Impact of emissions of VOCs and NO x on trends of ground-level O 3 in Mexico during 1993–2014: Comparison of Monterrey with Mexico City and Guadalajara

In developed countries, long-term trends in O 3 have been studied extensively. However, there has been relatively little focus on economically developing countries with significant emissions of pollutant precursors. Here, the dominant role of primary emissions on regional/urban O 3 mixing ratios in Mexico is addressed. High-precision and high-frequency UV-photometic measurements of ambient O 3 have been made since 1993 at 5 sites within the Monterrey metropolitan area (MMA), in Northeast Mexico. The data sets exhibit variations on time-scales of hours, days, months and years. The O 3 diurnal cycles vary with the length of daylight, which influences photochemistry. No differences are observed in the amplitudes of the diurnal cycle (AVd) during weekdays when fossil fuel use and combustion process are higher than during weekends, although larger AVd are observed at polluted sites close to industrial areas. During weekdays, cycle troughs and peaks are typically recorded at 07:00 and 14:00 CDT, respectively, and during weekends, at 06:00 and 13:00 CDT, respectively. The O 3 seasonal cycles are driven by the temporal variation of solar radiation, meteorological conditions and changes in emissions of precursors. Maximum O 3 mixing ratios are recorded in spring with minimum values in winter. The largest amplitudes of the seasonal cycles (AVs) are typically recorded downwind of an industrial area, whereas the lowest values are recorded in a highly populated area. At all sites, AVs declined during 1993–1998, followed by persistent increases from 1998 to 2014. Wind sector analysis show that, at all sites, the highest mixing ratios are recorded from the E and SE sectors, whilst the lowest ones are recorded in air masses from the W and NW. 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 the surrounding area. At all sites, the largest annual increases in O 3 are for the E and SE sectors, 0.50 and 0.66 ppb yr −1 , respectively. Overall, during 1993 to 2014, within the MMA, O 3 has increased at an average rate of 0.22 ppb yr −1 ( p −1 ( p  < 0.001) observed in the Mexico City metropolitan area (MCMA) for the same period. No clear trend is observed during 1996 to 2014 within the Guadalajara metropolitan area (GMA).