Fundamentals of Atmospheric Photochemistry

The prime driver of the chemical system of the earth s atmosphere is photochemical reactions caused by solar radiation. The atmosphere of the earth is divided into levels called the troposphere, stratosphere, mesosphere and thermosphere from nearest the ground to farthest according to the characteristics of the temperature gradient as shown in Fig. 3.1. The cause of the temperature inversion in the stratosphere, which characterizes the earth s atmosphere, is the formation of an ozone layer by the photolysis of oxygen, one of the major components of the atmosphere. In this chapter, the spectrum of solar radiation, actinic flux, and so on, that is necessary to calculate the photolysis rate of atmospheric molecules are explained. 

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Essential background material and presentation of the fundamentals of atmospheric photochemistry can be obtained from key textbooks such as those by Seinfeld and Pandis [1], Finlayson-Pitts and Pitts [2], Yung and De-More [3], and Warneck [4],... 

Meanwhile, what is necessary to get a feeling that a system composed of various elements is fully understood and to reliably predict the future In the field of atmospheric chemistry, a phenomenon would be felt fully understood when the controlling chemistry and physics are resolved in terms of their fundamental principles. Therefore, those who learn atmospheric chemistry have to learn fundamental chemistry and physics, which constructs the basis of the discipline. Reaction chemistry, which is a branch of physical chemistry, is one such area of the fundamentals of atmospheric chemistry. In reaction chemistry, it may be implied that the reaction is understood when a chemical reaction is fully explained by spectroscopy, photochemistry, and chemical kinetics that have bases in quantum chemistry. This book specializes in atmospheric reaction chemistry, skipping the vast fertile discussion of comprehensive atmospheric chemistry. For overall atmospheric chemistry, readers can refer to the existing textbooks complementarily. 

Models of chemical reactions of trace pollutants in groundwater must be based on experimental analysis of the kinetics of possible pollutant interactions with earth materials, much the same as smog chamber studies considered atmospheric photochemistry. Fundamental research could determine the surface chemistry of soil components and processes such as adsorption and desorption, pore diffusion, and biodegradation of contaminants. Hydrodynamic pollutant transport models should be upgraded to take into account chemical reactions at surfaces. 

In this chapter, we give a brief overview of the fundamentals of spectroscopy and photochemistry needed in atmospheric chemistry for detailed treatments, see Calvert and Pitts (1966), Okabe (1978), Turro (1978), Wayne (1988), and Gilbert and Baggott (1991). Specifics for individual molecules are found in Chapter 4. Excellent treatments of atmospheric radiation are given by Liou (1980), Goody and Yung (1989), and Lenoble (1993). 

Abstract Key features of tropospheric photochemistry are highlighted including both homogeneous gas-phase and heterogeneous reactions that are important in clouds and haze aerosol. Fundamental aspects of photochemical kinetics are reviewed and then extended to the major chromophores present in the multi-phasic, tropospheric atmosphere. Tables of up-to-date absorption cross sections and quantum yields as a function of wavelength range are presented. Primary emphasis is placed on reactions occurring within the troposphere and within clouds. 

Therefore, it is not surprising that laser spectroscopy has become an indispensable tool in research fields as far apart as studying the fundamental photochemistry of anionic species on the femtosecond and picosecond time-scales, or exploring the properties of complex species and clusters important to the understanding of atmospheric processes influencing our environment. Thus, when browsing the Web one invariably encounters topical description of research activities like ... 

Reviewed herein are some of the fundamental concepts associated with chemical equilibrium, chemical thermodynamics, chemical kinetics, aqueous solutions, acid-base chemistry, oxidation-reduction reactions and photochemistry, all of which are essential to an understanding of atmospheric chemistry. The approach is primarily from the macroscopic viewpoint, which provides the tools needed by the pragmatist. A deeper understanding requires extensive treatment of ihe electronic structure of matter and chemical bonding, topics that are beyond the scope of this introductory text. This book can be used for either self-instruction, or as the basis for a short introductory class... 

Since the sun s radiation is our primary source of energy, photochemistry is one of the most fundamental and important processes for understanding life on earth. From the highest altitudes down to the earth s surface gas-phase photochemistry controls the dynamic equilibrium of the atmosphere. Moreover, elementary biological processes in plants and animals such as photosynthesis and vision are photoinitiated intra- or inter-molecular processes. For understanding biological systems it is important to study not only the isolated molecules but also the influence of an environment. In... 

Detailed description on the fundamentals of physical chemistry, such as descriptions of spectroscopy, photochemistry, reaction kinetics, homogeneous and heterogeneous reactions are given. The destruction of the ozone layer, photochemical oxidants, acid deposition, hazardous air pollutants, indoor pollution, and so on are widely covered, and their countermeasures are explained based on atmospheric chemistry. 

James N. Pitts, Jr., is a Research Chemist at the University of California, Irvine, and Professor Emeritus from the University of California, Riverside. He was Professor of Chemistry (1954-1988) and cofounder (1961) and Director of the Statewide Air Pollution Research Center (1970-1988) at the University of California, Riverside. His research has focused on the spectroscopy, kinetics, mechanisms, and photochemistry of species involved in a variety of homogeneous and heterogeneous atmospheric reactions, including those associated with the formation and fate of mutagenic and carcinogenic polycyclic aromatic compounds. He is the author or coauthor of more than 300 research publications and three books Atmospheric Chemistry Fundamentals and Experimental Techniques, Graduate School in the Sciences—Entrance, Survival and Careers, and Photochemistry. He has been coeditor of two series, Advances in Environmental Science and Technology and Advances in Photochemistry. He served on a number of panels in California, the United States, and internationally. These included several National Academy of Science panels and service as Chair of the State of California s Scientific Review Panel for Toxic Air Contaminants and as a member of the Scientific Advisory Committee on Acid Deposition. 

From the preceding discussions, it should be clear that photochemistry within all phases of the atmosphere is a major driver of chemical transformations in relatively short time scales. With increasing knowledge of the ever-widening array of chromophoric compounds emitted and produced in the atmosphere, there is definitely room for much more fundamental research into primary and secondary photochemical reactions of relevance. In particular, the role of humic-like substances in aerosol, cloud and ice phases needs to be studied. 

Oxidation processes have played a major role in the evolution of Earth s atmosphere. Observations of trace gases and free radicals in the atmosphere in 1978-2003, and of chemicals in ice cores recording the composition of past atmospheres, are providing fundamental information about these processes. Also, basic laboratory studies of chemical kinetics, while not reviewed here, have played an essential role in defining mechanisms and rates. Models have been developed for fast photochemistry and for coupled chemical and transport processes that encapsulate current laboratory and theoretical understanding and help explain some of these atmospheric observations. 

Basic Physical Chemistry for the Atmospheric Sciences covers the fundamental concepts of chemical equilibria, chemical thermodynamics, chemical kineHcs, solution chemistry, acid and base chemistry, oxidation-reduction reactions, and photochemistry. Over 160 exercises are contained within the text, including 50 numerical exercises solved in the text and 112 exercises for the reader to work on with hints and solutions provided in an appendix. 

This book is dedicated to the late Prof. Ikuzo Tanaka, who was my Ph.D. supervisor in physical chemistry at the Tokyo Institute of Technology the late Prof. James N. Pitts, Jr., who was my postdoctoral supervisor in chemistry of the atmosphere then at the University of California, Riverside and Prof. Barbara Einlay son-Pitts, at the University of California, Irvine, a friend of mine with whom 1 worked together on lab kinetics in the Pitts group at the beginning of the 1970s. Prof. Pitts and Prof. Tanaka passed away recently, in June 2014 and Eebmary 2015, respectively, just before and after the publication of the Japanese Edition. It is my wish that the fundamental photochemistry and reaction kinetics related to atmospheric chemistry will be inherited by the next generation through this book. 

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