Diel Profile of Hydroperoxymethyl Thioformate: Evidence for Surface Deposition and Multiphase Chemistry

Dimethyl sulfide (DMS; CH₃SCH₃), a biogenically produced trace gas emitted from the ocean, accounts for a large fraction of natural sulfur released to the marine atmosphere. The oxidation of DMS in the marine boundary layer (MBL), via the hydrogen abstraction pathway, yields the short-lived methylthiomethylperoxy radical (MSP; CH₃SCH₂OO). In the remote MBL, unimolecular isomerization of MSP outpaces bimolecular chemistry leading to the efficient formation of hydroperoxymethyl thioformate (HPMTF; HOOCH₂SCHO). Here, we report the first ground observations and diurnal profiles of HPMTF mixing ratios, vertical fluxes, and deposition velocities to the ocean surface. Average daytime HPMTF mixing ratios, fluxes, and deposition velocities were recorded at 12.1 pptv, -0.11 pptv m s^-1, and 0.75 cm s^-1, respectively. The deposition velocity of HPMTF is comparable to other soluble gas phase compounds (e.g., HCOOH and HNO₃), resulting in a deposition lifetime of 30 h under typical windspeeds (3 m s^-1). A box model analysis incorporating the current mechanistic understanding of DMS oxidation chemistry and geostationary satellite cloud imagery data suggests that the lifetime of HPMTF in the MBL at this sampling location is likely controlled by heterogeneous loss to aerosol and uptake to clouds in the morning and evening.