Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 3 2005 10.5194/acpd-5-4083-2005 http://www.atmos-chem-phys-discuss.net/5/4083/2005/ http://www.atmos-chem-phys-discuss.net/5/4083/2005/acpd-5-4083-2005.html http://www.atmos-chem-phys-discuss.net/5/4083/2005/acpd-5-4083-2005.pdf 4083 4113 2005-06-23 Measurements of Black Carbon Specific Absorption in the Mexico City Metropolitan Area during the MCMA 2003 Field Campaign J. C. Barnard E. I. Kassianov T. P. Ackerman S. Frey K. Johnson B. Zuberi L. T. Molina M. J. Molina J. S. Gaffney N. A. Marley Pacific Northwest National Laboratory, Richland, Washington, USA Massachusetts Institute of Technology, Cambridge, Massachusetts, USA Argonne National Laboratory, Argonne, Illinois, USA now at: GEO2 Technologies, Weston, Massachusetts, USA During the Mexico City Metropolitan Area (MCMA) field campaign of 2003, measurements of the shortwave radiation field, lidar backscatter, and atmospheric concentrations of black carbon (BC) permitted the inference of the BC carbon specific absorption, α_λ, defined as the absorption cross section per unit mass (with units of m²/g). This diverse set of measurements allowed us to determine α_λ in two ways. These methods – labeled I and II – are distinguished from one another in the manner that the columnar concentration of BC (with units of mg/m² is determined. This concentration is found by using either surface measurements of BC concentration and lidar estimates of aerosol mixing heights, or a more rigorous method that relies on the columnar aerosol size distribution. The averaged values of α_λ derived from these methods agree to about 20%, although we expect that the values obtained from method I are underestimated. These results, along with those of Schuster et al. (2005), suggest that in the MCMA, α_λ is in a range of 8 to 10 m²/g at a wavelength of 550 nm. This range is somewhat lower than the commonly accepted value of 10 m²/g for a wavelength of 550 nm, but is consistent with the calculations of Fuller et al. (1999), who suggest that this value is too high.