Size distributions, scattering and absorption coefficients, and the bulk chemical composition of aerosols have been measured at a mountain site 400 m above the southwest sector of the Mexico City basin during a two-week period in November 1997. Variations in these properties are primarily related to local meteorology, i.e., wind direction and relative humidity; however, a link was found between carbon monoxide and ozone and the partitioning of aerosols between Aitken and accumulation mode sizes. Relative humidity was also found to affect this partitioning of aerosol size and volume. In addition, the fraction of sulfate in the aerosols was much higher on a high-humidity day than on a very low humidity day; however, the fraction of the mass contained in organic and elemental carbon was the same regardless of humidity levels. The daily variations of aerosol properties are associated with the arrival of new particles at the research site transported from the city basin and their subsequent mixture with aged aerosols that remain in the residual layer from the night before. ¿ 2000 American Geophysical Union