Aerosol Vertical Distributions Detected By an HSRL during ARM SAIL Campaign

Understanding the vertical and temporal distributions of aerosols is crucial for accurately quantifying their radiative effects and improving our process-level understanding of aerosol-cloud interactions. To provide this valuable information to Atmospheric Radiation Measurement (ARM) program users and the broader scientific community, we developed an aerosol layer feature identification Value-Added Product (VAP) utilizing ARM's High Spectral Resolution Lidar (HSRL) measurements based on Thorsen et al. (2015) method. Statistical results show improved detection accuracy compared to the ARM Raman Lidar (RL) RLPROF-FEX VAP.

We applied this aerosol identification technique to HSRL measurements collected during the ARM Surface Atmosphere Integrated Field Laboratory (SAIL) campaign near Crested Butte, Colorado, from September 2021 to June 2023. Preliminary analysis reveals pronounced seasonal variations in aerosol occurrence frequency. Aerosol layers are most frequently observed in summer, with an occurrence frequency exceeding 95% 2 km above the ARM site (~5 km above mean sea level), which diminishes exponentially with height. In contrast, winter exhibits the lowest detectable aerosol layers, with an occurrence frequency of less than 50% near the surface.

To further understand these patterns, we classified aerosol layers into two categories: elevated (aerosol layer base height above the planetary boundary layer height - PBLH) and near-surface (aerosol layer top height below the PBLH). Elevated aerosol layers are most common in spring, presumably due to the transport of smoke into the region, while they are least frequent in the fall.

Through back trajectory analysis, we observed that both elevated and near-surface aerosol layers exhibit an increase in measured lidar color ratio (CR) with increasing mean relative humidity (RH) along the trajectory, while the lidar linear depolarization ratio (LDR) decreases with increasing RH. This suggests the presence of larger and more spherical particles in higher RH air masses, likely due to hygroscopic growth.

Our analysis highlights the capability of ARM HSRL in studying aerosol vertical distributions and characterizing aerosol properties, particularly when combined with meteorological data such as those along back trajectories.

Thorsen, T. J., Fu, Q., Newsom, R. K., Turner, D. D., & Comstock, J. M. (2015). Automated Retrieval of Cloud and Aerosol Properties from the ARM Raman Lidar. Part I: Feature Detection. Journal of Atmospheric and Oceanic Technology, 32(11), 1977-1998.