Weekly patterns of Mexico City's surface concentrations of CO, NOx, PM10 and O-3 during 1986-2007

Surface pollutant concentrations in Mexico City show a distinct pattern of weekly variations similar to that observed in man), other cities of the world. Measurements of the concentrations of carbon monoxide (CO), nitrogen oxides (NOx = NO+NO2). particulate matter smaller than 10 mu m (PM10), and ozone (O-3) collected hourly over 22 years (1986-2007) at 39 urban monitoring locations were analyzed. Morning concentrations of CO, NOx, and PM10 are lower on Saturdays and even more so on Sundays, compared to workdays (Monday-Friday), while afternoon O-3 concentrations change minimally and are occasionally even higher. This weekend effect is empirical evidence that photochemical O-3 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 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/NOx and VOC/NOx ratios. Enhancements of photolysis frequencies resulting front smaller weekend aerosol burdens are not negligible. 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 primarily via formation of nitric acid and other NOx-related compounds. some of which (e.g. peroxy acyl nitrates) can contribute to the regional NOx budget. While VOC emission reductions would be most effective in reducing local O-3 production. NOx emission reduction may be more important for controlling regional oxidants.