[A-AS09] Atmospheric Chemistry

[AAS09-P15] Impact of detailed marine sulfur chemistry on the aerosol formation processes over the summertime Southern Ocean in GEOS-Chem-TOMAS

*Haruki Sato¹, Sakiko Ishino^1,2, Linia Tashmim³, William C Porter³ (1.School of Geosciences and Civil Engineering, Kanazawa University, 2.Institute of Nature and Environmental Technology, Kanazawa University, 3.Department of Environmental Sciences, University of California, Riverside, CA, USA)

Keywords:Southern Ocean, GEOS-Chem-TOMAS, DMS, HPMTF, Aerosol

Understanding the aerosol formation and loss processes over the summertime Southern Ocean is important for two reasons: (i) to figure out the source of an excessive radiative energy input across the region in climate models, which also matters global heat transport (Trenberth and Fassulo, 2010; Bodas-Salcedo et al., 2016), and (ii) as an analog of the preindustrial baseline of aerosol property, which is a leading driver of the uncertainty in radiative forcing caused by aerosol-cloud interactions (Carslaw et al., 2013; Hamilton et al., 2014). The current climate models underestimate aerosol number concentrations over the summertime Southern Ocean by about 50% or more compared to observations (Schmale et al., 2019; McCoy et al., 2021), contributing to the significant biases in clouds and solar radiation.
Aerosols in the summertime Southern Ocean are primarily composed of sulfate (SO₄^2-) produced via oxidation of marine biogenic dimethyl sulfide (DMS) (Quinn et al., 2017). The precise representation of DMS oxidation processes is thus an essential step toward understanding the cause of low bias in aerosol number concentrations. The recent development of understanding in multiphase chemistry (e.g., Hoffmann et al., 2016) and the discovery of a stable intermediate compound hydroperoxymethyl thioformate (HPMTF: HOOCH₂SCHO) (Veres et al., 2020) had triggered the great efforts in introducing the detailed DMS oxidation processes into global models upon the previous simplified chemical mechanisms.
GEOS-Chem-TOMAS is a global model that simulates the size-resolved aerosol number concentrations upon the thorough description of atmospheric chemistry, where the detailed DMS oxidation process including HPMTF has been also implemented (Tashmim et al., 2024). In this study, we evaluated GEOS-Chem-TOMAS using NASA ATom-2 aircraft observation data of multiple sulfur species and aerosol number concentrations (N₃) (Wofsy et al., 2018), with a specific focus on the summertime (February, 2017) Southern Ocean (latitude: 40 to 70°S). The difference between the model and observational data was evaluated with the mean fractional bias (MFB) (Boylan and Russell, 2006).
The base model with previous simplified DMS oxidation chemistry showed a significant underestimate in aerosol number concentration N₃ (MFB = -1.47), despite overestimates in DMS (MFB = 1.52) and mass concentration of SO₄^2- in PM₁ (MFB = 0.78). This result suggests that the SO₄^2- tends to be partitioned to the larger size particles in the model, and supplies an insufficient number of small, Aitken-mode particles that dominate aerosol number concentrations. Furthermore, the model with the detailed DMS oxidation process (Tashmim et al., 2024) resulted in a larger underestimate of N₃ and an overestimate of SO₄^2-, reflecting the newly added multiphase chemistry involving MSA and HPMTF that contributes to the growth of particles. The addition of multiphase chemistry also reduced the gas-phase H₂SO₄ and its precursor SO₂ by 19 and 56 %, respectively. These results highlight the need for a better understanding of marine sulfur chemistry to improve the aerosol size distribution over the summertime Southern Ocean.