On the evolution of aerosol properties at a mountain site above Mexico City

Size distributions, scattering and absorption coefficients, and the bulk ch emical 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 perio d 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 ae rosols between Aitken and accumulation mode sizes. Relative humidity was al so found to affect this partitioning of aerosol size and volume. In additio n, the fraction of sulfate in the aerosols was much higher on a high-humidi ty day than on a very low humidity day; however, the fraction of the mass c ontained in organic and elemental carbon was the same regardless of humidit y levels. The daily variations of aerosol properties are associated with th e arrival of new particles at the research site transported from the city b asin and their subsequent mixture with aged aerosols that remain in the res idual layer from the night before.