Implication of Planetary Boundary Layer Variation and Meteorology on Air Quality in Mexico City during the Dry Season

The diurnal evolution of the planetary boundary layer (PBL) is crucial to air quality as it impacts the exchange and distribution of pollutants close to the surface. The emissions of pollutants near the surface contribute to poor air quality during stable atmospheric conditions (calm winds and cool temperature) where the pollutants accumulate near the surface and can rise to unhealthy levels within the PBL. Meteorology impacts ozone (O₃) concentrations especially during the presence of a high pressure system (associated with calm winds, warm, and clear sky conditions), which enhances the formation of O₃ due to strong insolation and limited atmospheric exchange processes. These conditions frequently occur in Mexico City, in particular during the dry warm season (March to May). These processes are enhanced due to the location of Mexico City in a high altitude basin, which often prevents proper ventilation and favors intense solar radiation for photochemical reactions.

This study presents temporally highly resolved PBL heights derived from potential temperature and humidity profiles measured continuously by a MP-3000A microwave radiometer at a monitoring site in Mexico City in conjunction with air quality data for the month of March for the years 2015-2017. The instrument was deployed and maintained for several years by SEDEMA (Secretaría del Medio Ambiente), the Environment Ministry of Mexico City. The month of March is the most representative month for the dry season in Mexico City. In particular, our study focused on March 2016, which showed the most severe smog episode for almost a decade in Mexico City with peak hourly O₃ concentrations reaching 200 ppb. The PBL heights were based on various approaches ( The results show that peak pollutant concentrations correlate well with the meteorological observations (weak winds, stable PBL) as well as air mass circulation within the Mexico basin.