Modeling Particulate Evolution from Urban to Synoptic Scales Downwind of Mexico City

High concentrations of pollutants, such as ozone, ozone precursors, and particulates, have been observed in Mexico City for many years. Field campaign measurements and modeling studies over the past twenty years have examined the local scale processes responsible for the evolution of these pollutants. However, the effect of these pollutants on air quality and climate downwind of Mexico City was not known. To address this issue, extensive meteorological, chemical, and particulate measurements were made during March 2006 over central Mexico and the Gulf of Mexico as part of the Megacity Initiative Local and Global Regional Observations (MILAGRO). MILAGRO was comprised of several complementary field campaigns including MCMA (supported by Mexican agencies, NSF, and DOE), MAX-Mex (supported by DOE), MIRAGE-Mex (supported by NSF), and INTEX-B (supported by NASA) that were conducted at different spatial scales ranging from local-scale measurements over the Mexico City basin to synoptic-scale measurements several hundred kilometers downwind. The impact, or "footprint", of Mexico City emissions on synoptic-scale trace gases and particulate distributions may be somewhat different since gas-aerosol partitioning processes and aerosol-radiation effects couple trace gas and particulate evolution. The "footprint" also varies from day to day as a result of the evolving meteorological conditions in the region. The downwind impact of Mexico City pollutants is investigated by combining MILAGRO measurements with a fully-coupled meteorology-chemistry-particulate model (WRF-chem) to examine the how particulate mass, composition, and size distribution evolves over several days downwind of Mexico City. Understanding the regional- to synoptic-scale impacts requires understanding local-scale processes that affect non-linear chemistry in the vicinity of urban primary emissions; therefore, nested domains are employed that consist of an outer grid encompassing all of Mexico with a grid spacing of 12 km and an inner grid that encompasses central Mexico with a grid spacing of 3 km. The performance of the model is evaluated with measurements obtained from surface, aircraft, and satellite instrument platforms. The contribution of anthropogenic emissions from Mexico City in relation to other sources, including anthropogenic emissions outside of Mexico City, as well as biogenic, biomass burning, and dust sources is examined. Other modeling studies that are currently being conducted to examine the impact of Mexico City on air quality and climate will also be described.