Weekly patterns of México City's surface concentrations of CO, NO<sub>x</sub>, PM<sub>10</sub> and O<sub>3</sub> during 1986–2007
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<sub>x</sub>=NO+NO<sub>2</sub>), particulate...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2008-09-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/8/5313/2008/acp-8-5313-2008.pdf |
| Summary: | 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<sub>x</sub>=NO+NO<sub>2</sub>), particulate matter smaller than 10 μm (PM<sub>10</sub>), and ozone (O<sub>3</sub>) collected hourly over 22 years (1986–2007) at 39 urban monitoring locations were analyzed. Morning concentrations of CO, NO<sub>x</sub>, and PM<sub>10</sub> are lower on Saturdays and even more so on Sundays, compared to workdays (Monday–Friday), while afternoon O<sub>3</sub> concentrations change minimally and are occasionally even higher. This weekend effect is empirical evidence that photochemical O<sub>3</sub> production is NO<sub>x</sub>-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<sub>x</sub> and VOC/NO<sub>x</sub> 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 NO<sub>x</sub>-related compounds, some of which (e.g. peroxy acyl nitrates) can contribute to the regional NO<sub>x</sub> budget. While VOC emission reductions would be most effective in reducing local O<sub>3</sub> production, NO<sub>x</sub> emission reduction may be more important for controlling regional oxidants. |
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| ISSN: | 1680-7316 1680-7324 |