DMS oxidation experiments in the SAPHIR* simulation chamber: Constraining chemical kinetics of the HPMTF channel
Authors: Alber Sascha , Steffes Wilhelm , Stroh Fred , Wang Hui , Wu Rongrong , Zanders Annika , Zorn Sören R. 
Published: March 2024

The oxidation pathways of dimethyl sulfide (DMS), the largest natural source of sulfur in theatmosphere (Bates et al., 1992), and its impact on Earth&#8217;s radiative balance are not fully understood&#160;yet. One of the major open questions in the DMS oxidation scheme is the chemical kinetics of&#160;hydroperoxymethyl thioformate (HPMTF) formation and breakdown, which also leads to&#160;substantial HPMTF abundances in the global atmosphere as detetcted by Veres et al. in 2020 by&#160;means of aircraft observations. HPMTF is formed from methylthiomethyl peroxy radical (MTMP),&#160;one of the major oxidation products of DMS, and was first suggested by Wu et al. (2015) based on&#160;ab initio calculations. Meanwhile, several atmospheric simulation chamber studies have taken place&#160;to better understand DMS oxidation (e.g., Jernigan et al. (2022), Ye et al. (2022), Shen et al. (2022)&#160;and, von Hobe et al. (2023)).In autumn 2023 a DMS oxidation campaign took place at the SAPHIR* chamber at the&#160;Forschungszentrum Ju&#776;lich to address these issues. SAPHIR* (SAPHIR-STAR; Simulation of&#160;Atmospheric PHotochemistry In a large Reaction Chamber &#8211; STirred Atmospheric tank Reactor) is&#160;a continuously stirred tank reactor made of glass with a total volume of 2000 l. All experiments in&#160;SAPHIR* are carried out under steady state conditions.&#160;The chamber is inside a thermostat, features two independent UV light systems at 254nm (UV-C)&#160;and 365nm (UV-A), a state of the art control and gas supply system. Measurements are performed&#160;under extremely controlled conditions regarding temperature, relative humidity (RH), flows and&#160;composition/constituents. SAPHIR* features a set of standard measurements (O3, NO/NOx, CO,&#160;CO2, CH4). During the campaign three Chemical Ionisation Mass Spectrometers (CIMS) were used&#160;during the campaign: a CIMS-API-LTOF with a nitrate and bromide ion source, which was also&#160;used for frequent voltage scanning during the steady states, a CIMS-HTOF with iodide ion source&#160;and, FunMass-C, a rebuilt aircraft CTOF instrument employing CF3O- ion chemistry. Every of the&#160;four reagent ions is capable to detect HPMTF. Moreover, a proton-transfer-reaction mass&#160;spectrometer (PTR-MS) and a gas chromatography mass spectrometer (GC-MS) measured DMS.&#160;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&#160;dry conditions (RH < 1%) as well as the investigation of the influence of additional water vapour&#160;(RH = 20, 40 and 60 %) under atmospherically relevant OH levels (~1*107 molecules*cm-3). In&#160;addition, the dependence of several NO levels (5, 10, 20 and 30 ppbv NO) on HPMTF formation&#160;under dry conditions was investigated. Lastly, the HPMTF formation caused by nitrate radicals in&#160;the dark (100 ppb O3 and 25 ppb NO) was investigated, as well as the influence of water vapour on&#160;this reaction pathway (RH = 20 %). For all experiments the initial DMS mixing ratio in the chamber&#160;was 10 or 20 ppb. The presentation gives an overview of these experiments and discusses&#160;preliminary results.