Vertical Profiling of Aerosols With a Combined Raman‐Elastic Backscatter Lidar in the Remote Southern Ocean Marine Boundary Layer (43–66°S, 132–150°E)

Abstract

A combined Raman-elastic backscatter lidar, deployed aboard the research vessel RV Investigator for two campaigns for a total of 10 week's ship time, is used to quantify the properties of aerosols within the remote Southern Ocean marine boundary layer between Australia and Antarctica in the region 43-66 • S and 132-150 • E. Eleven Raman case studies are identified for analyses. Particle linear depolarization ratio and height-resolved lidar ratio S, calculated from the Raman retrievals, are consistent with values expected within the surface mixed layer for clean marine conditions. We determine S = (19 ± 7) sr across the Southern Ocean with the Raman lidar observations. Aerosol optical properties in the marine boundary layer close to Tasmania (43 • S) sometimes indicate the influence of continental air masses. Aerosol optical depth at 355 nm calculated from the retrieved Raman extinction profiles within the surface mixed layer is = (0.11 ± 0.04). Boundary-layer height is determined from the lidar observations and decreases from (0.9 ± 0.4) km north of the Polar Front (around 55 • S) to (0.7 ± 0.2) km south of the Polar Front. Dried sea salt is present above the midlatitude ocean in the dehumidified decoupled layer in different synoptic-scale atmospheric conditions including beneath a high-pressure system and in a post-frontal air mass. At all latitudes across the Southern Ocean, large aerosol backscatter, low depolarization ratio, and high relative humidity indicate the presence of sea salt droplets within the well-mixed near-surface layer.

Southern Ocean Cloud Properties Derived From CAPRICORN and MARCUS Data

“…Both systems provide an elastic backscatter and depolarization channel. While the RMAN system provides a Raman scattering channel, the system's sensitivity is such that long integration times in cloud‐free tropospheric air are required for calibration (Alexander & Protat, 2019). In our earlier work (MP18b), we relaxed the profile to theoretical Rayleigh profiles from clear sky nights.…”

Study on variations in lidar ratios for Shanghai based on Raman lidar

“…Oklahom, America Continental 68±12 (Ferrare et al, 2001) Tokyo, Japan Dust 49 (Murayama et al, 2004) Anmyeon (Alexander and Protat, 2019) https://doi.org/10.5194/acp-2020-1162 Preprint. Discussion started: 14 November 2020 c Author(s) 2020.…”

Polarimetric Backscatter Sonde Observations of Southern Ocean Clouds and Aerosols

“…The Southern Ocean is often considered as a "pristine" region (e.g., [6,7]), where aerosols are purely natural, having no anthropogenic component, or that they are mainly of marine origin and have no continental source (e.g., dust or smoke). Measuring aerosols in pristine environments is important for providing a baseline for resolving uncertainties in the impact of anthropogenic aerosols on cloud albedo and the earth's radiation budget [8], as well as their direct radiative effects, though continental aerosols do sometimes intrude into this region [9].…”

“…The structure of the marine boundary layer will also affect the vertical distribution of aerosols so that measurements of the aerosol distribution serve as a probe for unmixed layers within the boundary layer. Sulphate aerosol is a major component of concentrations of total aerosol (CCN) [10], and vertical profiles of aerosols could be useful in elucidating pathways for oxidation of biogenic dimethyl sulphide to sulphate [7,11].…”

“…That the background attributed to aerosols shows a significant amount of cross-polarised scattering signal in both the lin90 and lin45 channels indicates that the particles are not spherical and therefore solid. The literature on aerosols in this region generally highlights CCN (mostly sea salt and sulphate [6,7,25,26] and a dearth of ice nuclei [27,28]. The aerosol background for this profile below 2000 m cannot be due to solid sea salt or sulphate (sulphuric acid or ammonium sulphate), as these are deliquescent and the relative humidity is near saturation [29].…”