Polarimeter + Lidar–Derived Aerosol Particle Number Concentration

In this study, we propose a simple method to derive vertically resolved aerosol particle number concentration (N_a) using combined polarimetric and lidar remote sensing observations. This method relies on accurate polarimeter retrievals of the fine-mode column-averaged aerosol particle extinction cross section and accurate lidar measurements of vertically resolved aerosol particle extinction coefficient such as those provided by multiwavelength high spectral resolution lidar. We compare the resulting lidar + polarimeter vertically resolved N_a product to in situ N_a data collected by airborne instruments during the NASA aerosol cloud meteorology interactions over the western Atlantic experiment (ACTIVATE). Based on all 35 joint ACTIVATE flights in 2020, we find a total of 32 collocated in situ and remote sensing profiles that occur on 11 separate days, which contain a total of 322 cloud-free vertically resolved altitude bins of 150 m resolution. We demonstrate that the lidar + polarimeter N_a agrees to within 106% for 90% of the 322 vertically resolved points. We also demonstrate similar agreement to within 121% for the polarimeter-derived column-averaged N_a. We find that the range-normalized mean absolute deviation (NMAD) for the polarimeter-derived column-averaged N_a is 21%, and the NMAD for the lidar + polarimeter-derived vertically resolved N_a is 16%. Taken together, these findings suggest that the error in the polarimeter-only column-averaged N_a and the lidar + polarimeter vertically resolved N_a are of similar magnitude and represent a significant improvement upon current remote sensing estimates of N_a.