Solar radiation at visible and ultraviolet wavelengths drives the chemistry of the atmosphere, by photo-dissociating relatively stable molecules into highly reactive radical fragments. Knowledge of photo-dissociation rate coefficients (J values) is crucial to understanding the behavior of global stratospheric and tropospheric ozone, the atmospheric lifetimes of gases such as carbon monoxide, methane,...
This Chapter provides an introduction to the spectroscopic techniques used to study transient species produced during the photolysis of environmentally relevant chemicals. After a brief discussion of the photochemical processes which an excited state can undergo, the techniques of conventional (microsecond) flash photolysis and laser flash photolysis (nanosecond to femtosecond) are described, together...
This chapter provides an overview of the atmospheric photochemical degradation of important anthropogenic molecules. Starting with an introduction to atmospheric chemistry, the concept of atmospheric lifetime is presented and the degradation mechanisms of alkanes, aromatics, nitrogen oxides, SO2, chlorofluorocarbons, halons, hydrofluorocarbons, and hydrochlorofluorocarbons are discussed. Areas of...
The kinetic characterizations of direct photolysis and indirect photoreactions in natural waters are described, including the reactions of multiple species in rapid reversible equilibrium. The photochemical sources, fates, and environmental effects of various photo-reactants are discussed. It is concluded that in the oceans, hydroxyl radical (*OH) and the aquated electron (eaq -) will ultimately...
Singlet oxygen (1O2) is produced in the environment almost exclusively by photochemical pathways. Although it can be detected in many environmental compartments, its potential roles in chemical reactions under environmental conditions are not completely understood. For example, in the atmosphere, it is formed at lower elevations (<40km) by photolysis of ozone and by energy transfer to ground-state...
The photochemistry of numerous polycyclic aromatic compounds (PAHs) and polychlorobiphenyls (PCBs) has been studied in water and on solids in environmental and laboratory settings. Photoproducts have been identified in many instances. In the presence of O2, the photochemistry of PAHs is dominated by oxidation, with either singlet O2 or Superoxide serving as oxidant. The factors which dictate whether...
This review describes the reaction mechanisms of three typical classes of compounds which play a role in the photochemical processes occurring in natural waters, namely nitrate ions, FeIII salts and humic substances.
This article reviews recent research on the mechanism of the photolysis of phenolic compounds. Relevant photophysical studies are also included. The main emphasis is on photo-degradation of compounds of actual environmental relevance or structurally related to such compounds, such as halogen-, nitro-, or cyano-substituted phenols in aqueous solutions, and on the photosensitizing action of phenolic...
The role of the light on the transformation of pesticides in water depends on many parameters. Transformation are categorized as being direct or indirect photodegradation. We investigated the influence of the spectroscopic properties of pesticides and their effects on the nature of the photochemical reactions with the oxygen species, adjuvants of formulation, humic acids, and water. Chemical reactions,...
This paper reviews the rates, pattern and possible mechanisms of light-induced degradation of lipidic components in detritus derived from phytoplankton. After a brief discussion of the main photooxidation reactions which can occur in the euphotic zone of the oceans, emphasis is given to the photodegradation of lipids during the senescence of phytoplankton. Production and quenching of excited states...
During the past 15 years research and development in the area of photocatalysis have been tremendous. The present review describes the basic principles of photocatalysis, focusing in particular on important mechanistic and kinetic aspects. The properties and requirements for efficient photocatalysts are discussed in detail. A few representative model compounds have been selected to illustrate the...