Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 6 4 2006 10.5194/acp-6-925-2006 http://www.atmos-chem-phys.net/6/925/2006/ http://www.atmos-chem-phys.net/6/925/2006/acp-6-925-2006.html http://www.atmos-chem-phys.net/6/925/2006/acp-6-925-2006.pdf 925 946 2006-03-24 Characterization of ambient aerosols in Mexico City during the MCMA-2003 campaign with Aerosol Mass Spectrometry: results from the CENICA Supersite D. Salcedo T. B. Onasch K. Dzepina M. R. Canagaratna Q. Zhang J. A. Huffman P. F. DeCarlo J. T. Jayne P. Mortimer D. R. Worsnop C. E. Kolb K. S. Johnson B. Zuberi L. C. Marr R. Volkamer L. T. Molina M. J. Molina B. Cardenas R. M. Bernabé C. Márquez J. S. Gaffney N. A. Marley A. Laskin V. Shutthanandan Y. Xie W. Brune R. Lesher T. Shirley J. L. Jimenez Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Cuernavaca, Morelos, México Cooperative Institute for Research in the Environmental Sciences (CIRES), Univ. of Colorado at Boulder, Boulder, CO, USA Center for Aerosol and Cloud Chemistry, Aerodyne Research Inc., Billerica, MA, USA Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO, USA Program in Atmospheric and Oceanic Sciences, University of Colorado at Boulder, Boulder, CO, USA Department of Earth, Atmospheric and Planetary Sciences and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA, USA Molina Center on Energy and Environment, CA, USA Centro Nacional de Investigación Capacitación Ambiental, Instituto Nacional de Ecología, México D.F., México Argonne National Laboratory, Argonne, IL, USAU William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Lab., Richland, WA, USA Pennsylvania State University, University Park, PA, USA now at: Atmospheric Science Research Center, State University of New York-Albany, Albany, NY, USA now at: John Hopkins University, Baltimore, MD, USA now at: Department of Chemistry and Biochemistry, University of California San Diego, San Diego, CA, USA now at: GEO2 Technologies, Woburn, MA, USA now at: Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA now at: Molina Center on Energy and Environment, CA, USA An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite, during the Mexico City Metropolitan Area field study (MCMA-2003) from 31 March-4 May 2003 to investigate particle concentrations, sources, and processes. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 µm (NR-PM₁) with high time and size-resolution. In order to account for the refractory material in the aerosol, we also present estimates of Black Carbon (BC) using an aethalometer and an estimate of the aerosol soil component obtained from Proton-Induced X-ray Emission Spectrometry (PIXE) analysis of impactor substrates. Comparisons of AMS + BC + soil mass concentration with other collocated particle instruments (a LASAIR Optical Particle Counter, a PM_2.5 Tapered Element Oscillating Microbalance (TEOM), and a PM_2.5 DustTrak Aerosol Monitor) show that the AMS + BC + soil mass concentration is consistent with the total PM_2.5 mass concentration during MCMA-2003 within the combined uncertainties. In Mexico City, the organic fraction of the estimated PM_2.5 at CENICA represents, on average, 54.6% (standard deviation σ=10%) of the mass, with the rest consisting of inorganic compounds (mainly ammonium nitrate and sulfate/ammonium salts), BC, and soil. Inorganic compounds represent 27.5% of PM_2.5 (σ=10%); BC mass concentration is about 11% (σ=4%); while soil represents about 6.9% (σ=4%). Size distributions are presented for the AMS species; they show an accumulation mode that contains mainly oxygenated organic and secondary inorganic compounds. The organic size distributions also contain a small organic particle mode that is likely indicative of fresh traffic emissions; small particle modes exist for the inorganic species as well. Evidence suggests that the organic and inorganic species are not always internally mixed, especially in the small modes. The aerosol seems to be neutralized most of the time; however, there were some periods when there was not enough ammonium to completely neutralize the nitrate, chloride and sulfate present. The diurnal cycle and size distributions of nitrate suggest local photochemical production. On the other hand, sulfate appears to be produced on a regional scale. There are indications of new particle formation and growth events when concentrations of SO₂ were high. Although the sources of chloride are not clear, this species seems to condense as ammonium chloride early in the morning and to evaporate as the temperature increases and RH decreases. The total and speciated mass concentrations and diurnal cycles measured during MCMA-2003 are similar to measurements during a previous field campaign at a nearby location.