Modeling of Mexico City aerosol air pollution

Mexico City has one of the world's most serious air pollution problems. There are 20 million people, 3 million private vehicles, and 300,000 industries in the basin enclosing the urban area. Most of the cars lack catalytic converters and light industrial emissions have been difficult to regulate. Since the surrounding mountains hinder ventilation, concentrations of ozone (O₃), nitrogen dioxide (NO₂), carbon monoxide (CO), sulfur dioxide (SO₂), hydrocarbons (HC), total suspended particulate (TSP), and particulate matter less than 10 microns (PM₁₀) regularly exceed local standards. Visibility and human health are both strongly impacted. Tightened controls have resulted in some improvements in Mexico City air quality. For example, as a result of sulfur reductions visibility measured in terms of visual range has risen from a low of around 5 km in 1991 to almost 20 km. However, given that the valley of Mexico is 70 km wide the surrounding mountains are rarely visible.

Since several key pollutants (e.g. ozone and secondary aerosol) are not emitted directly but are produced within the atmosphere, the interactions resulting in their formation can be complicated. Advanced computer models, such as the CIT (Carnegie/California Institute of Technology) three dimensional airshed model, calculate the evolution of pollutants over the entire domain by accurately describing the processes responsible for chemical transformation, transport, and deposition. These models offer a foundation for testing the relative effectiveness of alternative control strategies in advance of the adoption of regulations. However, the simulations require an enormous amount of data. Although a fairly extensive monitoring network exists across the Valley of Mexico for criteria pollutants, intensive measurement campaigns generate more detailed informationon meteorological parameters, pollutant concentrations, and boundary and initial conditions. A 1991 collaboration between scientists at U.S. institutions and the Mexico Petroleum Institute (IMP) led to the collection of raw data for modeling the major gas phase oxidants. A follow-on to the gas phase study was conducted in the spring of 1997 and focused on particulate pollution issues. Measurements included size resolved ionic/salt composition of the inorganic aerosol.

In order to examine the secondary inorganic aerosol concentrations which exist in Mexico City, two detailed thermodynamic inorganic aerosol phase mechanisms, which have been successfully used in other research studies, have been incorporated into the CIT model to follow particle chemistry and transport. The first mechanism, SEQUILIB, is computationally intensive but accurate. The second mechanism, ISOPORRIA, decreases the computational time (and accuracy) for the module through making approximations. Both mechanisms successfully represent the major steps in the daily evolution of the particulate system: 1) overnight buildup of precursors such as ammonia and SO_x, 2) photochemical conversion of SO₂ to SO₃ after sunrise, 3) titration of sulfate and nitrate by ammonia shortly after sunrise, and 4) dilution as the boundary layer rises. This paper will discuss the major findings of model evaluation of secondary inorganic aerosols in Mexico City, the amount of pollutants which are being ventilated into the regional area, and the major differences and similarities of the two aerosol mechanisms on model predictions.