Chapter 2 - On the Nature of Light and Its Interaction with Atmospheric Particles

Abstract

This chapter focuses on the physio-optical properties of particles and gases. It starts with basic concepts such as color, electromagnetic energy properties such as wavelength, frequency, and speed, and the idea of radiometric and photometric units. A qualitative discussion of the interaction of light with particles is presented along with the ideas of particle diffraction, refraction, and absorption of electromagnetic energy. Rayleigh and Mie scattering are introduced, and the consequential effects of particle size on the efficiency with which particles scatter light and the effects of size on directional scattering are discussed. These effects are highlighted with real-world photos. The next sections present a mathematical treatment of the relationship between mass size distributions and their physical characteristics with particle scattering and absorption. Many commonly used physio-optical variables such as extinction, scattering, and absorption coefficients, volume scattering function, mass extinction efficiency, and the relative humidity enhancement factor f(RH) are introduced. The concept of partial mass extinction/scattering efficiency is defined and explored as it relates to the change expected from removing (or adding) a chemical species to (or from) the atmosphere. Real-world examples are given. A summary of hundreds of measurements and calculations of mass extinction coefficients is presented. The chapter closes with a discussion of two simple methods for estimating atmospheric extinction from filter-based, species-specific, aerosol mass concentrations.