PM-10 observations and cloud characteristics in Mexico City

Mexico City is located in an elevated basin surrounded by mountains and is notorious for its bad air pollution problem, mainly high ozone concentrations and particulate matter. Pollution in the city is being monitored by an automated netwark with over 30 stations distributed around the city. Particulate matter with diameter less than 10 micrometers (PM-10) has been measured in hourly intervals systematically since 1995, using the TEOM method.

A recent study on the electrification of convective clouds published in Science (1998) indicated that the smoke from Mexican fires during the 1998 dry season was responsible for the large number of positive ground to cloud flashes observed. It is not clear what is the mechanism that would cause this to happen. In this study we want to investigate if the urban aerosol particles could have a similar effect on convective cloud characteristics. We utilize surface PM-10 data from the monitoring network and combine it with (GOES) satellite-derived cloud chracteristics. The study area was divided into 4 sections, 3 located in the urban area (labeled NW, SW and Central) and the fourth outside the city, used as a reference for cloud characteristics. The results indicate that the PM-10 frequency distributions are significantly different in the dry and rainy seasons. A subset of PM-10 are obviously cloud condensation nuclei during the rainy season and thus, the frequency distribution is modified. Furthermore, in 1998 the SW and NW regions show significantly different results for the rainy season. This result is correlated with the observation of high electrical activity in the same region observed by the LIS sensor in TRMM by Smith and Baker (personal communication, 1999), as part as a ongoing collaborative study between the University of Washington and UNAM.