Retrieval of the Sea Spray Aerosol Mode from Submicron Particle Size Distributions and Supermicron Scattering during LASIC

Abstract. Improved quantification of sea spray aerosol concentration and size is important for determining aerosol effects on clouds and climate, though accurately capturing the size distribution of the sea spray mode remains limited by the availability of supermicron size distributions. In this paper we introduce a new approach to retrieve lognormal mode fit parameters for a sea spray aerosol mode by combing 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 3-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 look-up table of Mie theory-simulated scattering coefficients for low error solutions that minimized the 0.4–1 µm residual in the UHSAS size distribution. The UHSAS-NEPH method retrieved sea spray mode properties for approximately 95 % of background periods during LASIC when scattering variability was low and particle concentrations were typical of the clean marine boundary layer (< 400 cm−3), ranging from 0.6 to 1.6 µm in mass mean diameter (1.3 ± 0.15 µm), 1.2 to 3.7 in modal width (2.2 ± 0.2), and mass concentration of 0.13 to 20.7 µg m−3 (6.6 ± 3.5 µg m−3). The measured nephelometer scattering at 3 wavelengths was found to only marginally constrain the mode width 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. We also show that measured supermicron size distributions are needed to adequately characterize the sea spray number concentration, though mass concentration can be comparably characterized using the supermicron scattering. 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.