Effects of Atmospheric Dimethyl Sulfide Oxidation on Methanesulfonic Acid and Sulfate Formation over the Tropical Western Indian Ocean

To investigate the influence and contribution of dimethylsulfide (DMS) oxidation released from the tropical western Indian Ocean to the formation of methane sulfonic acid (MSA) and sulfate (\mathrmSO_4^2- ) in marine aerosols, atmospheric DMS samples and aerosol particle samples were collected in the tropical western Indian Ocean from December 2021 to February 2022. The distribution characteristics of mass concentrations of atmospheric DMS and both MSA and \mathrmSO_4^2- (including non-sea-salt \mathrmSO_4^2- : nss-\mathrmSO_4^2- ) in aerosols were studied. The relationship between atmospheric DMS and aerosol MSA and \mathrmSO_4^2- was explored, and the quantification of DMS oxidation on atmospheric nss-\mathrmSO_4^2- generation was assessed. Results showed that the atmospheric DMS mass concentration (mean±standard deviation) over the tropical western Indian Ocean was (353.4±169.1) ng·m⁻³, comparable to that of the aerosol \mathrmSO_4^2- (224.6±88.46 ng·m⁻³), but significantly higher than that of the MSA (4.12 ng·m⁻³). The mass concentration ratios, c(MSA)/c(DMS) (R_m) and c(nss-\mathrmSO_4^2- )/c(DMS) (R_s), were (0.012±0.010) and (0.465±0.464), respectively. The difference indicated that the main product of DMS oxidation in the tropical waters of the western Indian Ocean is nss-\mathrmSO_4^2- rather than MSA. Furthermore, the contribution of biogenic sulfur to nss-\mathrmSO_4^2- in aerosols could reach 65%, indicating that the marine biological processes in tropical western Indian Ocean play an important regulatory role in the atmospheric nss-\mathrmSO_4^2- above it.