MAXMEX

2006 MAX-Mex-Megacity Aerosol eXperiment - Mexico City

3 March 2006 - 28 March 2006

Lead Scientist: Jeffrey Gaffney

Observatory: OSC

A 4-week field campaign was conducted in and downwind of Mexico City during March 2006. The Megacity Aerosol eXperiment - MEXico City (MAX-MEX) characterized aerosol formation and changes in aerosol composition, size distribution, light scattering coefficient, absorption coefficient, optical depth, soot-specific absorption, and radiative fluxes at selected vertical and horizontal locations in the outflow from a well-characterized urban core. Detailed analyses were made of the meteorological conditions during the field campaign to distinguish features of the observed changes attributable to factors such as transport, diffusion and dilution, and relative humidity from intrinsic modifications in the chemical and microphysical properties of the aerosols resulting from chemical processes. The field campaign focused on chemical, physical, and optical characterization of the aerosols, on aerosol transformations including aging of the black carbon during outflow from the region, and on the effects of the megacity aerosol plume on the regional radiative balance in and near this megacity source. The G-1 aircraft was based out of the Veracruz International Airport, where the lower altitude allowed heavier take offs. The G-1 had its standard instrument package to measure aerosols, gases, radiation, and atmospheric state variables, and flew on 11 separate days a total of 15 missions. Flight patterns had the aircraft flying primarily within the Valley of Mexico to examine the near-source aerosols and aerosol precursors over the Mexico City urban area, and to follow them as they are advected downwind towards the north. Analysis of past weather patterns indicated that a boundary-layer plume will be carried in this direction about 25-30% of the time. These flights were coordinated with the NSF C-130 flights that sampled the plume farther downwind, and NASA King Air B-200 flights that took lidar aerosol profile measurements from about 27000 feet. The G-1 flew over two ground sites outside of the urban center in order to obtain vertical profile information on the chemical and physical composition and optical properties from ground to above the boundary layer. An urban ground site was located in the Mexico City metropolitan area, a second site was 15 km to the NE outside the city, and a third site was another 20 km farther north (downwind based on predominant climatological flow patterns.) To the extent possible, the ground sites made measurements similar to those in the aircraft, plus surface and boundary layer meteorological measurements. The measurement strategies were designed to examine the evolution of aerosols and aerosol precursors over time scales of approximately 1 to 12 hours. These time scales were chosen to achieve a better understanding of the aging of primary aerosols and the formation of secondary organic and inorganic aerosols. It is hypothesized that over this time scale there is a substantial shift from primary emitted aerosols to an internal mixture of primary and secondary aerosols, resulting in a substantial modification to the optical and cloud nucleating properties of aerosols as they age and are advected downwind from the urban source.