Formation of semivolatile inorganic aerosols in the Mexico City Metropolitan Area during the MILAGRO campaign

Abstract. One of the most challenging tasks for chemical transport models (CTMs) is the prediction of the formation and partitioning of the major semi-volatile inorganic aerosol components (nitrate, chloride, ammonium) between the gas and particulate phases. In this work the PMCAMx-2008 CTM, which includes the recently developed aerosol thermodynamic model ISORROPIA-II, is applied in the Mexico City Metropolitan Area in order to simulate the formation of the major inorganic aerosol components. The main sources of SO₂ (such as the Miguel Hidalgo Refinery and the Francisco Perez Rios Power Plant) in the Mexico City Metropolitan Area (MCMA) are located in Tula, resulting in high predicted PM₁ (particulate matter with diameter less than 1 μm) sulfate concentrations (over 25 μg m^-3) in that area. The average predicted PM₁ nitrate concentrations are up to 3 μg m⁻³ (with maxima up to 11 μg m⁻³) in and around the urban center, mostly produced from local photochemistry. The presence of calcium coming from the Tolteca area (7 μg m⁻³) as well as the rest of the mineral cations (1 μg m⁻³ potassium, 1 μg m⁻³ magnesium, 2 μg m⁻³ sodium, and 3 μg m⁻³ calcium) from the Texcoco Lake resulted in the formation of a significant amount of aerosol nitrate in the coarse mode with concentrations up to 3 μg m⁻³ over these areas. PM₁₋₁₀ (particulate matter with diameter between 1 and 10 μm) chloride is also high and its concentration exceeds 2 μg m⁻³ in Texcoco Lake. PM₁ ammonium concentrations peak at the center of Mexico City (2 μg m⁻³) and the Tula vicinity (2.5 μg m⁻³). The performance of the model for the major inorganic PM components (sulfate, ammonium, nitrate, chloride, sodium, calcium, and magnesium) is encouraging. At the T0 measurement site, located in the Mexico City urban center, the average measured values of PM₁ sulfate, nitrate, ammonium, and chloride are 3.5 μg m⁻³, 3.5 μg m⁻³, 2.1 μg m⁻³, and 0.36 μg m⁻³, respectively. The corresponding predicted values are 3.7 μg m⁻³, 2.7 μg m⁻³, 1.7 μg m⁻³, and 0.25 μg m⁻³. High sulfate concentrations are associated with the transport of sulfate from the Tula vicinity, while in periods where southerly winds are dominant; the concentrations of sulfate are low. The underprediction of nitrate can be attributed to the underestimation of OH levels by the model during the early morning. Ammonium is sensitive to the predicted sulfate concentrations and the nitrate levels. The performance of the model is also evaluated against measurements taken from a suburban background site (T1) located north of Mexico City. The average predicted PM_2.5 (particulate matter with diameter less than 2.5 μm) sulfate, nitrate, ammonium, chloride, sodium, calcium, and magnesium are 3.3, 3.2, 1.4, 0.5, 0.3, 1.2, and 0.15 μg m⁻³, respectively. The corresponding measured concentrations are 3.7, 2.9, 1.5, 0.3, 0.4, 0.6, and 0.15 μg m⁻³. The overprediction of calcium indicates a possible overestimation of its emissions and affects the partitioning of nitric acid to the aerosol phase resulting occasionally in an overprediction of nitrate. Additional improvements are possible by improving the performance of the model regarding the oxidant levels, and revising the emissions and the chemical composition of the fugitive dust. The hybrid approach in which the mass transfer to the fine aerosol is simulated using the bulk equilibrium assumption and to the remaining aerosol sections using a dynamic approach, is needed in order to accurately simulate the size distribution of the inorganic aerosols. The bulk equilibrium approach fails to reproduce the observed coarse nitrate and overpredicts the fine nitrate. Sensitivity tests indicate that sulfate concentration in Tula decreases by up to 0.5 μg m⁻³ after a 50% reduction of SO₂ emissions while it can increase by up to 0.3 μg m⁻³ when NO_x emissions are reduced by 50%. Nitrate concentration decreases by up to 1 μg m⁻³ after the 50% reduction of NO_x or NH₃ emissions. Ammonium concentration decreases by up to 1 μg m⁻³, 0.3 μg m⁻³, and 0.1 μg m⁻³ after the 50% reduction of NH₃, NO_x, and SO₂ emissions, respectively.