Aerosol transport and associated boundary layer thermodynamics under contrasting synoptic conditions over a semiarid site.

Abstract

Understanding the kinematics of aerosol horizontal transport and vertical mixing near the surface, within the atmospheric boundary layer (ABL), and in the overlying free troposphere (FT) is critical for various applications, including air quality and weather forecasting, aviation, road safety, and dispersion modeling. Empirical evidence of aerosol mixing processes within the ABL during synoptic-scale events over arid and semiarid regions (i.e., drylands) remains sparse. We explored how synoptic-scale weather systems impact aerosol mixing processes within the daytime ABL over a site located in a dryland. We used ground-based Doppler lidar measurements collected during three events: a cold-front passage, a fair-weather day, and a dryline passage over Lubbock, Texas. The measurements of backscatter and vertical velocity fields were obtained with temporal and vertical resolutions of 1 s and 60 m, respectively. Here, we documented observations of aerosol transport and mixing within the ABL and found that frontal passages are crucial for understanding ABL features and aerosol mixing processes. For example, our findings suggest that during a dryline passage yielding a water vapor mixing ratio drop of 10 g kg^-1, the boundary layer characteristics transition from being shallow and stratified throughout a stable, pre-dryline ABL aerosol regime (300 m deep) to a deep and well-mixed structure within the post-dryline ABL (2200 m deep) confirming a higher ABL depth growth rate (∼300 mh^-1) than under quiescent conditions (∼125 m h^-1). The results for the frontal case reported aerosol mixing via frontal lifting to an altitude of 1250 m from the ground due to strong updrafts (>7 m s^-1). Additionally, Doppler lidar measurements helped to characterize the aerosol mixing and transport processes in dry regions under different weather conditions which yielded close correspondence with the observed variability in near-surface particulate matter (i.e., PM_2.5) concentrations (e.g., increase in PM_2.5 from 9 μg m^-3 to 27 μg m^-3 due to a cold front passage). The aerosol transport, along with the derived properties of the mean up- and downdraft observations and variance-based (both vertical velocity and aerosol backscatter) turbulence profiling helped explain how frontal airmass exchanges impact aerosol loading near the surface. The results obtained emphasize the need to consider the impact of synoptic-scale events over drylands in both observational and atmospheric modeling studies.