Retrieval of the sea spray aerosol mode from submicron particle size distributions and supermicron scattering during LASIC
Authors: Dedrick Jeramy L. , Saliba Georges , Williams Abigail S. , Russell Lynn M. , Lubin Dan 
Published: July 2022

Abstract. Improved quantification of sea spray aerosol
concentration and size is important for determining aerosol effects on
clouds and the climate, though attempts to accurately capture the size distribution of the
sea spray mode remain limited by the availability of supermicron size
distributions. In this work, we introduce a new approach to retrieving
lognormal mode fit parameters for a sea spray aerosol mode by combining
submicron size distributions with supermicron scattering measurements using
a Mie inversion. Submicron size distributions were measured by an ultra-high-sensitivity aerosol spectrometer (UHSAS), and supermicron scattering was
taken as the difference between <10 µm and <1 µm three-wavelength integrating nephelometer measurements (NEPH). This
UHSAS-NEPH method was applied during background marine periods of the
Department of Energy Atmospheric Radiation Measurement Layered Atlantic
Smoke Interactions with Clouds (LASIC) campaign on Ascension Island
(November 2016–May 2017), when the contribution of sea spray aerosol was
expected to represent a large fraction of the aerosol mass and total
scattering. Lognormal sea spray modal parameters were retrieved from
comparisons between nephelometer measurements and a lookup table of Mie
theory-simulated scattering coefficients for low-error solutions that
minimized the 0.4–1 µm residual in the UHSAS size distribution. We
evaluated the UHSAS-NEPH method with a set of clean marine measurements in
the North Atlantic that included supermicron size and chemical measurements,
showing that measured supermicron size distributions are needed to constrain
the sea spray number concentration but that mass concentration was reasonably
characterized using supermicron scattering. For LASIC, the UHSAS-NEPH method
retrieved sea spray mode properties for approximately 88 % of the background
marine times when the scattering variability and total particle
concentration were low (<± 5 Mm−1 and <400 cm−3, respectively), with mass mean
diameter ranging from 0.6 to 1.9 µm (1.47 ± 0.17 µm), modal width ranging from 1.1 to 3.97 (2.4±0.3), and mass concentration ranging from 0.18 to 23.0 µg m−3
(8.37. ± 4.1 µg m−3). The measured nephelometer scattering
at three wavelengths was found to constrain the mode width marginally at the
largest particle sizes in the absence of additional size and chemical
measurements for defining parameters for the Mie solutions. Comparing
UHSAS-NEPH retrievals to those of a fitting algorithm applied only to the
submicron UHSAS number size distribution showed that correlations between
retrieved mass concentration and the available mass-based sea spray tracers
(coarse scattering, wind speed, and chloride) are low when supermicron
measurements are not considered. This work demonstrates the added value of
supermicron scattering measurements for retrieving reasonable sea spray mass
concentrations, providing the best-available observationally constrained
estimate of the sea spray mode properties when supermicron size distribution
measurements are not available.