PNNL

Abstract

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, a?, defined as the absorption cross section per unit mass (with units of m2/g). This diverse set of measurements allowed us to determine a? 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 a? 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, a? is in a range of 8 to 10 m2/g at a wavelength of 550 nm. This range is somewhat lower than the canonical value of 10 m2/g often assumed for a wavelength of 550 nm, but is consistent with the calculations of Fuller et al. (1999), who suggest that the canonical value is too high.