Retrieval of the sea spray aerosol mode from submicron particle size distributions and supermicron scattering during LASIC

Abstract

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.

Aerosol‐Correlated Cloud Activation for Clean Conditions in the Tropical Atlantic Boundary Layer During LASIC

“…Two‐hour averaged aerosol measurements from the Department of Energy Atmospheric Radiation Measurement Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign on Ascension Island (8°S, 14.5°W, 360 m ASL) (Zuidema et al., 2018) provided particle concentration and composition measurements (Text S1, Table S2, Figures S1, S2 in Supporting Information ), of which we classified ∼40% as “clean” and the remaining 60% as “smoky” using criteria previously developed (Dedrick et al., 2022a) (Text S2, Figures S3, S4 in Supporting Information ). Submicron aerosol size distribution measurements were fit with three modes comprising of Aitken, accumulation, and sea‐spray modes, with the latter constrained by supermicron nephelometer scattering during clean conditions (Text S3, Figure S5 in Supporting Information ).…”

Aerosol Size Distribution Properties Associated with Cold-Air Outbreaks in the Norwegian Arctic

“…The linear regression line is shown in solid black. Modini et al (2015), as in Dedrick et al (2022a). Three lognormal modes were then fit to the merged size distributions at both sites using an automated algorithm that calculates the modal number concentration (N), geometric mean diameter (D g ), and geometric standard deviation (σ g ) as parameters of the fitted distribution Dedrick et al (2022a); Modini et al (2015); Saliba et al (2019).…”

Biomass-burning smoke properties and its interactions with marine stratocumulus clouds in WRF-CAM5 and southeastern Atlantic field campaigns

“…3b-c), suggesting that its emergence at ASI is driven not by smoke. The likely source is sea spray in the MBL (Clarke et al, 1998;Dedrick et al, 2022;Saliba et al, 2019). A caveat in this dataset is that the LASIC ARM emplacement was within ~500 meters of a sea cliff, where winds and breaking waves may represent a large, localized particle source that is much less influential elsewhere in the SEA BL.…”