[A-AS11] Atmospheric Chemistry

[AAS11-P05] Contribution of Sea Spray Aerosols to a Large Low Bias in Aerosol Number Concentrations over the summertime Southern Ocean

*Tatsuya Fukuta¹, Sakiko Ishino¹, Haruki Sato¹, Hélène Angot^2,3, Betty Croft⁴, Jeffrey Pierce⁵, Linia Tashmim⁶, William Porter⁶, Julia Schmale² (1.School of Geosciences and Civil Engineering, Kanazawa University, 2.Extreme Environments Research Laboratory, École Polytechnique Fédérale de Lausanne, Switzerland, 3.Univ. Grenoble Alpes, CNRS, INRAE, IRD, Grenoble INP, IGE, France, 4.Department of Physics and Atmospheric Sciences, Dalhousie University, Canada, 5.Department of Atmospheric Science, Colorado State University, USA, 6.Department of Environmental Sciences, University of California, Riverside, USA)

Keywords:Southern Ocean, GEOS-Chem-TOMAS, CCN, Sea spray aerosols

During the summer, the Southern Ocean (SO) experiences minimal influence from anthropogenic aerosols, maintaining preindustrial-like pristine atmospheric conditions (Hamilton et al., 2014). Understanding aerosol properties in this region is of particular importance for constraining the preindustrial state of aerosols and subsequently reducing uncertainties in the anthropogenic aerosol radiative forcing through aerosol-cloud interactions (Carslaw et al., 2013). However, the current global scale aerosol models often underestimate the number concentration of cloud condensation nuclei (CCN) by approximately 50% (Schmale et al., 2019; McCoy et al., 2021; Fiddes et al., 2024), suggesting that CCN production processes may not be adequately represented in models.
One of the major sources of CCN in the SO is sea spray aerosol (SSA) primarily emitted from the ocean. The SSA emission flux widely used in current models is tuned to reproduce global SSA mass concentrations (Jaeglé et al., 2011). However, it was later found that the same model underestimates SSA mass concentrations over the SO by ~60% (Jiang et al., 2021). The magnitude of the bias varies depending on the temperature inversion between sea surface (SST) and surface atmosphere (T2M), suggesting that unique SSA emission processes may be at play in cold sea surface environments (Jiang et al., 2021). Additionally, the size distribution of SSA may not be adequately represented in models. The commonly used distribution developed by Gong et al. (2003) includes a high proportion of coarse-mode particles, whereas adopting the distribution developed by Sofiev et al. (2011) increases the fraction of fine-mode particles, improving the reproducibility of aerosol optical depth (AOD) (Witek et al., 2016). However, AOD is not a direct measure of number concentrations of fine-mode particles, leaving uncertainties in this evaluation.
In this study, we use the GEOS-Chem-TOMAS model and ship-based observations of fine-mode particle number concentrations by ACE-SPACE project (Schmale et al., 2019), to examine the effect of these two SSA flux parameterizations, (i) the SST-T2M dependency and (ii) the different size distributions, on a low bias of CCN over the summertime SO. In the base simulation, both the mass concentration (NaCl; -71 to -21%) and the number concentration of CCN size (> 80 nm) particles (N₈₀; -84 to -54%) were underestimated. By applying the above two parameterizations ((i) and (ii)) in the model, the biases were successfully reduced for both NaCl mass (-55 to +12%) and N₈₀ (-27 to +101%). However, the number concentration of mid-sized (> 700 nm) particles remained largely underestimated (-61 to -18%), possibly indicating an issue in capturing the jet drop process, which supplies particles in this mid-size range (Lewis and Schwartz, 2004).