A thorough knowledge of the behaviour of certain chemical compounds responsible for atmospheric pollution, and a correct evaluation of the state of air quality, represent the key points on which to base specific interventions that aim to reduce and/or eliminate possible repercussions on the ecosystem and on human health. This thesis work, based on the historical studies carried out on the formation and reaction of NOx in the atmosphere and on the empirical reports produced in previous studies, analysed the historical series of NOx, NO2, NO and O3 concentrations detected in the period 2015 - 2019 by two monitoring stations of the Regional Air Quality Monitoring System of the Piedmont Region, both located in the city of Turin.