Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 8 17 2008 10.5194/acp-8-5313-2008 http://www.atmos-chem-phys.net/8/5313/2008/ http://www.atmos-chem-phys.net/8/5313/2008/acp-8-5313-2008.html http://www.atmos-chem-phys.net/8/5313/2008/acp-8-5313-2008.pdf 5313 5325 2008-09-05 Weekly patterns of México City's surface concentrations of CO, NO_x, PM₁₀ and O₃ during 1986–2007 S. Stephens S. Madronich sasha@ucar.edu F. Wu J. B. Olson R. Ramos A. Retama R. Muñoz Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, USA TDA Research, Golden, Colorado, USA William J. Clinton Foundation, Clinton Climate Initiative, México D. F., México, USA Dirección de Monitoreo Atmosférico, Gobierno del Distrito Federal, México D. F., México, USA Surface pollutant concentrations in México City show a distinct pattern of weekly variations similar to that observed in many other cities of the world. Measurements of the concentrations of carbon monoxide (CO), nitrogen oxides (NO_x=NO+NO₂), particulate matter smaller than 10 μm (PM₁₀), and ozone (O₃) collected hourly over 22 years (1986–2007) at 39 urban monitoring locations were analyzed. Morning concentrations of CO, NO_x, and PM₁₀ are lower on Saturdays and even more so on Sundays, compared to workdays (Monday–Friday), while afternoon O₃ concentrations change minimally and are occasionally even higher. This weekend effect is empirical evidence that photochemical O₃ production is NO_x-inhibited, and to the extent that emissions of CO are proportional to those of reactive volatile organic compounds (VOCs), it is VOC-limited, at least in the urban areas for which the monitoring stations are representative. The VOC-limitation has increased in the past decade, due to decreases in the concentrations of CO (and presumably VOCs) and consequent decreases in the CO/NO_x and VOC/NO_x ratios. 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