Relative Humidity Dependence of Growth Factor and Real Refractive Index for Sea Salt/Malonic Acid Internally Mixed Aerosols. Issue 6 (18th March 2023) - Liu, Huichao
Abstract: Sea salt aerosols (SSAs) often mix with organic matter via sea‐to‐air emission, photochemical oxidation, and atmospheric transport, which change particle hygroscopicity and real refractive index (RRI), thereby the radiative effects. However, the influence of water uptake and organic matters on radiative effects is not well understood. In this study, relative humidity (RH)‐dependent growth factor (GF), light scattering enhancement factor, RRI ( n mea ), and radiative forcing efficiency (RFE) in the visible spectrum of SSAs/malonic acid (MA) internally mixed aerosols were investigated. Our results showed that MA‐mediated SSAs readily absorbed water even at low RH but suppressed hygroscopic growth at high RH. The discrepancy between Zdanovskii‐Stokes‐Robinson predictions and measured GF indicated that the suppressing effect on size caused by produced malonate salts in chloride depletion became more evident as MA content increased. RRI in the visible spectrum decreased as MA got enriched and approached toward water's RRI at high RH, resulting in a less cooling effect. The increasing RH reduced RFE both by decreasing RRI and increasing particle size substantially. Ignoring the effect of MA on particle size and RRI under varied RH, errors in RFE at 525 nm up to 25% could be induced. Volume mixing rule performs well in predicting the RRI of mixed aerosols in dry conditions, while the retrieved difference of 0.061 between volume mixing RRI n vol and n mea would make the error in RFE up to 1.15 W·m −2 ·Aerosol optical depth −1 at 87% RH. Our study helps understand the impact of water content and dicarboxylic acids on the direct radiative effects of SSAs. Plain Language Summary: Sea salt aerosols often mix with organic matter during the process of generation and transportation, which changes particles' hygroscopicity and real refractive index (RRI), thereby the radiative effects. In this study, the hygroscopicity, scattering properties, RRI, and radiative forcing efficiency (RFE) in the visible spectrum of both sea salt aerosols and sea salt/malonic acid aerosols under varied relative humidity are investigated. It was found that malonic acid enhanced the water uptake of sea salt aerosols at low relative humidity but suppressed hygroscopic growth at high relative humidity. RRI of mixed aerosols decreased along with the increase in relative humidity and malonic acid content leading to a lower RFE. Ignoring the effect of malonic acid on particle size and RRI under varied relative humidity, large errors in RFE could be induced. The volume mixing rule used to calculate the RRI and thereby RFE performs well at low relative humidity but is invalid at high relative humidity. Our study helps understand the impact of water content and dicarboxylic acids on the direct radiative effects of sea slat aerosols. Key Points: The relative humidity‐dependent refractive index of submicron sea salt/malonic acid internally mixed aerosols was first retrieved The interactions between sea salt and malonic acid are responsible for the change in water uptake and refractive index It is necessary to consider the impact of relative humidity on sea salt/malonic acid mixed aerosols in climate models