Sunlight-initiated photochemical formation

The major source of H202 in the epilimnion of lakes is a sunlight-initiated photochemical process. The mechanism for the photochemical formation of... 

As described below, the absorption of sunlight by CDOM also initiates the formation of a variety of photochemical intermediates and products. The photochemical reactions producing these species ultimately lead to the degradation of the CDOM and the loss, or bleaching, of its absorption. This process can act as a feedback to alter the aquatic light field. 

The chlorination of methyl chloroformate in sunlight was first reported by Hentschel, but without a detailed description of either the procedure or the results. The first step of the present procedure for the preparation of trichloromethyl chloroformate utilizes an ultraviolet light source and affords a simple and reproducible way to obtain this reagent. Although trichloromethyl chloroformate may also be synthesized by photochemical chlorination of methyl formate,the volatility of methyl formate causes losses during the reaction and increases the hazard of forming an explosive mixture of its vapor and chlorine gas. The preparation of trichloromethyl chloroformate by chlorination of methyl chloroformate in the dark with diacetyl peroxide as initiator has been reported. However, the procedure consists of several steps, and the overall yield is rather low. 

Photochemical air pollution of the earth s troposphere and stratosplu i. involves a series of complex reactions initiated by sunlight. Thanks to ili< large body of information accumulated in recent years, the main proas., leading to the formation of photochemical smog are well understood although the details of some reactions are still unknown. 

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). 

The emissions from smoldering combustion contain all the necessary compounds needed for the formation of photochemical smog. We have mentioned in Chapter 3 that four main components are necessary for the initiation of this tropospheric phenomenon sunlight, temperature above 18 °C, hydrocarbons and nitrogen oxides, and all of them are present during forest fires in Southeast Asia. 

We have seen that formation of photochemical smog depends on the diurnal variation of important chemical species. Sunlight is also among the most important parameters since all chemical reactions are initiated by photons. 

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