Atmos. Chem. Phys. Discuss., 10, 3347-3399, 2010
© Author(s) 2010. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
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
P. B. Voss1, R. A. Zaveri2, F. M. Flocke3, H. Mao4, T. P. Hartley1, P. DeAmicis1, I. Deonandan1, G. Contreras-Jiménez5, O. Martínez-Antonio5, M. Figueroa Estrada6, D. Greenberg7, T. L. Campos3, A. J. Weinheimer3, D. J. Knapp3, D. D. Montzka3, J. D. Crounse8, P. O. Wennberg9,10, E. Apel3, S. Madronich3, and B. de Foy11
1Picker Engineering Program, Smith College, Northampton, MA, USA
2Pacific Northwest National Laboratory, Richland, WA, USA
3National Center for Atmospheric Research, Boulder, CO, USA
4Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA
5Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Cuernavaca, Mexico
6Instituto Nacional de Ecologia, Delegación Coyoacán, México D.F., Mexico
7Mohawk Trail Regional School District, Shelburne Falls, MA, USA
8Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA
9Division of Engineering and Applied Science and Geological Science and Planetary Science, California Institute of Technology, Pasadena, CA, USA
10Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
11Department of Earth and Atmospheric Sciences, Saint Louis University, St. Louis, MO, USA

Abstract. 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 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 based on 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 different 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, and (c) low-altitude outflow with entrainment into a cleaner westerly jet below the plateau. The C-130 aircraft intercepted the balloon-based trajectories three times on 19 March, once along each of these pathways. In all three cases, distinct peaks in the urban tracer signatures and LIDAR backscatter imagery were consistent with MCMA pollution. The coherence of the high-altitude outflow was well preserved after one day whereas that lower in the atmosphere was more widely dispersed over the same time period. Other C-130 intercepts of polluted air are shown to have likely originated outside of MCMA. These findings, and the aircraft intercepts in particular, should prove useful in answering a range of scientific questions pertaining to the transport, transformation, and downwind impacts of megacity air pollution.

Citation: Voss, P. B., Zaveri, R. A., Flocke, F. M., Mao, H., Hartley, T. P., DeAmicis, P., Deonandan, I., Contreras-Jiménez, G., Martínez-Antonio, O., Figueroa Estrada, M., Greenberg, D., Campos, T. L., Weinheimer, A. J., Knapp, D. J., Montzka, D. D., Crounse, J. D., Wennberg, P. O., Apel, E., Madronich, S., and de Foy, B.: 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, Atmos. Chem. Phys. Discuss., 10, 3347-3399, doi:10.5194/acpd-10-3347-2010, 2010.
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