Atmospheric Chemistry and Physics Discussions
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10.5194/acpd-8-8357-2008
http://www.atmos-chem-phys-discuss.net/8/8357/2008/
http://www.atmos-chem-phys-discuss.net/8/8357/2008/acpd-8-8357-2008.html
http://www.atmos-chem-phys-discuss.net/8/8357/2008/acpd-8-8357-2008.pdf
8357
8384
2008-05-06
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
S. Stephens
S. Madronich
F. Wu
J. 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
Dirección de Monitoreo Atmosférico, Gobierno del Distrito Federal, México D. F., México
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 32 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 correlated with 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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