Simulation of Mexico City plumes during the MIRAGE-Mex field campaign using the WRF-Chem model

The quantification of tropospheric O? 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 O? and its precursors in Mexico City and the surrounding region during March 2006, and compared the model with in-situ aircraft measurements of O?, 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 O? 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 O?-NOx, O?-CO, and O?-NOz correlations generally agree with the measured values, and the analyses of these correlations suggest that photochemical O? production continues in the plume downwind of the city (aged plume), adding to the O? 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 O? production in the aging plume. Furthermore, O? production per NOx molecule consumed (O? production efficiency) is more efficient in the aged plume than in the young plume near the city. The major contributor to the high O? 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.