The Time Evolution of Aerosol Size Distribution Over the Mexico City Plateau

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 determin- ing growth characteristics of aerosols from a megacity ur- ban 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 accu- mulation 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 ac- cumulation particles. Condensation and volume growth laws were examined to see whether either is consistent with obser- vations. 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 distribu- tion 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 infor- mation on the volatility and properties of secondary organic aerosol.