Particulate Polycyclic Aromatic Hydrocarbons and Aerosol Active Surface Area in Different Environments of Mexico City Dwight A. Thornhill 1, Linsey C. Marr 1, Luisa T. Molina 2 1 Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, Virginia Molina Center for Strategic Studies in Energy and the Environment, La Jolla, California Introduction Previous studies have shown that Mexico City has some of the highest ambient concentrations ever measured of particle-phase polycyclic aromatic hydrocarbons (PAHs), which are potent carcinogens.  PAH pollution is a major problem throughout Mexico City. PAH concentrations vary considerably in space; therefore PAHs cannot be treated as a regional-scale pollutant.  PAHs at T0 are more likely influenced by local emissions as compared to T1, which is most likely influenced by emissions that have been transported and undergone dilution and aging. 2. Experimental Methods 3.Results 5. Acknowledgments 4. Conclusions This research was funded by the Molina Center for Strategic Studies in Energy and the Environment. D. Thornhill is a Fulbright Scholar from Barbados. We also acknowledge the financial support of the U.S. National Science Foundation and the U.S. Department of Energy. Improving the understanding of spatial and temporal variations in PAHs and of transformations they may undergo in the atmosphere is critical to further assessing their impact on health. The objective of this study is to compare and contrast concentrations of PAHs, aerosol active surface area (AS), and other gaseous and aerosol compounds in fresh, mixed, and aged emission environments within Mexico City. The results will help to identify different sources of particulate matter and contribute to the knowledge of aerosol processing. Figure 1. Measurement sites around Mexico City. T0 is located near the city center and is surrounded by a dense network of roads. T1 sits along the northern edge of the city. The AML visited numerous suburban and boundary sites including Pedregal, Pico Tres Padres, PEMEX and Santa Ana. During the MILAGRO/Mexico City Metropolitan Area field campaign in March 2006, we used photoemission aerosol sensors (EcoChem Analytical) to measure PAHs and diffusion chargers (EcoChem Analytical) to measure aerosol active surface area (AS). The instruments were based at Instituto Mexicano del Petrόleo (T0 supersite), Universidad Tecnológica de Tecámac (T1 supersite), and in the Aerodyne Mobile Laboratory (AML), which visited a variety of sites. Pedregal Pico Tres Padres As shown in Figure 2, concentrations of both PAH and AS were higher at T0 compared to T1. There is a marked increase in concentrations during the morning rush hour (6-9 a.m.) at both locations, with this effect being more pronounced at T0. This site is located near the city center and is surrounded by a high density of road networks, and the majority of particles here are likely to be freshly emitted from vehicles. T1 is located along the northern edge of the city road network, and likely represents a combination of fresh and aged emissions that have been transported from the city center. PAHs at T0 averaged 50 ng m ­ 3, while PAH concentrations at T1 averaged 12 ng m ­3 over the entire campaign. AS concentrations averaged 80 mm 2 m -3 at T0 and 10 mm 2 m -3 at T1. Figure 3 shows PAH and AS concentrations at the different sites visited by the AML. The sites with the denser road networks showed higher values of PAHs (T0 and Pemex-Tula-T0), while some sites with sparser road networks (Pico Tres Padres, Pedregal) showed lower values of PAHs but similar levels of AS. At these sites, sources of particles other than fresh vehicle emissions may be more important, including secondary aerosol formation. The temporal variations between PAH and AS measurements were only similar at T0, and to a lesser extent at Pedregal. PEMEX T1 T0 Figure 2. PAH and AS concentrations at the T0 and T1 supersites during the entire field campaign. Raw 1-min measurements are shown by the colored lines, where color indicates wind transport episodes, and 1-hr averages are shown in black. Pedregal T0 Pico Tres PadresT1 Santa Ana PEMEX-TULA-T0 Figure 3. PAH and AS measured by the AML at 10-min frequencies at different sites throughout Mexico City.