Abstract
Abstract. 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 (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 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 O3 concentrations change minimally and are occasionally even higher. This weekend effect is empirical evidence that photochemical 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 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 from 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 O3 production, NOx emission reduction may be more important for controlling regional oxidants.
Highlights
The atmosphere of México City has received considerable scientific attention in recent years, foremost because of concerns about the potential health effects of air pollutants on its ∼20 million inhabitants, and because it may be to some extent representative of current and future conditions in other megacities undergoing rapid economic development
For carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter smaller than 10 μm (PM10) the maximum values occur during the morning rush hours, followed by a decrease in the late morning due to lower emissions and the rapid growth of the planetary boundary layer (PBL) as reAtmos
México City experiences a weekend effect in its air quality similar to that found in many cities around the world: concentrations of O3 precursors NOx, CO, and volatile organic compounds (VOCs) are significantly lower on Saturday and even more so on Sunday compared to workdays, the concentrations of O3 change only minimally, and in some cases are even larger
Summary
The atmosphere of México City has received considerable scientific attention in recent years, foremost because of concerns about the potential health effects of air pollutants on its ∼20 million inhabitants, and because it may be to some extent representative of current and future conditions in other megacities undergoing rapid economic development. An air quality monitoring network was established in 1986, and has helped document long-term reductions of some pollutants following the institution of various emission-reduction programs (INE, 1998). One of the issues most relevant to the design of emission reduction policies for urban areas is whether the formation of O3 is more sensitive to emissions of nitrogen oxides (NOx ) or volatile organic compounds (VOCs). Finlayson-Pitts and Pitts, 1986) that O3 formation depends non-linearly on these emissions, and is maximal when VOC/NOx molar ratios are in the range of 5–15, the exact value depending on various conditions.
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