Atmospheric Chemistry and Physics
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9
2006
10.5194/acp-6-2753-2006
http://www.atmos-chem-phys.net/6/2753/2006/
http://www.atmos-chem-phys.net/6/2753/2006/acp-6-2753-2006.html
http://www.atmos-chem-phys.net/6/2753/2006/acp-6-2753-2006.pdf
2753
2765
2006-07-07
Atmospheric oxidation in the Mexico City Metropolitan Area (MCMA) during April 2003
T. R. Shirley
W. H. Brune
brune@ems.psu.edu
X. Ren
J. Mao
R. Lesher
B. Cardenas
R. Volkamer
L. T. Molina
M. J. Molina
B. Lamb
E. Velasco
T. Jobson
M. Alexander
Pennsylvania State University, University Park, PA, USA
Autonomous Metropolitan University, Mexico City, Mexico
Massachusetts Institute of Technology, Cambridge, MA, USA
Washington State University, Pullman, WA, USA
Pacific Northwest National Laboratory, Richland, WA, USA
The Mexico City Metropolitan Area (MCMA) study in April 2003 had
measurements of many atmospheric constituents, including OH and HO<sub>2</sub>. It
provided the first opportunity to examine atmospheric oxidation in a
megacity in a developing country that has more pollution than typical U.S. and
European cities. At midday, OH typically reached 0.35 pptv (~7×10<sup>6</sup> cm<sup>−3</sup>), comparable to amounts observed in U.S. cities, but
HO<sub>2</sub> reached 40 pptv, more than observed in most U.S. cities. The OH
reactivity was also measured, even during the highly polluted morning rush
hour. MCMA's OH reactivity was 25 s<sup>−1</sup> during most of the day and 120 s<sup>−1</sup>
at morning rush hour, which was several times greater than has been
measured in any U.S. city. Median measured and modeled OH and HO<sub>2</sub>
agreed to within combined uncertainties, although for OH, the model exceeded
the measurement by more than 30% during midday. OH production and loss,
which were calculated from measurements, were in balance to within
uncertainties, although production exceeded loss during morning rush hour.
This imbalance has been observed in other cities. The HO<sub>2</sub>/OH ratio from
measurements and steady-state analyses have essentially the same dependence
on NO, except when NO was near 100 ppbv. This agreement is unlike other
urban studies, in which HO<sub>2</sub>/OH ratio decreased much less than expected
as NO increased. As a result of the active photochemistry in MCMA 2003, the
median calculated ozone production from measured HO<sub>2</sub> reached 50 ppb h<sup>−1</sup>,
a much higher rate than observed in U.S. cities.
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