Atmos. Chem. Phys., 10, 219-237, 2010
www.atmos-chem-phys.net/10/219/2010/
doi:10.5194/acp-10-219-2010
© Author(s) 2010. This work is distributed
under the Creative Commons Attribution 3.0 License.
Black carbon over Mexico: the effect of atmospheric transport on mixing state, mass absorption cross-section, and BC/CO ratios
R. Subramanian1, G. L. Kok1, D. Baumgardner2, A. Clarke3, Y. Shinozuka3, T. L. Campos4, C. G. Heizer4, B. B. Stephens4, B. de Foy5, P. B. Voss6, and R. A. Zaveri7
1Droplet Measurement Technologies, Boulder, CO 80301, USA
2Universidad Nacional Autónoma de México, Mexico City, Mexico
3University of Hawaii, Honolulu, HI 96822, USA
4National Center for Atmospheric Research, Boulder, CO 80305, USA
5St. Louis University, St. Louis, MO 63108, USA
6Smith College, Northampton, MA 01063, USA
7Pacific Northwest National Laboratory, Richland, WA 99352, USA

Abstract. A single particle soot photometer (SP2) was operated on the NCAR C-130 during the MIRAGE campaign (part of MILAGRO), sampling black carbon (BC) over Mexico. The highest BC concentrations were measured over Mexico City (sometimes as much as 2 μg/m3) and over hill-fires to the south of the city. The age of plumes outside of Mexico City was determined using a combination of HYSPLIT trajectories, WRF-FLEXPART modeling and CMET balloon tracks. As expected, older, diluted air masses had lower BC concentrations. A comparison of carbon monoxide (CO) and BC suggests a CO background of around 65 ppbv, and a background-corrected BC/COnet ratio of 2.89±0.89 (ng/m3-STP)/ppbv (average ± standard deviation). This ratio is similar for fresh emissions over Mexico City, as well as for aged airmasses. Comparison of light absorption measured with a particle soot absorption photometer (PSAP) and the SP2 BC suggests a BC mass-normalized absorption cross-section (MAC) of 10.9±2.1 m2/g at 660 nm (or 13.1 m2/g @ 550 nm, assuming MAC is inversely dependent on wavelength). This appears independent of aging and similar to the expected absorption cross-section for aged BC, but values, particularly in fresh emissions, could be biased high due to instrument artifacts. SP2-derived BC coating indicators show a prominent thinly-coated BC mode over the Mexico City Metropolitan Area (MCMA), while older air masses show both thinly-coated and thickly-coated BC. Some 2-day-old plumes do not show a prominent thickly-coated BC mode, possibly due to preferential wet scavenging of the likely-hydrophilic thickly-coated BC.

Citation: Subramanian, R., Kok, G. L., Baumgardner, D., Clarke, A., Shinozuka, Y., Campos, T. L., Heizer, C. G., Stephens, B. B., de Foy, B., Voss, P. B., and Zaveri, R. A.: Black carbon over Mexico: the effect of atmospheric transport on mixing state, mass absorption cross-section, and BC/CO ratios, Atmos. Chem. Phys., 10, 219-237, doi:10.5194/acp-10-219-2010, 2010.
 
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