Evolution of Particulates and Direct Radiative Forcing Downwind of Mexico City during the 2006 MILAGRO Field Campaign

Much has been learned about the boundary layer structure and circulations associated with the complex terrain surrounding Mexico City from previous field experiments and modeling studies. Heating of the higher terrain surrounding Mexico City produces daytime upslope flows that draw air from the plateau north of the city into the basin [Jauregui, 1988; de Foy et al., 2005]. Late in the afternoon, the temperature gradient between the warm basin CBL and the cool ambient air south of the basin produces strong winds through the gap in the terrain near Chalco [Doran and Zhong, 2000]. A propagating density current [Bossert, 1997; Whiteman et al., 2000] is also produced that brings in cooler air from the Gulf of Mexico into the north end of the basin. These converging flows may enhance vertical mixing and venting of pollutants out of the basin [Fast et al., 1998]. While extensive chemistry and particulate measurements have been collected at the surface to characterize air quality over the city, relatively few measurements have been made aloft and downwind of the city to characterize the vertical variations of anthropogenic trace gases and aerosols emitted from Mexico City and their impact on the local and regional environment. This knowledge gap has been addressed by the recent field campaigns conducted in Mexico during March 2006, as part of the Megacities Initiative: Local and Global Research Observations (MILAGRO). This study focuses on simulating the evolution of particulates and their impact of radiative forcing up to 100 km downwind of Mexico city using WRF-chem [Grell et al., 2006] with PNNL’s modules [Fast et al., 2006] for periods in which southerly to westerly synoptic winds transported anthropogenic pollutants over surface measurement sites. The ambient flow during these periods was strong enough to counteract the daytime thermally-driven circulations that would normally draw air from the plateau into the city and away from the surface measurement sites. Various in-situ and remote sensing measurements are used to evaluate the predicted boundary layer structure, vertical mixing and dilution of trace gases and particulates, and the magnitude and spatial variation in aerosol direct radiative forcing.