Abstract
Here, we present an assessment of long-term trends in the O3 weekend effect (WE) occurrences and spread within the Mexico City (MCMA), Guadalajara (GMA), and Monterrey (MMA) metropolitan areas, which are the three largest metropolitan areas (MAs) of Mexico and concentrate around 33% of the total population in the country. Daytime averages and peak differences in O3 concentrations from weekdays to weekends were used as a proxy of WE occurrence. All MAs exhibited the occurrence of WE in all years at least in one monitoring site. Substantial differences in O3 daytime averages and peaks from weekdays to weekends have decreased over time in all MAs, and since 1998 and 2013 for the MCMA and GMA, respectively, higher O3 levels during weekends are typical during most of the year. The largest variations in the O3 WE were observed at downwind and urban core sites of the MCMA and GMA. Significant increasing trends (p < 0.05) in the O3 WE magnitude were observed for Sundays at all sites within the MCMA, with trends in annual averages ranging between 0.33 and 1.29 ppb O3 yr−1. Within the GMA, for Sundays, fewer sites exhibited increasing trends in the WE occurrence and at lower growth rates (0.32 and 0.48 ppb yr−1, p < 0.1) than within the MCMA, while within the MMA no apparent trends were observed in marked contrast with the MCMA and GMA. Our findings suggest that policies implemented have been successful in controlling weekday ground-level O3 within the MCMA and GMA, but further actions must be introduced to control the increases in the O3 WE magnitude and spread.
Highlights
Tropospheric O3 is a secondary air pollutant formed by photo-oxidation of volatile organic compounds (VOCs), carbon monoxide (CO), and methane (CH4) in the presence of nitrogen oxides (NOX = NO + NO2)
The O3 weekend effect” (WE) is explained by non-linearity of the system of O3 production, which may lead to increases in production rates of O3 with increasing NOX, whereas for other conditions the production rates may decrease with increases in NOX and/or VOCs [1]
The later WE increase at the Guadalajara metropolitan area (GMA) and Mexico City metropolitan area (MCMA) is opposite to the apparent decrease observed since 2013 for the Monterrey metropolitan area (MMA), which suggests significant variations across metropolitan areas (MAs) of Mexico
Summary
For the Santiago metropolitan area in Chile, Seguel et al reported a significant increase since 2004 in diurnal average concentrations in ground-level O3 during weekends than during weekdays [16] Such increase was associated with an increase in the VOC/NOx ambient ratio due to lower NOX emissions from motor vehicles during weekends. In the Mexico City metropolitan area (MCMA), non-significant changes during 2001–2007 in ground-level O3 from weekdays to Saturday and Sunday were reported by Stephens et al with only occasional higher O3 concentrations during weekends despite relatively lower emissions of precursors [17]. IInn oorrddeerr ttoo bbeetttteerr aasssseessss tteemmppoorraall aanndd ssppaattiiaall cchhaannggeess iinn tthhee WWEE ffrroomm yyeeaarr ttoo yyeeaarr,, aa ttiimmee wwiinnddooww ffoorr tthhee ppeeaakk ooccccuurrrreennccee iinn OO33 wwaass ddeeffiinneedd uussiinngg aann eexxpplloorraattoorryy aannaallyyssiiss ooffthtehecoclloelclteecdteddatdaasteatss. eFtisn.aFlliyn,awlleyd, ewsceridbeeshcoriwbedehcorewasdinegcrtereansidnsgintrOen dpsrecinurOso3r pemreicsusirosnosr hemaviessrieosnusltheadvienrleoswuletredOi3nlelovwelesrdOur3ilnegvewlsedekudrianygs wbuetekleddaytos hbiugthleerdletovehlisgohnerwleevekelesnodns.weekends
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