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Abstract. 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. OverviewAerosol particles in urban areas are a major issue with respect to their impact on public health, damage to the environment, and changes to regional and global climate. 1999]), but these were from filters where the particle mass was dominated by coarse mode aerosols, i.e., mechanically generated particles like dust or road debris. A more detailed analysis of aerosol physical and chemical properties, especially in the submicron size range, is necessary to understand how these aerosols form and evolve.The scarcity of information about the physical characteristics of Mexico City aerosols motivated a project in November 1997 to study the formation and growth of these particles. One objective was to acquire detailed size distributions, sizeresolved particle composition, and bulk optical properties, i.e., absorption and scattering coefficients, as the first step in the creation of a database of information on Mexico City aerosols. The complementary objective was to evaluate the relationship of aerosol properties to precursor gases like CO and SO2 and other environmental factors, e.g., water vapor, temperature, and winds. The present paper summarizes the properties of 22,243