Abstract
Abstract. The quantification of tropospheric O3 production in the downwind of the Mexico City plume is a major objective of the MIRAGE-Mex field campaign. We used a regional chemistry-transport model (WRF-Chem) to predict the distribution of O3 and its precursors in Mexico City and the surrounding region during March 2006, and compared the model with in-situ aircraft measurements of O3, CO, VOCs, NOx, and NOy concentrations. The comparison shows that the model is capable of capturing the timing and location of the measured city plumes, and the calculated variability along the flights is generally consistent with the measured results, showing a rapid increase in O3 and its precursors when city plumes are detected. However, there are some notable differences between the calculated and measured values, suggesting that, during transport from the surface of the city to the outflow plume, ozone mixing ratios are underestimated by about 0–25% during different flights. The calculated O3-NOx, O3-CO, and O3-NOz correlations generally agree with the measured values, and the analyses of these correlations suggest that photochemical O3 production continues in the plume downwind of the city (aged plume), adding to the O3 already produced in the city and exported with the plume. The model is also used to quantify the contributions to OH reactivity from various compounds in the aged plume. This analysis suggests that oxygenated organics (OVOCs) have the highest OH reactivity and play important roles for the O3 production in the aging plume. Furthermore, O3 production per NOx molecule consumed (O3 production efficiency) is more efficient in the aged plume than in the young plume near the city. The major contributor to the high O3 production efficiency in the aged plume is the reaction RO2+NO. By contrast, the reaction of HO2+NO is rather uniformly distributed in the plume.
Highlights
The export of air pollutants from urban to regional and global environments is of major concern because of wide-ranging potential consequences for human health and for cultivated and natural ecosystems, visibility degradation, weather modification, radiative forcing, and changes in tropospheric oxidation capacity
The results suggest that the calculated O3 is consistently underestimated in both the city plumes and in the background compared to the measured O3 concentrations
Alkane species generally react with OH more slowly than do aromatics, but their concentrations are normally higher than aromatics in citiesAs a result, both volatile organic compounds (VOCs) classes could be important for ozone productions in the city plumes
Summary
The export of air pollutants from urban to regional and global environments is of major concern because of wide-ranging potential consequences for human health and for cultivated and natural ecosystems, visibility degradation, weather modification, radiative forcing, and changes in tropospheric oxidation (self-cleaning) capacity. Thielmann et al (2002) suggested that in the rural area of Milan, Italy, ozone production per NOx consumed is less efficient when the advected air masses originate from Milan. Their study suggests that the influence of emissions from Mexico City could lead to the enhancements in surface NOx, CO, and O3 concentrations to distances of about 200 km. These studies provide useful discussions on the ozone chemistry in the vicinity of Mexico City. The main focus is to investigate the ozone evolution in the downwind plume of Mexico City and the ozone chemical production in air masses with different chemical ages.
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