1.  enigenda@atmosfera.unam.mx
MCCM-WEPS: Coupling of Meteorological, Air Quality and Erosion Models for Mexico City.
1M.I. Emmanuel Díaz, 2Dr. John Tatarko, 1Dr. Arón Jazcilevich, 1Dr. Agustín García, 1Dr. Ernesto Caetano
1Centro de Ciencias de la Atmósfera, UNAM. 2Wind Erosion Research Unit, USDA. 
I. INTRODUCTION.
Since natural dust emissions are an important factor in the air quality of Mexico City, a modeling effort to quantify their sources and evaluate their impact on the population is presented.
The meteorological and air quality model Multiscale Climate and Chemistry Model (MCCM) provides the meteorological inputs to the erosion model Wind Erosion Prediction System (WEPS) that then provides the natural PM10 emissions to be transported.
The system was developed to study the particles dispersion from natural sources (unprotected soils) as agricultural lands and Lake of Texcoco. These sources are located around the Valley of Mexico City. As a result of this research we developed a system with the capability of modeling the phenomenon of air pollution by natural particles emitted by wind erosion and to generate case study scenarios useful to propose control policies. Some of them are presented here. Also an effort to predict with anticipation this phenomenon is under way.
Figure 2 shows the location of the Official Monitoring Network (RAMA) stations that were selected for comparison.
Figure 5 shows the time series of observed and calculated PM10 concentrations in the monitoring stations. We can see that MCCM-WEPS had the capability of reproducing the two pollution events. The first one from 21:00 GMT March 15 to 02:00 GMT March 16 and the second one from 22:00 GMT March 16 to 03:00 GMT March.
Figure 2. Location of selected RAMA stations over Mexico City.
Figure 5. Comparison of MCCM-WEPS with measurements. Discontinuities represent no data reported by stations.
1ST. event
2nd. event
The only station under the influence of the first event was San Agustin due to its proximity to Lake of Texcoco, see Fig. 3.
II. METHODOLOGY.
Modifications to the source code coupling the meteorological and transport model MCCM and the erosion model WEPS were performed.
At each time step, the wind components (u,v,w) calculated by MCCM are processed to obtain wind intensity and direction needed by WEPS. In fact WEPS becomes another subroutine of MCCM. The PM10 particles “emitted” by WEPS are added directly to the MCCM particle transport module. The flow diagram of the system obtained by coupling MCCM and WEPS is shown in Figure 1.
Also, two different erosion subroutines were developed for WEPS: one for agricultural lands, and another for the Lake of Texcoco. Each subroutine has its own soil parameters.
Figure 3. Wind field and concentrations modeled during the first pollution event. White lines represent level isolines.
IV. CONCLUSIONS.
MCCM-WEPS shows that the north of Mexico City is the most heavily impacted area by natural dust particles.
MCCM-WEPS was able to capture the air pollution events but with not sufficient intensity and out of phase.   
The system allowed to calculate the emission and transport of dust particles in one time step, thus improving the system performance.
Future work
MCCM-WEPS is being improved by adding new soil parameters obtained by field experiments, see Fig. 6. Addition of drag parameters in the MCCM momentum equations is also being considered. See poster “Air Quality Forecasting Performance During the MILAGRO Campaign” by Agustin Garcia.
Figure 6. Field equipment (BSNE) installed during measurement campaign.
The second event was captured by Tlalnepantla, San Agustin, Merced and Hangares stations. MCCM-WEPS showed that the pollutants came from agricultural lands on the northwest of the valley, see Fig. 4.
Figure 1. MCCM-WEPS flow diagram. Dust particle emissions are now considered in the particle transport module.
Figure 4. Wind field and concentrations modeled for the second pollution event. White lines represent level isolines.
V. ACKNOWLEDGMENTS
The authors gratefully acknowledge Dra. Mireya Moya, M.C. Norma Fernández, Dra. María de la Isla, Dr. Mario Martínez, Ing. Francisco Magallanes for their help and comments. To CONACyT for the financial support.
Special thanks to Ing. Manuel García and Ing. Jorge Escalante of the Instrumentation Department at CCA-UNAM.
III. RESULTS.
An air pollution event reported on March 16th, 2006, over Mexico City was simulated. Local newspapers reported that the pollution was generated by the presence of dust particles and smoke. Using MCCM-WEPS we describe this and a previous event.