Atmos. Chem. Phys. Discuss., 7, 15693-15721, 2007
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Particulate polycyclic aromatic hydrocarbon spatial variability and aging in Mexico City
D. A. Thornhill1, S. C. Herndon2, T. B. Onasch2, E. C. Wood2, M. Zavala3, L. T. Molina3, J. S. Gaffney4, N. A. Marley4, and L. C. Marr1
1Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA
2Aerodyne Research Inc., Billerica, Massachusetts, USA
3Molina Center for Energy and the Environment, La Jolla, California and Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
4Department of Chemistry, University of Arkansas, Little Rock, Arkansas, USA

Abstract. As part of the Megacities Initiative: Local and Global Research Observations (MILAGRO) study in the Mexico City Metropolitan Area in March 2006, we measured particulate polycyclic aromatic hydrocarbons (PAHs) and other gaseous species and particulate properties at six locations throughout the city. The measurements were intended to support the following objectives: to describe spatial and temporal patterns in PAH concentrations, to gain insight into sources and transformations of PAHs, and to quantify the relationships between PAHs and other pollutants. Total particulate PAHs at the Instituto Mexicano del PetrĂ³leo (T0 supersite) located near downtown averaged 50 ng m−3, and aerosol active surface area averaged 80 mm2 m−3. PAHs were also measured on board the Aerodyne Mobile Laboratory, which visited six sites encompassing a mixture of different land uses and a range of ages of air parcels transported from the city core. Weak intersite correlations suggest that local sources are important and variable and that exposure to PAHs cannot be represented by a single regional-scale value. The relationships between PAHs and other pollutants suggest that a variety of sources and ages of particles are present. Among carbon monoxide, nitrogen oxides (NOx), and carbon dioxide, particulate PAHs are most strongly correlated with NOx. Mexico City's PAH-to-black carbon mass ratio of 0.01 is similar to that found on a freeway loop in the Los Angeles area and approximately 8–30 times higher than that found in other cities. Ratios also indicate that primary combustion particles are rapidly coated by secondary aerosol in Mexico City. If so, the lifetime of PAHs may be prolonged if the coating protects them against photodegradation or heterogeneous reactions.

Citation: Thornhill, D. A., Herndon, S. C., Onasch, T. B., Wood, E. C., Zavala, M., Molina, L. T., Gaffney, J. S., Marley, N. A., and Marr, L. C.: Particulate polycyclic aromatic hydrocarbon spatial variability and aging in Mexico City, Atmos. Chem. Phys. Discuss., 7, 15693-15721, doi:10.5194/acpd-7-15693-2007, 2007.
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