ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-4261-2009 The time evolution of aerosol size distribution over the Mexico City plateau Kleinman L. I. 1 Springston S. R. 1 Wang J. 1 Daum P. H. 1 Lee Y.-N. 1 Nunnermacker L. J. 1 Senum G. I. 1 Weinstein-Lloyd J. 2 Alexander M. L. 3 Hubbe J. 3 Ortega J. 3 Zaveri R. A. 3 Canagaratna M. R. 4 Jayne J. 4 Brookhaven National Laboratory, Upton, New York, USA SUNY, Old Westbury, NY, USA Pacific Northwest National Laboratory, Richland, WA, USA Aerodyne Research Inc., Billerica, MA, USA 03 07 2009 9 13 4261 4278 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/9/4261/2009/acp-9-4261-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/4261/2009/acp-9-4261-2009.pdf

As part of the MILAGRO field campaign, the DOE G-1 aircraft was used to make measurements over and downwind of Mexico City with the objective of determining growth characteristics of aerosols from a megacity urban source. This study focuses on number concentration and size distributions. It is found that a 5-fold increase in aerosol volume is accompanied by about a 5-fold increase in accumulation mode number concentration. There is growth in aerosol volume because there are more accumulation mode particles, not because of an increase in the average size of accumulation particles. Condensation and volume growth laws were examined to see whether either is consistent with observations. Condensation calculations show that the growth of Aitken mode particles into the accumulation mode size range gives the required increase in number concentration. There are minimal changes in the accumulation mode size distribution with age, consistent with observations. Volume-growth in contrast yields a population of large particles, distinctly different from what is observed. Detailed model calculations are required to translate our observations into specific information on the volatility and properties of secondary organic aerosol.

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