DMS oxidation experiments in the SAPHIR* simulation chamber: Constraining chemical kinetics of the HPMTF channel

The oxidation pathways of dimethyl sulfide (DMS), the largest natural source of sulfur in theatmosphere (Bates et al., 1992), and its impact on Earth’s radiative balance are not fully understood yet. One of the major open questions in the DMS oxidation scheme is the chemical kinetics of hydroperoxymethyl thioformate (HPMTF) formation and breakdown, which also leads to substantial HPMTF abundances in the global atmosphere as detetcted by Veres et al. in 2020 by means of aircraft observations. HPMTF is formed from methylthiomethyl peroxy radical (MTMP), one of the major oxidation products of DMS, and was first suggested by Wu et al. (2015) based on ab initio calculations. Meanwhile, several atmospheric simulation chamber studies have taken place to better understand DMS oxidation (e.g., Jernigan et al. (2022), Ye et al. (2022), Shen et al. (2022) and, von Hobe et al. (2023)).

In autumn 2023 a DMS oxidation campaign took place at the SAPHIR* chamber at the Forschungszentrum Jülich to address these issues. SAPHIR* (SAPHIR-STAR; Simulation of Atmospheric PHotochemistry In a large Reaction Chamber – STirred Atmospheric tank Reactor) is a continuously stirred tank reactor made of glass with a total volume of 2000 l. All experiments in SAPHIR* are carried out under steady state conditions. The chamber is inside a thermostat, features two independent UV light systems at 254nm (UV-C) and 365nm (UV-A), a state of the art control and gas supply system. Measurements are performed under extremely controlled conditions regarding temperature, relative humidity (RH), flows and composition/constituents. SAPHIR* features a set of standard measurements (O₃, NO/NO_x, CO, CO₂, CH₄). During the campaign three Chemical Ionisation Mass Spectrometers (CIMS) were used during the campaign: a CIMS-API-LTOF with a nitrate and bromide ion source, which was also used for frequent voltage scanning during the steady states, a CIMS-HTOF with iodide ion source and, FunMass-C, a rebuilt aircraft CTOF instrument employing CF₃O^- ion chemistry. Every of the four reagent ions is capable to detect HPMTF. Moreover, a proton-transfer-reaction mass spectrometer (PTR-MS) and a gas chromatography mass spectrometer (GC-MS) measured DMS. For FunMass-C, this was the first time measuring under non-laboratory conditions over weeks.

As precursor of OH, H₂O₂ photolysis was used. This allowed for HPMTF production even at mostly dry conditions (RH < 1%) as well as the investigation of the influence of additional water vapour (RH = 20, 40 and 60 %) under atmospherically relevant OH levels (~1*10⁷ molecules*cm^-3). In addition, the dependence of several NO levels (5, 10, 20 and 30 ppbv NO) on HPMTF formation under dry conditions was investigated. Lastly, the HPMTF formation caused by nitrate radicals in the dark (100 ppb O₃ and 25 ppb NO) was investigated, as well as the influence of water vapour on this reaction pathway (RH = 20 %). For all experiments the initial DMS mixing ratio in the chamber was 10 or 20 ppb. The presentation gives an overview of these experiments and discusses preliminary results.