Atmos. Chem. Phys. Discuss., 7, 1623-1653, 2007
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
On the diurnal variability of particle properties related to black carbon in Mexico City
D. Baumgardner1, G. L. Kok2, and G. B. Raga1
1Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, México
2Droplet Measurement Technologies, México

Abstract. The black carbon mass (BCM) of individual, internally mixed aerosol particles was measured with the Single Particle Soot Photometer (SP2) in April of 2003 and 2005. The average BCM, single particle BC mass fraction and BCM equivalent diameter were evaluated with respect to concentrations of carbon monoxide (CO), particle bound polycyclic aromatic hydrocarbons (PPAH) and condensation nuclei (CN). The BCM and CO have matching diurnal trends that are linked to traffic patterns and boundary layer growth. The PPAH reaches a maximum at the same hour as CO and BCM but returns rapidly back to nighttime values within three hours of the peak. The number of particles containing BCM ranges between 10% to 40% of all particles between 200 nm and 700 nm and the BCM is between 4% and 12% of the total mass in this size range. The average BC equivalent mass diameter varies between 300 and 400 nm and reaches its daily minimum value when BCM is a maximum. The BC particles have the thinnest coating of non-light absorbing material during periods of maximum BCM. The scattering and absorption coefficients, Bscat and Babs , derived from the SP2 measurements were compared with direct measurements from a nephelometer and soot photometer. The measured and derived Babs are in close agreement whereas the Bscat comparisons show larger discrepancies in absolute value and daily trends. Even though approximately 40% of the BCM is in particles with diameters smaller than 200 nm, the extinction coefficient is dominated by the BCM in particles larger than this size. The BCM contributes up to 20% of the total extinction in this size range. BCM is emitted at a rate of 1200 metric tons per year in Mexico City, based upon the SP2 measurements and correlations between BCM and CO.

Citation: Baumgardner, D., Kok, G. L., and Raga, G. B.: On the diurnal variability of particle properties related to black carbon in Mexico City, Atmos. Chem. Phys. Discuss., 7, 1623-1653, doi:10.5194/acpd-7-1623-2007, 2007.
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