Formation of semivolatile inorganic aerosols in the mexico city metropolitan area during the milagro campaign

Abstract

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 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 chloride is also high and its concentration exceeds 2 μg m−3 in Texcoco Lake. PM 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 T0, the average measured values of PM1 sulfate, nitrate, ammonium, and chloride are 3.6 μg m−3, 3.6 μg m−3, 2.1 μg m−3, and 0.35 μg m−3 respectively. The corresponding predicted values are 3.7 μg m−3, 2.8 μg m−3, 1.7 μg m−3, and 0.25 μg m−3. Additional improvements are possible by (i) using a day-dependent emission inventory, (ii) improving the performance of the model regarding the oxidant levels, and (iii) revising the emissions and the chemical composition of the fugitive dust. 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 NOx emissions are reduced by 50 %. Nitrate concentration decreases by up to 1 μg m−3 after the 50 % reduction of NOx 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, NOx, and SO2 emissions respectively.