Airborne HSRL-2 measurements of elevated aerosol depolarization associated with non-spherical sea salt

Abstract

Airborne NASA Langley Research Center (LaRC) High Spectral Resolution Lidar-2 (HSRL-2) measurements acquired during the recent NASA Earth Venture Suborbital-3 (EVS-3) Aerosol Cloud Meteorology Interactions over the Western Atlantic Experiment (ACTIVATE) revealed elevated particulate linear depolarization associated with aerosols within the marine boundary layer. These observations were acquired off the east coast of the United States during both winter and summer 2020 and 2021 when the HSRL-2 was deployed on the NASA LaRC King Air aircraft. During 20 of 63 total flight days, particularly on days with cold air outbreaks, linear particulate depolarization at 532 nm exceeded 0.15–0.20 within the lowest several hundred meters of the atmosphere, indicating that these particles were non-spherical. Higher values of linear depolarization typically were measured at 355 nm and lower values were measured at 1,064 nm. Several lines of evidence suggest that these non-spherical particles were sea salt including aerosol extinction/backscatter ratio (“lidar ratio”) values of 20–25 sr measured at both 355 and 532 nm by the HSRL-2, higher values of particulate depolarization measured at low (< 60%) relative humidity, coincident airborne in situ size and composition measurements, and aerosol transport simulations. The elevated aerosol depolarization values were not correlated with wind speed but were correlated with salt mass fraction and effective radius of the aerosol when the relative humidity was below 60%. HSRL-2 measured median particulate extinction values of about 20 Mm−1 at 532 nm associated with these non-spherical sea salt particles and found that the aerosol optical depth (AOD) contributed by these particles remained small (0.03–0.04) but represented on average about 30%–40% of the total column AOD. Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) spaceborne lidar aerosol measurements during several cold air outbreaks and CALIOP retrievals of column aerosol lidar ratio using column AOD constraints suggest that CALIOP operational aerosol algorithms can misclassify these aerosols as dusty marine rather than marine aerosols. Such misclassification leads to ∼40–50% overestimates in the assumed lidar ratio and in subsequent retrievals of aerosol optical depth and aerosol extinction.

Optical Characterization of Marine Aerosols Using a Morphologically Realistic Model With Varying Water Content: Implications for Lidar Applications and Passive Polarimetric Remote Sensing

“…This can be understood in terms of an increase in the emission flux with wind speed, and an increase in water content with increasing relative humidity (which makes the particles more spherical, thus less depolarizing). A similar relation between depolarization and relative humidity in marine aerosols has been reported from lidar campaign measurements (Haarig et al, 2017) and from airborne campaigns (Ferrare et al, 2023). This illustrates the potential usefulness of satellite data in studying emission and water adsorption processes of marine aerosols.…”

Retrievals of aerosol optical depth over the western North Atlantic Ocean during ACTIVATE

“…The Second Generation High Spectral Resolution Lidar (HSRL-2) is an airborne multi-wavelength instrument which uses the HSRL technique to separate the aerosol and molecular backscatter signals at 355 and 532 nm and the standard backscatter lidar technique at 1064 nm. HSRL-2 can vertically resolve the backscatter coefficient β and depolarization δ at 355, 532, and 1064 nm and extinction coefficient α at 355 and 532 nm (Burton et al, 2018;Hair et al, 2008;Ferrare et al, 2023). Because of the incorporation of three backscatter and two extinction measurements, the instrument is also referred to as a "3β+2α" lidar (Burton et al, 2016).…”

“…Eight types of aerosols are classified, including dust, polluted marine, fresh smoke, smoke, urban, marine, a dusty mix, and ice. The algorithm has been shown to improve a similar scheme for the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument (Burton et al, 2013;Ferrare et al, 2023).…”

“…Therefore, the shape of a sea salt particle depends not only on the magnitude but also the history of ambient RH (Haarig et al, 2017). Ferrare et al (2023) reported that sea salt may cause high depolarization at low RH (i.e., more likely to be nonspherical) within the boundary layer for around one-third of flights during the first 2 years of ACTIVATE based on HSRL-2 and dropsonde measurements. Here we extend the result by collocating the maximum HSRL-2 depolarization at the lowest 500 m with the interpolated MERRA-2 RH for the whole campaign period (Fig.…”

Sea salt reactivity over the northwest Atlantic: an in-depth look using the airborne ACTIVATE dataset

“…For this reason, the northwest Atlantic (NWA) is an opportune region for observing and studying Cl − depletion. Cities extending along the East Coast of North America consistently emit sulfur dioxide (SO 2 ), nitrogen oxides (NO x ), and VOCs, which can oxidize to form H 2 SO 4 , nitric acid (HNO 3 ), and organic acids, respectively, while sea salt particles are ubiquitous over the region due to wave breaking (Reid et al, 2001;Ferrare et al, 2023). Occasional longrange transport from BB in Alaska, Canada, and the western United States (US; Fehsenfeld et al, 2006;Mardi et al, 2021), agricultural fires throughout the eastern and southeastern US (Jaffe et al, 2020;McCarty et al, 2007), wintertime wood burning for residential heating (Corral et al, 2021;Sullivan et al, 2019), and seasonally varying emissions from vegetation and ocean biological activity (Savoie et al, 2002;Corral et al, 2022) can also introduce acidic species to this region.…”