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In this work, based on the existing studies on photochemical reactions in the lower atmosphere, an analysis of the historical series of NO x , NO 2 , and O 3  concentrations measured in the period 2015–2019 by two monitoring stations located in the urban area of Turin, Italy, was elaborated. The objective was to investigate the concentration trends of the contaminants and evaluate possible simplified relationships based on the observed values. Concentration trends of these pollutants were compared in different time bands (diurnal or seasonal cycles), highlighting some differences in the dispersion of the validated data. Calculated [NO 2 ]/[NO x ] ratios were in agreement with the values observed in other urban areas worldwide. The influence of temperature on the [NO 2 ]/[NO x ] ratio was investigated. An increase of [NO 2 ]/[NO x ] concentration ratio was found with increasing temperature. Finally, a set of empirical relationships for the preliminary determination of NO 2  concentration values as a function of the NO x  was elaborated and compared with existing formulations. Polynomial functions were adapted to the average concentration values returned by the division into classes of 10 μg/m 3  of NO x . The choice of an empirical function to estimate the trend of NO 2  concentrations is potentially useful for the preliminary data analysis, especially in case of data scarcity. The scatter plots showed differences between the two monitoring stations, which may be attributable to a different urban context in which the stations are located. The dissonance between a purely residential context (Rubino station) and another characterised by the co-presence of residential buildings and industries of various kinds (Lingotto station) leads to the need to consider a greater contribution to the calculation of the concentrations emitted in an industrial/residential context due to a greater presence of industrial chimneys but also to more intense motorised vehicle transport. The analysis of the ratio between nitrogen oxides and tropospheric ozone confirmed that, as O 3  concentration increases, there is a consequent reduction of NO x  concentration, due to the chemical reactions of the photo-stationary cycle that takes place between the two species. This work highlighted that the use of an empirical formulation for the estimation of [NO x ] to [NO 2 ] conversion rate could in principle be adopted. However, the application of empirical models for the preliminary estimation of [NO x ] conversion to [NO 2 ] cannot replace advanced models and should be, in principle, restricted to a limited area and a limited range of NO x  concentrations.

Air quality and photochemical reactions: analysis of NOx and NO2 concentrations in the urban area of Turin, Italy