adhesion and its stability in the atmosphere. The electron dynamics of these charged particles could also have an adverse effect on human’s health. The recent research suggested that during the process of explosive growth, the reverse growth of volume concentration of aerosol may be influenced by fluctuation of air moisture under stable atmospheric conditions, which means that in a very short period of time, under the condition of stable or declining number concentration of aerosol particles, adsorbing moisture could increase the diameter of aerosols. In fact, water molecules adsorbed on the surface of particles in atmospheric environment could play a very complicated role in the process of particles collision. If thin water film is present on the surfaces of suspended aerosol particles, a ‘water bridge’ could be formed at the contact interface when particles are moving and colliding in the air, by which hydrogen ions would be