Abstract: This presentation will travel back to some of the key studies in urban air pollution carried out by our research group at UNAM since 1997 as background for the recent studies in aerosol-cloud interactions at local and regional scales. While urban pollution in Mexico City has declined since the late 1980’s, fine particles and ozone have remained fairly constant over the last few years, exceeding the national standards AQ and WHO guidelines. The results of the recent project ADABBOY: African Dust and Biomass Burning over the Yucatan will provide a more regional view, introducing non-urban sources of aerosols that affect Mexico and discuss insights on how they can act as nucleating particles for droplets and ice crystals in clouds. The final segment of the presentation will cover the ongoing research on how fine particles from biomass burning led to an event of unusual deep convection that resulted in hail in Mexico City during a week of severely degraded air quality associated with regional biomass burning. Results from measurements and regional sensitivity modelling will assess the role of ice nucleating particles sampled during the pollution event on the development and evolution of precipitation.