Observed trends in ground-level O3 in Monterrey, Mexico, during 1993-2014: comparison with Mexico City and Guadalajara

Here, we present an assessment of long-term trends in O-3 and odd oxygen (O-3 + 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 O-3 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 O-3 mixing ratios are observed in spring, with minimum values in winter. The largest annual variations in O-3 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 O-3 precursors revealed that the dominant sources of emissions are located in the industrial regions within the MMA and surrounding area. Significant increasing trends in O-3 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, O-3 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).