Tropospheric Reservoir Molecules

In the CO oxidation cycle, hydrogen peroxide, H202, is a reservoir molecule for HO,  

In the CH4 oxidation cycle, methyl hydroperoxide, CH3OOH, is also a HO., reservoir  

Nitrous acid. HONO, which is formed by a heterogeneous reaction involving N02 and H20 (Calvert et al. 1994), is a reservoir for both HO, and NO,. HONO dissociates by photolysis to regenerate OH and NO  

Formed overnight, HONO photodissociates upon sunrise to inject a pulse of OH into the early-morning atmosphere. 

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Chlorine nitrate CIONO2 and bromine nitrate B1ONO2 are important reservoir molecules formed by the chain termination reactions, CIO + NO2 and BrO + NO2, in the CIO and BrOx cycles in the stratosphere, respectively. Iodine nitrate IONO2 plays a similar role in the in the iodine chemistry in the troposphere. 

The reaction of OH radical with nitric acid (HONO2, HNO3) in the stratosphere is important as it reproduces active nitrogen from the reservoir molecule HONO2 in the NOx cycle. Although in the troposphere, water-soluble nitric acid is mainly removed by wet deposition into cloud and fog, and dry deposition on earth s surface, the OH reaction as well as photolysis are also important as removal processes and as active nitrogen regenerating process in the upper troposphere where clouds are not abundant. 

Irreversible destruction of the source molecules to initially form the free radicals, is a slow process compared with any other exchange reactions. Conversion from the source is matched at steady state by the downward mixing of the soluable reservoir term for wash out below the troposphere. 

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