Indirect photochemical process

An important result from aerosol chamber studies was the discovery of the indirect photochemical process. Thus, Bricard et al. (1968) found that intense aerosol particle production can be observed in the chamber in the dark if ambient filtered air is sampled from a sunlit atmosphere. It is speculated that in the atmosphere some gaseous substance is excited by sunlight and is not collected by the filter used to obtain air which is free of aerosol particles. In the chamber these photochemically excited molecules initiate secondary thermal reactions leading to the formation of some supersaturated vapour (e.g. H,S04) which subsequently condenses (see also Subsection 3.6.3). 

Meszaros, E., 1973 Evidence of the role of indirect photochemical processes in the formation of atmospheric sulphate particulate. J. Aerosol Sci. 4 429-434. 

In the following paragraphs we will summarize the recent developments in understanding direct and indirect photochemical processes in the light of this... 

The (indirect) photodegradation of chemicals (i.e., reactions with reactive species formed by photochemical processes) has been recognized as the major transformation pathway for chemicals in the troposphere. The electrophilic addition of tropospheric radicals constitutes the principal... 

Much of the confusion in textbooks and to a certain extent in the photochemical literature follows, either directly or indirectly, from the ambiguity in the use of the two key terms "primary photochemical process" and "quantum yield." We shall first review the various meanings which have been attributed to these quantities in the literature. When an example is needed, reference will be made to Figure 1, which shows schematically the most important routes by which a photoexcited molecule may lose its energy. This scheme is, however, relatively simple, and we shall have occasion later to refer to more complicated schemes. 

The field of aquatic photochemistry encompasses a wide diversity of areas within environmental science. Natural waters receiving solar radiation are active photochemical reactors. Within these reactors, primary and secondary processes are occurring. Heterogeneous reactions are associated with both living and nonliving particulate matter. Naturally occurring humic substances are relatively efficient initiators of photochemical reactions. Many xenobiotic chemicals in natural waters undergo either direct or indirect photochemical transformations. 

The (indirect) photodegradation - reaction with reactive species formed by photochemical processes - has been recognized as the major transformation pathway for chemicals in the troposphere. The electrophilic addition of tropospheric radicals constitutes the principal degradation pathway. The relevant reactive species are hydroxyl radicals (OH ) and ozone (O3) during day time and N03 radicals at night. The hydroxyl radicals result from reactions of oxygen atoms with water vapour, photolysis of HNO2 and reactions of HO2 (a... 

The decarboxylation of carboxylic acids is a photochemical process of significance in several areas of study thus, photodecarboxylation reactions are used in synthetic chemistry to generate radical intermediates or in agriculture to produce pesticides. In the case of pharmaceuticals, this process is responsible for a decreased stability of some compounds and seems to be directly or indirectly involved in adverse photosensitizing side effects. 

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