Agents of Chemical Attack in the Troposphere

Atmospheric (gas-phase) compounds undergo chemical transformation by three possible routes (1) photolysis (2) chemical attack by free radical and other species and (3) dissolution in droplets followed by aqueous-phase chemical reaction. In this section we briefly discuss photolysis and then introduce the most important gas-phase species responsible for chemical attack in the troposphere. We will discuss aqueous-phase processes in Section X. 

Atmospheric species may undergo transformation by photolysis. For this to occur the compound must absorb light in the so-called actinic portion of the spectrum, from 290 nm to 1000 nm, and, after absorption of a photon, undergo chemical change. For most compounds, breakage of chemical bonds requires energies in excess of about 40 kcal mole-1 (Benson, 1976) photolytic wavelengths less than about 700 nm correspond to this limit. 

Chemical attack on atmospheric compounds is frequently an oxidation step. Principal oxidizing species in the troposphere include the hydroxyl radical (OH), ozone (03), and the nitrate radical (N03). 

Hydroxyl radicals are formed in the troposphere via the following processes (Ehhalt et al., 1991)  

Most of the excited 0( D) atoms are physically quenched by N2 and Oz back to the ground state 0(3P), 

---