The impact of biogenic carbon sources on aerosol absorption in Mexico City 1Graduate Institute of Technology, University of Arkansas at Little Rock, Little Rock, AR, USA
2Chemistry Department, University of Arkansas at Little Rock, Little Rock, AR, USA
3Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, IL, USA
4Swarthmore College, 500 College Ave., Swarthmore, PA, USA
5University of Puerto Rico, Mayaguez, Mayaguez, PR, Puerto Rico
6Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA, USA
7Chemistry Department, University of Central Arkansas, Conway, AR, USA
Received: 13 Aug 2008 – Published in Atmos. Chem. Phys. Discuss.: 24 Oct 2008 – Published: 02 Mar 2009Abstract. In order to determine the wavelength dependence of fine (<1 micron)
atmospheric aerosol absorption in the Mexico City area, the absorption
Ångstrom exponents (AAEs) were calculated from hourly averages of
aerosol absorption measured at seven wavelengths (370, 450, 520, 590, 660,
880, and 950 nm) with an aethalometer during two field campaigns, the Mexico
City Metropolitan Area study in April 2003 (MCMA 2003) and the Megacity
Initiative: Local and Global Research Observations in March 2006 (MILAGRO).
These results were compared to AAEs determined in the laboratory from
850–280 nm (350 points) on 12-h fine aerosol samples collected at the
same sites. The aerosol AAEs varied from 0.76 to 1.5 in 2003 and from 0.63
to 1.4 in 2006. The AAE values determined in the afternoon were consistently
higher than the corresponding morning values, suggesting the photochemical
aging of the aerosols leading to the formation of more highly UV absorbing
organic aerosol species in the afternoon.
The AAE values were compared to stable and radiocarbon isotopic measurements
of the 12-h aerosol samples to determine the sources of the aerosol
carbon. The fraction of modern carbon (fM) in the aerosol samples, as
determined fromC analysis, showed that an average of 70% of the
carbonaceous aerosols in Mexico City were from modern biomass sources during
both field campaigns. The 13C/12C ratios of the aerosol carbon
illustrate the significant impact of Yucatan forest fires (C-3 plants) in
2003 and local grass fires (C-4 plants) in 2006. A direct comparison of the
fM values, stable carbon isotope ratios, and aerosol AAEs suggested that the
wavelength dependence of the aerosol absorption was controlled by the
biogenically derived aerosol components.
Citation: Marley, N. A., Gaffney, J. S., Tackett, M., Sturchio, N. C., Heraty, L., Martinez, N., Hardy, K. D., Marchany-Rivera, A., Guilderson, T., MacMillan, A., and Steelman, K.: The impact of biogenic carbon sources on aerosol absorption in Mexico City, Atmos. Chem. Phys., 9, 1537-1549, doi:10.5194/acp-9-1537-2009, 2009.