ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-7137-2010 Long-range pollution transport during the MILAGRO-2006 campaign: a case study of a major Mexico City outflow event using free-floating altitude-controlled balloons Voss P. B. 1 Zaveri R. A. 2 Flocke F. M. 3 Mao H. 4 Hartley T. P. 1 DeAmicis P. 1 Deonandan I. 1 Contreras-Jiménez G. 5 Martínez-Antonio O. 5 Figueroa Estrada M. 6 Greenberg D. 7 Campos T. L. 3 Weinheimer A. J. 3 Knapp D. J. 3 Montzka D. D. 3 Crounse J. D. 8 Wennberg P. O. 9 10 Apel E. 3 Madronich S. 3 de Foy B. 11 Picker Engineering Program, Smith College, Northampton, MA, USA Pacific Northwest National Laboratory, Richland, WA, USA National Center for Atmospheric Research, Boulder, CO, USA Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Cuernavaca, Mexico Instituto Nacional de Ecologia, Delegación Coyoacán, México D.F., Mexico Mohawk Trail Regional School District, Shelburne Falls, MA, USA Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA Division of Engineering and Applied Science and Geological Science and Planetary Science, California Institute of Technology, Pasadena, CA, USA Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA Department of Earth and Atmospheric Sciences, Saint Louis University, St. Louis, MO, USA 04 08 2010 10 15 7137 7159 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/7137/2010/acp-10-7137-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/7137/2010/acp-10-7137-2010.pdf

One of the major objectives of the Megacities Initiative: Local And Global Research Observations (MILAGRO-2006) campaign was to investigate the long-range transport of polluted Mexico City Metropolitan Area (MCMA) outflow and determine its downwind impacts on air quality and climate. Six research aircraft, including the National Center for Atmospheric Research (NCAR) C-130, made extensive chemical, aerosol, and radiation measurements above MCMA and more than 1000 km downwind in order to characterize the evolution of the outflow as it aged and dispersed over the Mesa Alta, Sierra Madre Oriental, Coastal Plain, and Gulf of Mexico. As part of this effort, free-floating Controlled-Meteorological (CMET) balloons, commanded to change altitude via satellite, made repeated profile measurements of winds and state variables within the advecting outflow. In this paper, we present an analysis of the data from two CMET balloons that were launched near Mexico City on the afternoon of 18 March 2006 and floated downwind with the MCMA pollution for nearly 30 h. The repeating profile measurements show the evolving structure of the outflow in considerable detail: its stability and stratification, interaction with other air masses, mixing episodes, and dispersion into the regional background. Air parcel trajectories, computed directly from the balloon wind profiles, show three transport pathways on 18–19 March: (a) high-altitude advection of the top of the MCMA mixed layer, (b) mid-level outflow over the Sierra Madre Oriental followed by decoupling and isolated transport over the Gulf of Mexico, and (c) low-level outflow with entrainment into a cleaner northwesterly jet above the Coastal Plain. The C-130 aircraft intercepted the balloon-based trajectories three times on 19 March, once along each of these pathways; in all three cases, peaks in urban tracer concentrations and LIDAR backscatter are consistent with MCMA pollution. In comparison with the transport models used in the campaign, the balloon-based trajectories appear to shear the outflow far more uniformly and decouple it from the surface, thus forming a thin but expansive polluted layer over the Gulf of Mexico that is well aligned with the aircraft observations. These results provide critical context for the extensive aircraft measurements made during the 18–19 March MCMA outflow event and may have broader implications for modelling and understanding long-range transport.

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