Although there are abundant sources of NO and NO2 in the troposphere, few molecules make it to the stratosphere because these molecules are highly reactive, so they are stabilized in tropospheric reactions. But, N2O is very unreactive, so it does eventually make it to the stratospheric level. In the upper atmosphere, it can absorb ultraviolet radiation that splits nitric oxide into dinitrogen and an excited oxygen atom. A small portion of these excited oxygen atoms react with the N2O to produce NO, which can then serve to catalyze the ozone destruction reaction (O + O3 -->2O2).
N2O + hv = N2 + *O [g]
N2O + *O = 2NO [h]
NO + O3 = NO2 + O2 [i]
NO2 + O = NO + O2 [j]
Besides participating in the ozone destruction reaction, NO2 can also combine with OH· and ClO· to yield inactive forms that are unable to complete the catalytic loop of ozone destruction chemistry. The NO2 react to form chlorine nitrate and nitric acid, which serve as reservoir molecules. These reservoir molecules keep the reactive OH· and ClO· out of the ozone destruction reactions, thereby decreasing the amount of ozone depletion.
ClO· +NO2 = ClONO2 [k]
OH· +NO2 = HNO3 [l]
These reservoir molecules of ClONO2 and HNO3 keep the reactive catalytic molecules bound, which decreases ozone destruction because they are taken out of the cycle where they can possible be rained out of the stratosphere altogether. These radicals, OH· and ClO·, can be photolyzed by UV radiation.
HNO3 + hv = OH· +NO2 [m]
ClONO2 + hv = ClO· +NO2 [n]
Nitric oxide plays a double role in stratospheric ozone chemistry because it catalyzes the ozone destruction reaction, equations [d],[i],[j], as well as binding with highly reactive radicals to prevent the completion of ozone destructive cycles, equations [k] and [l]. Because of this duality, it had been difficult to ascertain the overall effect of nitric oxide on the stratospheric ozone concentration.
Because of the uncertainty in the nitric oxide chemistry, there must be another species destroying ozone. Perhaps, it is nature's "vacuum cleaner", the hydroxyl radical?