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Atmospheric Sciences & Global Change Division
Research Highlights

April 2006

PNNL Joins MILAGRO Project in Mexico City

PNNL participated in an international research collaboration to study the far-reaching, long-lasting effects of aerosol emissions on climate

The Pacific Northwest National Laboratory (PNNL) participated in one of the largest, most complex campaigns ever undertaken to intensively study the atmospheric chemistry of a megacity (a city with 10 million or more inhabitants). During March 2006, over 400 researchers from 17 countries conducted an investigation of aerosol and gaseous emissions from Mexico City and the changes that occur in the urban plume as it flows downwind.

The Megacity Initiative: Local and Global Research Observations (MILAGRO) is the name of an international collaboration of scientific institutions, universities, and government agencies that have contributed their researchers' time and expertise, unique instrumentation, and specialized modeling and simulation software to observe, collect, and quantify the emissions from Mexico City. Mexico City was selected as the case study because it produces significant quantities of aerosols on a daily basis and has been the site for previous collaborative atmospheric research campaigns.

The focus of PNNL in the MILAGRO campaign is to understand and describe modifications in the chemical and physical properties of atmospheric aerosols, how those modifications affect the scattering and absorption of solar radiation, and the consequences of those modifications on climate and global change. Because emissions undergo a complicated set of transformations over a range of temporal and spatial scales, the MILAGRO campaign was designed to carry out meteorology, chemistry, and particulate measurements encompassing all of those scales.

PNNL participated primarily in the Megacity Aerosol Experiment in Mexico City (MAX-Mex) component, funded by the U.S. Department of Energy (DOE), which concentrated on the evolution of aerosol properties from within the city out to distances of approximately one hundred kilometers (about 62 miles). Chris Doran and Jim Barnard are co-principal investigators studying how changes in aerosol composition affect solar radiation.

"Aerosols are important to climate because they are thought to contribute to global cooling, which may be mitigating part of the global warming effect," said Barnard. "We need to know the chemical properties of the aerosols and track the changes in that chemistry as the aerosols travel and age."

The ground monitoring sites where most of the PNNL participation took place were situated to capture the evolution of the emissions-site T0 sampled primarily fresh emissions within Mexico City, and sites T1 and T2 sampled more aged emissions at two successive downwind distances. Will Shaw was responsible for providing basic information on aerosol light scattering and absorption at the T1 and T2 sites as well as profiles of wind, temperature, and humidity at T2. "The profiles are essential for subsequent analyses of plume transport and dispersion, which are needed to interpret the aerosol and radiation measurements," said Shaw.

Mike Alexander, Alex Laskin, Yuri Desyaterik, and John Ortega, who work at DOE's Environmental Molecular Sciences Laboratory (EMSL) at PNNL and Xiao-ying Yu of PNNL's Atmospheric Science and Global Change Division, collected an extensive set of measurements of aerosol mass, size distribution, composition, and particle morphology using an array of in-situ techniques and aerosol sampling approaches.

"The aerosol samples will be analyzed using a suite of advanced analytical techniques at EMSL," said Laskin. "Comparing the data from each site will quantify how the chemical and physical properties of aerosols evolve, or 'age,' downwind of Mexico City."

DOE's Research Aircraft Facility's Gulfstream-1 (G-1) aircraft, managed by John Hubbe and under the command of chief pilot Bob Hannigan, measured properties of aerosols and their precursors aloft within and downwind of Mexico City. The G-1 instrumentation included a state-of-the-art Proton Transfer Reaction Mass Spectrometer and a newly developed Aerodyne Aerosol Mass Spectrometer, supported in part by EMSL, and operated by Alexander and Ortega.

"These new instruments make measurements at unprecedented speeds for a variety of gas phase and aerosol emissions," said Carl Berkowitz, Associate Director of the Atmospheric Science and Global Change Division at PNNL. "These measurements, coupled with those from the surface sampling sites will provide critical information on the vertical variations in particulate properties, which are central to understanding how these compounds affect the cooling and warming properties of the atmosphere, and hence the climate."

PNNL researcher Rahul Zaveri collaborated with scientists from Smith College to launch "smart" balloons whose altitude can be remotely controlled. Instrumentation on the balloons was used to help research aircraft track the emissions from Mexico City over extended periods and make measurements of aerosols and their precursors as they age. Zaveri and others on the MILAGRO project will use the measurements from the balloons and the aircraft to construct a detailed picture of how the chemical and radiative properties of Mexico City aerosols evolve over 1-2 days. Results will also be interpreted using a comprehensive gas and aerosol chemistry model developed by Zaveri at PNNL.

Jerome Fast led the multi-agency forecasting and modeling team that provided predictions of the weather conditions, the location of the particulate plume downwind of Mexico City, and the extent of plumes from agricultural burning. These predictions were used to devise sampling strategies for six research aircraft, including the G-1. After the field campaign, Fast will perform computer simulations to help evaluate all of the field campaign data and quantify the uncertainties associated with using coarse grid global climate models to study megacity emissions and to determine the radiative impact of the Mexico City particulates on the local and regional climate.

The Pacific Northwest National Laboratory (PNNL) is a DOE Office of Science national laboratory that solves complex problems in energy, national security, the environment, and life sciences by advancing the understanding of physics, chemistry, biology, and computation. PNNL employs more than 4,100 staff, has an annual budget of more than $700 million, and has been managed by Ohio-based Battelle since the lab's inception in 1965.

The William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) is located on the PNNL campus. Since its inception in 1997, the 200,000-square-foot facility has played host to more than 5,500 visiting scientists, professors and other individuals who requested use of the facility's resources through a peer-reviewed proposal process. These individuals—commonly referred to as "users"—come to EMSL from academia, other research and development laboratories, and industry.

The Department of Energy's Research Aircraft Facility (DOE-RAF), also located at PNNL, serves atmospheric scientists at DOE and other institutions in carrying out airborne research. Recent missions have focused on chemical reactions, gas-to-particle transformations, cloud processes, and long-range transport of airborne material. Central to this facility is an advanced sampling platform, the Grumman Gulfstream I (G-1), as well as its flight crew, technical and engineering support staff and state-of-the-art instrumentation. The G-1 is a large twin turboprop with performance characteristics of contemporary production aircraft. It is capable of measurements to altitudes approaching 30,000 feet over ranges of 1500 nautical miles, and can be operated at speeds that enable both relatively slow sampling and rapid deployment to field sites throughout the world.

 

Aircraft and aerial photo
(Left) DOE's G-1 aircraft and (right) an aerial photo of pollution in Mexico City, taken from the window of the G-1. This "pollution" consists mostly of particulates.

 

Instrumentation
PNNL team set up instrumentation at three surface monitoring sites (T0, T1, and T2), strategically located to measure changes in the aerosols as they were transported downwind from Mexico City. Shown here is some of the instrumentation for sampling particulates at the T1 Site at Tecamac Technical University north of Mexico City.

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