In this work we elucidate the origin of zinc oxide (ZnO) excitonic photoluminescence modifications induced by adsorption of nitrogen dioxide (NO2). In particular, we investigated the effects of NO2 exposure on excitons radiative recombination efficiency and lifetime in nano-structured vs. micro-structured ZnO films synthesized by wet chemical routes. By combining electrical and optical analyses and focusing on the role played by sample topology, we show that the presence of the oxidant species unambiguously gives rise to a decrease of photoluminescence efficiency and to an increment of the energy barrier at ZnO surfaces, while at the same time it leaves unaffected the probability of exciton radiative recombination. Comparisons between the results obtained for different kind of topologies allow us to infer about the basic fundamental mechanism underlying the effect, namely inhibition of free-excitons formation.