Atmospheric Chemistry and Physics Discussions
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10.5194/acpd-9-9081-2009
http://www.atmos-chem-phys-discuss.net/9/9081/2009/
http://www.atmos-chem-phys-discuss.net/9/9081/2009/acpd-9-9081-2009.html
http://www.atmos-chem-phys-discuss.net/9/9081/2009/acpd-9-9081-2009.pdf
9081
9115
2009-04-06
Black carbon over Mexico: the effect of atmospheric transport on mixing state, mass absorption cross-section, and BC/CO ratios
R. Subramanian
random@dropletmeasurement.com
G. L. Kok
D. Baumgardner
A. Clarke
Y. Shinozuka
T. L. Campos
C. G. Heizer
B. B. Stephens
Droplet Measurement Technologies, Boulder, CO 80301, USA
Universidad Nacional Autónoma de México, Mexico City, Mexico
University of Hawai'i, Honolulu, HI 96822, USA
National Center for Atmospheric Research, Boulder, CO 80305, USA
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 2μg/m<sup>3</sup>)
and over hill-fires to the south of the city. 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/CO<sub>net</sub> ratio of 3.1 (ng/m<sup>3</sup> STP)/ppbv (±25%). 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 10.3 m<sup>2</sup>g<sup>−1</sup> (±30%) at 660 nm (or
12.4 m<sup>2</sup>g<sup>−1</sup> @ 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. BC mass per particle of the thinly-coated
mode appears to increase as the air mass ages, possibly due to coagulation and/or increased
coating of the particles containing smaller BC masses. Differences in the coating indicator
patterns for similarly-aged air masses may be due to differences in atmospheric processing
on each day, including mixing with non-MCMA air masses.
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