Estimating the Lidar Ratios of Modeled Aerosol Species to Help Inform Selection for Clean Marine Aerosol Lidar Ratios in a Future CALIPSO Data Products Release

[11-Jan-2023] Abstract  The NASA Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument, aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite, has collected vertical observations of aerosols and clouds in the atmosphere since 2006. As an elastic backscatter lidar (EBL), assumptions of aerosol lidar ratios (i.e., extinction-to-backscatter ratios) are generally necessary to retrieve vertical profiles of aerosol extinction and subsequently column-integrated aerosol optical depth (AOD). In a future data release, the CALIPSO project aims to improve the current lidar ratio selection algorithm for seven tropospheric aerosol types. In this study, aerosol lidar ratios are inferred through CALIOP backscatter profiles constrained by collocated AOD from Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) datasets in a Fernald inversion method. Our analysis is subsampled for only those profiles that are cloud-free and contain clean marine aerosols (based on CALIOP feature classification). In addition, we collocate the CALIOP profiles with the aerosol volume fractions simulated by the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model. Preliminary results for the 2006-2017 period reveal smaller sea salt volume fractions and larger lidar ratios near coastlines. This is likely due to the influence of aerosols in these regions from over-land sources (e.g., pollution and smoke). In contrast, we find larger sea salt volume fractions and smaller lidar ratios in the remote oceans, in agreement with more pristine conditions. We then assume the lidar ratios can be empirically represented as a volume average of the lidar ratio for each modeled aerosol component. Assuming all clean marine aerosol layers are composed of only two aerosol components (sea salt and non-sea salt), we use an empirical relationship to estimate the lidar ratios of these two modeled aerosol components. For 2006-2017, we find a preliminary global sea salt lidar ratio of ~31 sr and non-sea salt lidar ratio of ~74 sr. While these initial values are larger than anticipated, our current work investigates the impact of stratospheric AOD and free tropospheric background aerosols on the derived lidar ratios. The aim of this study is to improve clean marine aerosol lidar ratio selections in the CALIOP algorithms and provide important lidar ratio information for future space-based lidars, including the EBL proposed for the NASA Atmosphere Observing System (AOS) satellite mission.