The T1-T2 study: evolution of aerosol properties downwind of Mexico City J. C. Doran1, J. C. Barnard1, W. P. Arnott2, R. Cary3, R. Coulter4, J. D. Fast1, E. I. Kassianov1, L. Kleinman5, N. S. Laulainen1, T. Martin4, G. Paredes-Miranda2, M. S. Pekour1, W. J. Shaw1, D. F. Smith3, S. R. Springston5, and X.-Y. Yu1 1Pacific Northwest National Laboratory, Richland, WA, USA 2Desert Research Institute, Reno, NV, USA 3Sunset Laboratory, Inc., Tigard, OR, USA 4Argonne National Laboratory, Argonne, IL, USA 5Brookhaven National Laboratory, Upton, NY, USA
Abstract. As part of a major atmospheric chemistry and aerosol field program carried
out in March 2006, a study was conducted in the area to the north and
northeast of Mexico City to investigate the evolution of aerosols and their
associated optical properties in the first few hours after their emission.
The focus of the T1-T2 aerosol study was to investigate changes in the
specific absorption αABS (absorption per unit mass, with unit
of m2 g−1) of black carbon as it aged and became coated with
compounds such as sulfate and organic carbon, evolving from an external to
an internal mixture. Such evolution has been reported in previous studies.
The T1 site was located just to the north of the Mexico City metropolitan
area; the T2 site was situated approximately 35 km farther to the northeast.
Nephelometers, particle soot absorption photometers, photoacoustic
absorption spectrometers, and organic and elemental carbon analyzers were
used to measure the optical properties of the aerosols and the carbon
concentrations at each of the sites. Radar wind profilers and radiosonde
systems helped to characterize the meteorology and to identify periods when
transport from Mexico City over T1 and T2 occurred. Organic and elemental
carbon concentrations at T1 showed diurnal cycles reflecting the nocturnal
and early morning buildup from nearby sources, while concentrations at T2
appeared to be more affected by transport from Mexico City. Specific
absorption during transport periods was lower than during other times,
consistent with the likelihood of fresher emissions being found when the
winds blew from Mexico City over T1 and T2. The specific absorption at T2
was larger than at T1, which is also consistent with the expectation of more
aged particles with encapsulated black carbon being found at the more
distant location. In situ measurements of single scattering albedo with an
aircraft and a ground station showed general agreement with column-averaged
values derived from rotating shadowband radiometer data, although some
differences were found that may be related to boundary-layer evolution.
Citation: Doran, J. C., Barnard, J. C., Arnott, W. P., Cary, R., Coulter, R., Fast, J. D., Kassianov, E. I., Kleinman, L., Laulainen, N. S., Martin, T., Paredes-Miranda, G., Pekour, M. S., Shaw, W. J., Smith, D. F., Springston, S. R., and Yu, X.-Y.: The T1-T2 study: evolution of aerosol properties downwind of Mexico City, Atmos. Chem. Phys., 7, 1585-1598, doi:10.5194/acp-7-1585-2007, 2007.