Insights into the real part of natural sea spray aerosol refractive index in the Pacific Ocean

Abstract

Sea spray aerosols (SSAs) play a pivotal role in influencing radiative effects over oceanic regions, making it essential to accurately quantify their optical properties, particularly the real part of the refractive index (RRI) under varying relative humidity (RH) conditions. This study employs an aerosol optical tweezer (AOT) system coupled with Mie scattering theory to precisely measure the RRI of sea spray aerosols at 650 nm across a range of RH levels. First, standard ammonium sulfate particles were used to validate the AOT measurements against thermodynamic models and previously established parameterizations, confirming the reliability of the optical tweezers' measurements. Measurements of SSA from offshore and open-sea samples show consistent RRI values, independent of seawater salinity, with artificial sea salt particles effectively representing the optical properties of real SSA at RH > 70 %. A least-squares linear regression scheme linking RRI and RH was developed, allowing for accurate RRI estimation under varying RH conditions. Additionally, our scheme's intercept at RH = 0 potentially represents the molten-state RRI for sea spray aerosols, validated against standard particles. Results highlight that traditional volume-weighted mixing rules underestimate RRI and aerosol optical depth (AOD), thus reinforcing the need for direct measurement-based parameterizations. This study underscores the importance of accurately representing sea spray aerosols' radiative properties in climate models. We suggest incorporating the proposed linear regression scheme into aerosol and radiative transfer models to improve model accuracy and enhance the understanding of the effects of sea spray aerosols on radiative processes.