Connecting Lidar-Derived Aerosol Hygroscopicity to Estimated CCN Concentrations during the Combined HSRL and Raman Lidar Measurement Study (CHARMS)

Lidar remote sensing of the atmosphere is a useful method to observe the physical characteristics of both aerosols and clouds, providing continuous, vertically resolved measurements both day and night. However, challenges remain on how to translate lidar observables to atmospherically and climatically relevant information. In an effort to explore emerging capabilities made possible from advanced remote sensing platforms, the Atmospheric Radiation Measurement (ARM) facility at the Department of Energy’s measurement site in northern Oklahoma implemented a novel system that leveraged the benefits of multiple types of lidar. The focus of this work is motivated by the necessity for measurements of ambient hygroscopic aerosol properties to better understand aerosol-cloud interactions and what correlations might exist between lidar observables and cloud condensation nuclei (CCN) concentrations. Here, we use vertically-resolved lidar retrievals as a function of simultaneously retrieved relative humidity (RH) for two months of data analyzed during the summer of 2015. Ground-based Raman lidar made retrievals of aerosol extinction coefficient (α) at 355 nm and water vapor mixing ratio (r_v). The RH was obtained by combining r_v with concurrent retrievals of collocated temperature from the AERI interferometer. High Spectral Resolution Lidar (HSRL) retrieved aerosol depolarization ratio and α at 532 nm. The humidification factor f(RH), defined as α at some RH normalized by α at a reference RH (here 40%), was a primary retrieval of interest. Aerosol f(RH) from lidar was compared to ground-based measurements from nephelometers and to modeled retrievals based on the submicron aerosol chemical composition. For a case study on August 2^nd, 2015, lidar and modeled retrievals of f(RH) at 85% compared well to nephelometer measurements with an RMSE = 1 (~50%) and 0.2 (~10%), respectively. Surface aerosol chemistry was determined to be a good approximator for the aerosol chemical composition within the mixed layer where the lidar retrievals of f(RH) were obtained. The ARM surface measured CCN-supersaturation spectra were used with an activation model and Doppler lidar-retrieved updraft velocity to calculate the number of activated CCN (N_d). Anti-correlations were found to exist between N_d and the lidar-retrieved dry extinction coefficient with an R² = 0.4. Regimes of updraft velocity are essential when estimating the activated CCN number from the dry extinction coefficient. Values for correlation when arranged by updraft velocity were shown to increase up to R² = 0.8. These results are for maximum updraft velocities of approximately 30 cm/s and correspond to maximum supersaturations < 0.2%. Other investigators found positive correlation between dry extinction coefficients and N_d at supersaturations of 0.4%, in apparent contrast to results presented here. However, competition for available water vapor and the number of particles in these low-supersaturation environments show the importance of these measurements for placing lidar observables and ambient conditions into correct context. These results can provide significant improvements to our understanding of aerosol-cloud interactions in ambient environments as seen by state-of-the-art lidars.

Figure 1. The aerosol extinction coefficient at 40% RH as derived by lidar f(RH) at 532 nm is plotted on the x-axis against the estimated activated CCN concentration normalized by the bin Aerosol Optical Depth (AOD) also at 40% RH. Colors represent the collocated Doppler lidar-retrieved updraft velocity. All data are for August 2^nd, 2015. Data is on a log-log scale for the x and y axes and linear scale for updraft velocity.