Simulation of regional dust and its effect on photochemistry in the Mexico City area during MILAGRO experiment

Dust particles have important effects on solar radiation, climate, and photochemistry in the troposphere. Dust events were observed from March 16 to 20, 2006 in the Mexico City (MC) area during the MILAGRO (Megacity Initiative: Local and Global Research Observations) field experiment. In order to study the effects of dust aerosols on total aerosol mass concentrations and photochemistry, a regional chemical/dynamical model (Weather Research and Forecasting Chemical model version 3 - WRF/Chem-v3) was used in this study, and a dust module was implemented in the model. Analysis of model and observational data suggests that the large area of coastal dry lands to the northeast of MC is an important source of dust particles for the entire MC area. The simulations of both PM_2.5 (total particle mass with radius less than 2.5 μm) and PM₁₀ (total particle mass with radius less than 10 μm) concentrations are more consistent with observations than the results of the model without the dust module. During this dust period (March 16-20, 2006), the simulated dust aerosol mass accounts for about 70% of the total PM₁₀ aerosol mass concentrations, with a strong diurnal variation. The results also suggest that dust aerosols have important effects on actinic fluxes and therefore photochemistry, especially on hydroxyl radical (OH) and ozone (O₃) concentrations, in the MC area and the surrounding region. The dust particles decrease the photochemical production of OH, with a maximum reduction of 60% in the dust source region (northeast of the MC area). Near the city area, the reduction of OH concentrations is about 5-20%. The strongest effects on O₃ concentrations are near MC, where the maximum reduction of O₃ is about 10 ppbv. In the dust source region, O₃ concentrations are reduced by about 3-5 ppbv.