Some aspects of boundary layer evolution in Mexico City

Measurements of chemical species and meteorological parameters were made at a site located 440 m above the mean basin level of Mexico City, over a two -week period in November during Project Azteca. Data from three of the stat ions of Mexico City's air quality monitoring network (Red Automatica de Mon itoreo Ambiental, RAMA) were also used to estimate the dilution in concentr ation experienced by pollutants as they are transported upslope during the course of the day. Both carbon monoxide and sulfur dioxide show a dilution of up to 50%, while ozone is usually more concentrated at the elevated sire . These comparisons clearly highlight the intrinsic differences between pri mary and secondary gases, which are supported also by time-space, cross cor relation analysis. The thermal mesoscale wind circulation dominates concent rations of pollutants at the research site: upslope during the day and down slope during the night. The data present clear evidence that downslope flow s during the night contribute to stone concentration at basin sites. (C) 19 99 Elsevier Science Ltd. All rights reserved.