Optical Properties of Prebiotic Seawater Analogs and Sea-Spray Aerosols

Abstract

Optical constants for plausible Hadean seawater and seawater-derived marine aerosols are needed to constrain early Earth radiative transfer and surface photochemistry, yet they remain poorly characterized. Here we report the wavelength-, temperature-, and water-activity-dependent complex refractive index, m(λ)=n(λ)+ik(λ), for modern seawater and three model prebiotic seawater compositions chosen to represent the leading hypotheses for primordial ocean chemistry, including sulfate-depleted, chloride-dominated analogs and a Ca-enriched, crust-controlled analog. Using single-particle measurements via optical trapping (OT) and an electrodynamic balance (EDB), we retrieve n and quantify weak visible absorption (k on the order of 10^-9) across water activities relevant to concentrated sea-spray aerosol and near-surface waters. Complementary bulk refractometry extends the dataset to high water activity and provides the temperature dependence of n from 10–70 °C. The measurements are consistent with an effective-oscillator description for weakly absorbing aqueous aerosols. EDB measurements additionally confirm that the three prebiotic solutions exhibit similar hygroscopic growth, supporting their use as controlled analogs for seawater-derived aerosol particles. Together, these results provide physically grounded inputs for paleoclimate radiative-transfer calculations and for representing the optical impacts of marine aerosols generated from hypothesized early Earth seawater.