A null cycle neither produces nor destroys anything overall. If the reactions making up the null cycle are all fast (producing short residence times for the products and reactants), then each reactant/product will have small equilibrium concentrations (as it is in this case). A slow reaction in the scheme, however, serves as a bottleneck in the whole process and allows higher concentrations of the reactants of the slow reaction (this corresponds to a long residence time for the reactants, if the reaction is taken to be a "sink" process). Note that the NO + O2 going to NO2 step is slow, and is NOT part of the null cycle, which consists of relatively fast reactions.

Null cycle, illustrated.

Every time a nitrogen dioxide molecule (NO2) photodissociates, the resulting oxygen atom (O) quickly combines with molecular oxygen (O2) to form ozone (O3). So, it's as if an NO2 photodissociation event results in the production of ozone. The ozone molecule does not last long, as it is used to oxidize the nitric oxide (NO) back to NO2, starting the cycle over again.

Since the other reactions are fast, the concentrations of NO2 and ozone will not be high. So how does ozone reach high concentrations in L.A. smog, as we observe?