Diurnal variability of size-differentiated inorganic aerosols and their gas-phase precursors during January and February of 2003 near downtown Mexico City

Size-differentiated atmospheric aerosol particles along with gas-phase precursors were measured at a site near downtown Mexico City (MER site) during January and February 2003 to provide information regarding the diurnal variability and partitioning of semi-volatile inorganic compounds between the gas phase and different-size particles. The aerosols were sampled with cascade impactors (MOUDIʹs) in the following periods: 1st (06:00–09:00 h, LST), 2nd (09:00–12:00 h, LST), 3rd (12:00–15:00 h, LST) and 4th (15:00–18:00 h, LST). The gas-phase measurements were continuously recorded with an open-path FTIR spectrometer. Overall, inorganic aerosol size/composition measurements observed a bimodal distribution: one mode was present in the accumulation size range (0.18–0.32 μm, aerodynamic diameter) and the other in the coarse mode (over 1 μm, aerodynamic diameter). During the morning sampling periods, the highest concentrations occurred mainly over the accumulation mode while during the afternoon sampling periods, concentration peaks were observed over both accumulation and coarse modes. More than half of the ammonium was found in the accumulation mode. The rest of the ions (sodium, chloride, sulfate, nitrate, calcium and potassium) were prominent in both modes. The significant presence of sodium and crustals (calcium and potassium) are explained in terms of the potential influence of the dry salt-lake of Texcoco and resuspended dust/soil, respectively. Based on the analysis of the time-resolved PM and gas-phase composition, the significant presence of gas-phase ammonia (>35 ppb) during the morning sampling periods was of importance in neutralizing the aerosol particles.