In order to determine the wavelength dependence of fine (<1 micron)atmospheric aerosol absorption in the Mexico City area, the absorption?ngstrom exponents (AAEs) were calculated from hourly averages ofaerosol absorption measured at seven wavelengths (370, 450, 520, 590, 660,880, and 950 nm) with an aethalometer during two field campaigns, the MexicoCity Metropolitan Area study in April 2003 (MCMA 2003) and the MegacityInitiative: Local and Global Research Observations in March 2006 (MILAGRO).These results were compared to AAEs determined in the laboratory from850–280 nm (350 points) on 12-h fine aerosol samples collected at thesame sites. The aerosol AAEs varied from 0.76 to 1.5 in 2003 and from 0.63to 1.4 in 2006. The AAE values determined in the afternoon were consistentlyhigher than the corresponding morning values, suggesting the photochemicalaging of the aerosols leading to the formation of more highly UV absorbingorganic aerosol species in the afternoon.The AAE values were compared to stable and radiocarbon isotopic measurementsof the 12-h aerosol samples to determine the sources of the aerosolcarbon. The fraction of modern carbon (fM) in the aerosol samples, asdetermined from展开▼
