Production of Hydroxyl Radicals in the Troposphere

We begin this chapter with hydroxyl radical generation by 03 photolysis and then proceed first to the chemistry of NO as we develop the overall organic/NO chemistry of the troposphere. 

The key to understanding tropospheric chemistry begins with the hydroxyl (OH) radical. Because the OH radical is unreactive toward 02, once produced, it survives to react with virtually all atmospheric trace species. The most abundant oxidants in the atmosphere are 02 and 03, but these molecules have large bond energies and are generally unreactive, 

We learned in Section 3.6 and Chapter 5 that 03 photolysis (at wavelengths 319 nm) to produce both ground-state (O) and excited singlet (O( D)) oxygen atoms is important in both the stratosphere and troposphere  

The ground-state O atom combines rapidly with 02 to reform 03  

Since the ground-state O atom then just reacts with 02 by reaction 2 to replenish 03, this path, consisting of reactions lb, 3, and 2, is just another null cycle. Occasionally, however, O D) collides with a water molecule and produces two OH radicals  

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