Aerosols - an introduction

A gas with particles suspended therein is called an aerosol. Even the clean est air contains at least several millions of particles per cubic meter, th us we are always surrounded by an aerosol. But still the particles contribu te only a few ppb to the mass of the gas. Aerosol particles have sizes betw een a few nanometers and tens of micrometers. Outside this range the smalle r particles coagulate too rapidly and the larger particles sediment too fas t. Despite this small mass concentration the aerosol particles have conside rable effects: The majority of the atmospheric optical effects are caused b y particles, clouds can only be formed with particles acting as condensatio n nuclei. Particles are deposited in the lung and can transport toxic chemi cal compounds or radioactivity into the body. Soil material can be transpor ted over long distances to other regions via the aerosol phase. Aerosol par ticles are either emitted directly such as soil erosion, sea spray, fly ash , soot from diesel engines or iron oxide fumes from the steel industry, or are produced in the atmosphere by oxidation and reactions of chemical compo unds, such as sulfur dioxide and ammonia. The vegetation emits organic vapo rs, which eventually also form particles in the atmosphere. Aerosol particl es can be characterized by their size and chemical composition. Several mod els for size distributions and various nomenclatures exist. Particles betwe en 50 nm and 1 mu m have the longest lifetime of about a week. They are eit her emitted directly or can be formed by agglomeration from smaller particl es. During the life of these particles they can undergo transformations by incorporation in clouds, condensation of vapors or coagulation with smaller particles. Numerous instruments exist to measure and classify the particle s. The instruments use various principles to characterize the particles suc h as inertial deposition, condensation, light scattering or electrical mobi lity. Since most of the particles are non-spherical, equivalent diameters a re used to characterize the properties of the particles. (C) 2000 Elsevier Science Ltd. All rights reserved.