Planetary Boundary-Layer Properties and Implications on Air Quality in Mexico City

The planetary boundary layer (PBL) is critical for air quality because it strongly affects the concentration and dispersion of pollutants in the atmosphere. Despite the importance of the PBL, it cannot be directly measured as it can only be detected from the measured vertical profiles of temperature, moisture, and aerosol backscatter based on different retrieval mechanisms. This study investigates PBL structures i.e., residual layer (RL), stable boundary layer (SBL), and convective boundary layer (CBL) in Mexico City (CDMX) based on a composite retrieval method. First, an inter-comparison study was done for three PBL retrieval remote sensing devices i.e., microwave radiometer (MWR), mini micro pulse lidar (mini MPL), and conventional in-situ radiosondes (RS), with the aim of validating MWR-derived and mini MPL-derived PBL heights against that of RS. Both MWR and mini MPL showed excellent correlations with the RS data for RL and CBL heights, while SBL heights had modest correlation coefficients. In the second task, the MWR was used to provide a 3-year climatology of RL, SBL, and CBL heights in CDMX with the aim of investigating the variation of these PBL features and synoptic meteorology patterns, relating them to ozone (O3) exceedances. Results showed the presence of a strong, cold, north-westerly wind flow, which flushed O3 and its precursors out of the CDMX basin and suppressed PBL heights on days preceding severe O3 exceedances (≥ 130 ppb) in 2016 and 2017. However, strong, warm south-easterly winds flowing from the Pacific Ocean was evident on days preceding moderate O3 exceedances (101-120 ppb) in 2015, making it an overall cleaner year with respect to O3. In the last task, observed CBL heights were compared with those simulated by thermodynamic and encroachment models over 11 clear sky days with global radiation ≥ 700 W m-2 and wind speed ≤ 3 m s-1. The thermodynamic model showed excellent correlation of r ~ 0.94 with observed CBL heights, while the encroachment model showed poor correlation, which corresponds to days with global radiation ≤ 800 W m-2. The modeled CBL heights improved when days with maximum global radiation ≤ 800 W m-2 were not considered.