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.