The Effect of Anthropogenic and Biomass Burning Sources of Black Carbon on Observed and Predicted Aerosol Radiative Forcing over Central Mexico

Previous studies have suggested that black carbon concentrations are relatively high in Mexico City compared to U.S. urban areas and that secondary organic aerosols are a large fraction of the particulate mass transported out of Mexico City. While measurements of particulate mass, composition, and size distribution have been made in Mexico City, the impact of the particulates on air quality and climate downwind of the city is not known. To better understand the evolution of particulates originating from anthropogenic emissions in Mexico City and their impact on regional air quality and climate, a multi-agency field campaign called the Megacities Initiative: Local And Global Research Observations (MILAGRO) collected a wide range of meteorological, chemical, and particulate measurements during March 2006. One component of MILAGRO was the MAX-Mex (Megacity Aerosol eXperiment Mexico) experiment supported by the U.S. Department of Energy. MAX-Mex obtained particulate and radiation measurements at three surface sites and from aircraft flights up to 100 km downwind of Mexico City. We will present an initial analysis of the measurements that will quantify the aerosol radiative forcing and its variation downwind of the city during periods favorable for transport over the sampling network. Pacific Northwest National Laboratory's version of WRF-chem, a new fully-coupled meteorology-chemistry-aerosol model based on the chemistry version of the Weather Research and Forecasting (WRF) model will also be used to help interpret the meteorological processes that affect particulate evolution. We will focus on 1) the effect of black carbon and organic carbon on the predicted aerosol radiative forcing and 2) the uncertainties in the emission rates, transport, and transformation of black carbon and other particle types in the region. The model will also be used to quantify the relative roles of anthropogenic sources of black carbon in Mexico City and the numerous biomass burning sources fires that occurred over the central Mexican plateau. Surface and airborne measurements of particulate concentration, size distribution, and optical properties are used to evaluate the model performance.