Dimethyl sulfide (DMS), emitted from the oceans, is the most abundant biological source of sulfur to the marine atmosphere. Atmospheric DMS is oxidized to condensable products that form secondary aerosols that affect Earth’s radiative balance by scattering solar radiation and serving as cloud condensation nuclei. We report the atmospheric discovery of a previously unquantified DMS oxidation product, hydroperoxymethyl thioformate (HPMTF, HOOCH2SCHO), identified through global-scale airborne observations that demonstrate it to be a major reservoir of marine sulfur. Observationally constrained model results show that more than 30% of oceanic DMS emitted to the atmosphere forms HPMTF. Coincident particle measurements suggest a strong link between HPMTF concentration and new particle formation and growth. Analyses of these observations show that HPMTF chemistry must be included in atmospheric models to improve representation of key linkages between the biogeochemistry of the ocean, marine aerosol formation and growth, and their combined effects on climate.

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.

The oxidation of carbonyl sulfide (OCS) is the primary, continuous source of stratospheric sulfate aerosol particles, which can scatter shortwave radiation and catalyze heterogeneous reactions in the stratosphere. While it has been estimated that the oxidation of dimethyl sulfide (DMS), emitted from the surface ocean accounts for 8%–20% of the global OCS source, there is no existing DMS oxidation mechanism relevant to the marine atmosphere that is consistent with an OCS source of this magnitude. We describe new laboratory measurements and theoretical analyses of DMS oxidation that provide a mechanistic description for OCS production from hydroperoxymethyl thioformate, a ubiquitous, soluble DMS oxidation product. We incorporate this chemical mechanism into a global chemical transport model, showing that OCS production from DMS is a factor of 3 smaller than current estimates, displays a maximum in the tropics consistent with field observations and is sensitive to multiphase cloud chemistry.

Oceans emit large quantities of dimethyl sulfide (DMS) to the marine atmosphere. The oxidation of DMS leads to the formation and growth of cloud condensation nuclei (CCN) with consequent effects on Earth’s radiation balance and climate. The quantitative assessment of the impact of DMS emissions on CCN concentrations necessitates a detailed description of the oxidation of DMS in the presence of existing aerosol particles and clouds. In the unpolluted marine atmosphere, DMS is efficiently oxidized to hydroperoxymethyl thioformate (HPMTF), a stable intermediate in the chemical trajectory toward sulfur dioxide (SO2) and ultimately sulfate aerosol. Using direct airborne flux measurements, we demonstrate that the irreversible loss of HPMTF to clouds in the marine boundary layer determines the HPMTF lifetime (τHPMTF < 2 h) and terminates DMS oxidation to SO2. When accounting for HPMTF cloud loss in a global chemical transport model, we show that SO2 production from DMS is reduced by 35% globally and near-surface (0 to 3 km) SO2 concentrations over the ocean are lowered by 24%. This large, previously unconsidered loss process for volatile sulfur accelerates the timescale for the conversion of DMS to sulfate while limiting new particle formation in the marine atmosphere and changing the dynamics of aerosol growth. This loss process potentially reduces the spatial scale over which DMS emissions contribute to aerosol production and growth and weakens the link between DMS emission and marine CCN production with subsequent implications for cloud formation, radiative forcing, and climate.

We present an examination of the 248 nm VUV (vacuum ultraviolet) laser photolysis of an ozone (O3) and methylamine (CH3NH2) mixture as means to produce aminomethanol (NH2CH2OH). Aminomethanol is predicted to be the direct interstellar precursor to glycine and is therefore an important target for detection in the interstellar medium. However, due to its high reactivity under terrestrial conditions, aminomethanol evades gas-phase spectral detection. The insertion of O­(1D) into methylamine is one proposed pathway to form aminomethanol. However, this formation pathway is highly exothermic and results in a complex mixture of reaction products, complicating spectral assignment. Additional reactions between methylamine and the other products of ozone photolysis lead to further complication of the chemistry. Here, we present a systematic experimental study of these reaction pathways. We have used direct absorption millimeter/submillimeter spectroscopy in a supersonic expansion to probe the reaction products, which include formaldehyde (H2CO), methanimine (CH2NH), formamide (HCONH2), and hydrogen cyanide (HCN) and absorption signals arising from at least two additional unknown products. In addition, we examine the effects of reaction time on the chemical formation pathways and discuss them in the context of O­(1D) insertion chemistry with methylamine. We have built a kinetics box model to interpret the results that are observed. We then examine the implications of these results for future studies aimed at forming and detecting aminomethanol.