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
Received: 22 Jan 2010 – Discussion started: 09 Feb 2010
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, 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.
Revised: 28 Jun 2010 – Accepted: 16 Jul 2010 – Published: 04 Aug 2010
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., 10, 7137-7159, doi:10.5194/acp-10-7137-2010, 2010.