Modeling study of air pollution in the Mexico City area

The purpose of this research is to study the air pollution problem in the Mexico City (MC) area. The WRF/Chem model is used to simulate the chemical oxidants and dust aerosols, including meteorological conditions. The observations during the 2006 MILAGRO (Megacity Initiative: Local and Global Research Observations) experiments are used to evaluate the model results and to understand the characterizations of chemical species (CO, NO, N02, and O3): their magnitudes, diurnal variations, and horizontal distributions.

The main results are: (1) the chemical O3 formation is found to be under VOC-limited regime in the MC's center and south zone during March 12-15, 2006, and weakly sensitive to VOCs in the north, especially in the northeast zone. It is also found that the O3 production is possibly sensitive to both NOx and VOCs in the downwind suburb area, and NOx-limited in the further rural area. The OH reactivity with VOCs is calculated from simulations. It shows that alkenes dominate the OH reactivity at the city emission source site; and at the downwind site, oxygenated hydrocarbons make the largest contribution, and CO plays a relatively more important role in the OH reactivity. The most important VOCs in terms of OH reactivity is acetaldehyde in the all sites. (2) the diurnal variations of surface emissions play an important role in controlling the O3 concentrations in the MC area, and also in the downwind suburb area. The daytime O3 concentrations are sensitive to emissions of NOx and VOCs in the morning, increase in daytime O3 concentrations as well as the afternoon O3 maximum are mainly attributable to the increase of VOC emissions and the decrease of NOx emissions in the morning. The sensitivity experiments also suggest that without reduction of total emissions, the daytime concentrations of CO, NOx, and O3 and their maxima can be significantly reduced by changing the timing of surface emissions. (3) the large area of dry barren lands to the northeast of MC is found to be an major source of dust particles for the entire MC area. With dust module included in the WRF/Chem model, the simulated aerosols were significantly improved in terms of their mass concentrations and diurnal variations. Dust accounts for about 50% of total PM2.5 mass, and 70% of total PM10 mass during the period of March 16-20, 2006. The simulation results also show that dust aerosols have important effects on actinic fluxes and hence photochemistry, especially on hydroxyl radical (OH) and ozone (O3) concentrations, in the MC area and the surrounding region. The dust aerosols lead to a significant reduction of the surface concentrations of OH and O3. The maximum reduction of OH is about 0.3 pptv in the dust source region, and the maximum reduction of O3 is about 10 ppbv near Mexico City.