Aerosols

Science and Technology

Igor Agranovski

• Publisher: Wiley-VCH
• Year: 2011
• Print ISBN: 9783527326600
• eBook ISBN: 9783527632084
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

This self-contained handbook and ready reference examines aerosol science and technology in depth, providing a detailed insight into this progressive field. As such, it covers fundamental concepts, experimental methods, and a wide variety of applications, ranging from aerosol filtration to biological aerosols, and from the synthesis of carbon nanotubes to aerosol reactors.
Written by a host of internationally renowned experts in the field, this is an essential resource for chemists and engineers in the chemical and materials disciplines across multiple industries, as well as ideal supplementary reading in graduate level courses.

1

Introduction to Aerosols

Alexey A. Lushnikov

1.1 Introduction

Aerosol science studies the properties of particles suspended in air or other gases, or even in vacuum, and the behavior of collections of such particles. A collection of aerosol particles is referred to as an aerosol, although the particles may be suspended in some other gaseous medium, not just air. The term cosmosol is used for a collection of particles suspended in vacuum. Although attempts to give a strict definition of aerosol have appeared from time to time, to date no commonly acceptable and concise definition of an aerosol exists. In my opinion, it is better not to make any attempts in this direction, especially because intuitively it is clear what an aerosol is. For example, it is clear that birds or airplanes are not aerosol particles. On the other hand, smoke from cigarettes, fumes from chimneys, dust raised by the wind, and so on, are aerosols. Hence, there are some essential features that allow us to distinguish between aerosols and other objects suspended in the gas phase. There are at least two such features: (i) aerosol particles can exist beyond the aerosol for a sufficiently long time; and (ii) an aerosol can be described in terms of the concentration of aerosol particles, or, better, the concentration field. From this point of view, it is clear why birds are not aerosols. Interestingly, clouds are also not aerosols! Of course, we can introduce the concentration of cloud droplets. But if we isolate a cloud particle, it will immediately evaporate. The cloud creates a specially designed environment inside it – the humidity and the temperature fields – the conditions in which a water droplet does not evaporate during a long time.

Aerosols are divided into two classes, namely primary aerosols and secondary aerosols, according to the mechanisms of their origination. Primary aerosol particles result, for example, from fragmentation processes or combustion, and appear in the carrier gas as already well-shaped objects. Of course, their shape can change because of a number of physico-chemical processes such as humidification, gas–particle reactions, coagulation, and so on. Secondary aerosol particles appear in the carrier gas from “nothing” as a result of gas-to-particle conversion. For example, such aerosols regularly form in the Earth's atmosphere and play a key role in a number of global processes such as the formation of clouds. They serve as the centers for heterogeneous nucleation of water vapor. No aerosols – no clouds! One can imagine how our planet would look without secondary aerosol particles.

Primary and secondary aerosols are characterized by the size, shape, and chemical content of the aerosol particles. As for the shape, one normally assumes that the particles are spheres. Of course, this assumption is an idealization necessary for simplification of the mathematical problems related to the behavior of aerosol particles. There are very many aerosols comprising irregularly shaped particles. The non-sphericity of particles creates many problems. There exist also agglomerates of particles, which in some cases reveal fractal properties. We shall return to the methods for their description later on.

There are a number of classifications of particles with respect to their size. For example, if the particles are much smaller than the molecular mean free path, they are referred to as “fine” particles. This size range stretches from 1 to 10 nm under normal conditions. But from the point of view of aerosol optics, these particles are not small if the wavelength of the incident light is comparable with their size. This is the reason why such very convenient and commonly accepted classifications cannot compete with natural classifications based on the comparison of the particle size with a characteristic size that comes up each time when one solves a concrete physical problem.

1.2 Aerosol Phenomenology

1.2.1 Basic Dimensionless Criteria

It is convenient to characterize aerosols by dimensionless criteria. The most commonly used in the area of aerosol science are listed below. Each of these criteria contains the particle size a. In what follows we consider spherical particles of radius a.

1.2.1.1 Reynolds Number

The Reynolds number Re is introduced as follows:

1.1

Here ν is the kinematic viscosity of the carrier gas and u is the particle velocity with respect to the carrier gas. Small and large Re correspond to laminar or turbulent motion of the particle, respectively.

1.2.1.2 Stokes Number

The Stokes number Stk characterizes the role of inertial effects:

1.2

Here L is the characteristic length of the flow. Th...

Table of contents

• Cover
• Related Titles
• Title Page
• Copyright Page
• List of Contributors
• List of Symbols
• Introduction
• Chapter 1: Introduction to Aerosols
• Part I: Aerosol Formation
• Part II: Aerosol Measurement and Characterization
• Part III: Aerosol Removal
• Part IV: Atmospheric and Biological Aerosols
• Index