Processes Controlling DMS Variability in Marine Boundary Layer of the Arabian Sea During Post-Monsoon Season of 2021

Abstract

Ship-borne high time-resolution in situ measurements of dimethyl sulfide (DMS) in the marine air of the Arabian Sea reveal large spatiotemporal variations (20−709 ppt) during the post-monsoon season. The DMS mixing ratios in the southeast (98 ± 87 ppt) and northeast (116 ± 120 ppt) coastal regions were higher than in the oligotrophic central Arabian Sea (62 ± 53 ppt). The frequent DMS peaks over the coastal regions were associated with calm winds, lower salinity, higher SST, and elevated chlorophyll concentrations. The time series of DMS along the coastal regions exhibits stronger diurnal variability with higher values during early morning and evening hours compared to the open ocean. The lower daytime DMS throughout the campaign is attributed to the predominance of OH-oxidation loss, while nighttime radical chemistry was modulated by polluted air masses in coastal regions. The DMS levels showed a noticeable response to changes in meteorological and oceanic physiochemical parameters, including salinity gradients (−0.3 to 0.2 psu hr⁻¹), mostly associated with localized upwelling events. A weak negative correlation between DMS and seawater nitrate concentrations indicates the role of nitrogen availability in seawater DMS production. Despite relatively low phytoplankton biomass in the post-monsoon, the estimated DMS fluxes of ∼11 μmol m⁻² d⁻¹ were ∼3-times higher than the values determined two decades ago. Strong spatiotemporal variations and high levels of DMS could have significant implications for regional atmospheric chemistry, including the formation of sulfate aerosols and cloud condensation nuclei in the marine boundary layer over the Arabian Sea.