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Surface pollutant concentrations in M´ exico 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 ox- ides (NOx=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 ana- lyzed. Morning concentrations of CO, NOx, and PM10 are lower on Saturdays and even more so on Sundays, compared to workdays (Monday-Friday), while afternoon O3 concen- trations change minimally and are occasionally even higher. This weekend effect is empirical evidence that photochem- ical O3 production is NOx-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 represen- tative. The VOC-limitation has increased in the past decade, due to decreases in the concentrations of CO (and presum- ably VOCs) and consequent decreases in the CO/NOx and VOC/NOx ratios. Enhancements of photolysis frequencies resulting from smaller weekend aerosol burdens are not neg- ligible, but fall short of being an alternate explanation for the observed weekend effect. The strength of the weekend effect indicates that local radical termination occurs primar- ily via formation of nitric acid and other NOx-related com- pounds, some of which (e.g. peroxy acyl nitrates) can con- tribute to the regional NOx budget. While VOC emission reductions would be most effective in reducing local O3 pro- duction, NOx emission reduction may be more important for controlling regional oxidants.

atmospheric chemistry and physics

Weekly patterns of México City's surface concentrations of CO, NO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt;, PM&amp;lt;sub&amp;gt;10&amp;lt;/sub&amp;gt; and O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; during 1986–2007