Abstract. Here, we present an assessment of long-term trends in O₃ and odd oxygen (O₃ + NO₂) at the industrial Monterrey metropolitan area (MMA) in NE Mexico. Diurnal amplitudes in O_x (AV_d) are used as a proxy for net O₃ production, which is influenced by the NO₂ photolysis rate. No significant differences in the AV_d are observed between weekends and weekdays, although the largest AV_d values are observed at sites downwind of industrial areas. The highest O₃ mixing ratios are observed in spring, with minimum values in winter. The largest annual variations in O₃ are typically observed downwind of the MMA, with the lowest variations generally recorded in highly populated areas and close to industrial areas. A wind sector analysis of mixing ratios of O₃ precursors revealed that the dominant sources of emissions are located in the industrial regions within the MMA and surrounding area. Significant increasing trends in O₃ in spring, summer, and autumn are observed depending on site location, with trends in annual averages ranging between 0.19 and 0.33 ppb yr⁻¹. Overall, from 1993 to 2014, within the MMA, O₃ has increased at an average rate of 0.22 ppb yr⁻¹ (p < 0. 01), which is in marked contrast with the decline of 1.15 ppb yr⁻¹ (p < 0. 001) observed in the Mexico City metropolitan area (MCMA) for the same period. No clear trend is observed from 1996 to 2014 within the Guadalajara metropolitan area (GMA).