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In order to determine the wavelength dependence of fine (<1 micron)atmospheric aerosol absorption in the Mexico City area, the absorptionÅngstrom 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<sup14C analysis, showed that an average of 70% of thecarbonaceous aerosols in Mexico City were from modern biomass sources duringboth field campaigns. The 13C/12C ratios of the aerosol carbonillustrate the significant impact of Yucatan forest fires (C-3 plants) in2003 and local grass fires (C-4 plants) in 2006. A direct comparison of thefM values, stable carbon isotope ratios, and aerosol AAEs suggested that thewavelength dependence of the aerosol absorption was controlled by thebiogenically derived aerosol components

The impact of biogenic carbon sources on aerosol absorption in Mexico City