References

ACPD

Atmospheric Chemistry and Physics Discussions

ACPD

1680-7375

Copernicus GmbH

Göttingen, Germany

10.5194/acpd-9-9081-2009

Black carbon over Mexico: the effect of atmospheric transport on mixing state, mass absorption cross-section, and BC/CO ratios

Subramanian

¹ Kok

G. L.

¹ Baumgardner

² Clarke

³ Shinozuka

³ Campos

T. L.

⁴ Heizer

C. G.

⁴ Stephens

B. B.

⁴

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

06 04 2009

9 2 9081 9115

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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/m3) 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/COnet ratio of 3.1 (ng/m3 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 m2g−1 (±30%) at 660 nm (or 12.4 m2g−1 @ 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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