Atmos. Chem. Phys., 20, 2695–2707, 2020 https://doi.org/10.5194/acp-20-2695-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Kinetic and mechanistic study of the reaction between methane sulfonamide (CH3S.O/2NH2) and OH Matias Berasategui, Damien Amedro, Achim Edtbauer, Jonathan Williams, Jos Lelieveld, and John N. Crowley Division of Atmospheric Chemistry, Max-Planck-Institut für Chemie, 55128 Mainz, Germany Correspondence: John N. Crowley ([email protected]) Received: 8 November 2019 – Discussion started: 28 November 2019 Revised: 30 January 2020 – Accepted: 3 February 2020 – Published: 4 March 2020 Abstract. Methane sulfonamide (MSAM), CH3S.O/2NH2, The main organosulfur trace gases in the marine boundary was recently detected for the first time in ambient air over the layer are dimethyl sulfide (CH3SCH3, DMS) and its oxi- Red Sea and the Gulf of Aden where peak mixing ratios of dation products dimethyl sulfoxide (DMSO), dimethyl sul- ≈ 60 pptv were recorded. Prior to this study the rate constant fone (DMSO2), methyl sulfonic acid (MSA), and methyl for its reaction with the OH radical and the products thereby sulfinic acid (MSI) for which atmospheric lifetimes with re- formed were unknown, precluding assessment of its role in spect to their degradation by the OH radical vary between the atmosphere. We have studied the OH-initiated photo- hours (DMS) and several weeks (DMSO2). oxidation of MSAM in air (298 K, 700 Torr total pressure) Recently, the first detection of methane sulfonamide in a photochemical reactor using in situ detection of MSAM (CH3S.O/2NH2, MSAM) in ambient air was made during and its products by Fourier transform infrared (FTIR) absorp- the Air Quality and Climate Change in the Arabian Basin tion spectroscopy. The relative rate technique, using three (AQABA-2017) campaign. Mixing ratios of MSAM ap- different reference compounds, was used to derive a rate co- proached 60 pptv over the Arabian Sea; details of these mea- efficient of .1:4 ± 0:3/ × 10−13 cm3 molec:−1 s−1. The main surements and a discussion of the likely sources of MSAM end products of the photo-oxidation observed by FTIR were in these regions are given in a companion paper (Edtbauer et CO2, CO, SO2, and HNO3 with molar yields of (0:73±0:11), al., 2019). As MSAM had not been considered to be an atmo- (0:28 ± 0:04), (0:96 ± 0:15), and (0:62 ± 0:09), respectively. spheric trace gas prior to the observations of Edtbauer et al. N2O and HC.O/OH were also observed in smaller yields (2019), there have been no laboratory studies to investigate of (0:09 ± 0:02) and (0:03 ± 0:01). Both the low rate coef- either its spectroscopy or the kinetics of its reactions with ficient and the products formed are consistent with hydro- atmospheric radicals, such as OH, so that its atmospheric gen abstraction from the −CH3 group as the dominant initial lifetime and the products formed during its degradation in step. Based on our results MSAM has an atmospheric life- air were unknown. Combining carbon, nitrogen, sulfur, and time with respect to loss by reaction with OH of about 80 d. oxygen in a single, small molecule, MSAM is an intriguing species not only as an atmospheric trace gas but also from a spectroscopic and kinetic perspective. Unlike basic alkyl amines such as, for example, CH3NH2, MSAM contains an 1 Introduction acidic −NH2 group (Remko, 2003). This work presents the first kinetic and mechanistic study Natural emissions of organosulfur compounds from phyto- of the OH-induced oxidation of MSAM in air. A reaction plankton comprise up to 60 % of the total sulfur flux into mechanism is proposed that, through numerical simulation, the marine boundary layer (Andreae, 1990; Bates et al., describes the time-dependent formation of the end products 1992; Spiro et al., 1992), and in remote oceanic areas they we observed. From these results, we calculate the lifetime are the main source of climatically active sulfate aerosols, and the likely role of MSAM in the atmosphere. which can influence the radiation balance at the earth’s sur- face (Charlson et al., 1987; Andreae and Crutzen, 1997). Published by Copernicus Publications on behalf of the European Geosciences Union. 2696 M. Berasategui et al.: Kinetic and mechanistic study of the reaction between CH3S.O/2NH2 and OH 2 Methods calibrated), and the only nitrogen-containing products were HNO3 and N2O, which can also be calibrated. Experiments 2.1 Experimental set-up in which MSAM was almost completely converted to known amounts of SO2, HNO3, and N2O thus provided an indirect The experimental set-up used to study the reaction of OH calibration (via assumption of 100 % sulfur or nitrogen bal- with MSAM has been described in detail previously (Crow- ance) of its concentration and thus IR cross sections. ley et al., 1999; Bunkan et al., 2018). Briefly, the reac- tion vessel was a 44.39 L cylindrical quartz-wall chamber 2.2 Generation of OH equipped with a White-type multiple-reflection mirror sys- tem resulting in an 86.3 m optical path length for absorption OH was generated by the 254 nm photolysis of O3 in the spectroscopy in the infrared. The quartz reactor was at room presence of H2. temperature (296±2K) and for most experiments at 700 Torr 1 O3 C hν.254nm/ ! O. D/ C O2 (R1) total pressure (1Torr D 1:333hPa) using synthetic air bath 1 gas. Six external, radially mounted, low-pressure Hg lamps O. D/ C H2 ! OH C H (R2) emitting mainly at 253.65 nm provided a homogeneous light H C O2 C M ! HO2 C M (R3) flux within the reactor for radical generation. A 1000 Torr capacitance manometer was used to measure the pressure in- Further reactions that cycle OH and HO2 (e.g. OHCH2,HC side the reactor. O3, HO2 C O3) are listed in Table S1 in the Supplement. MSAM and other gases used to generate OH (see be- In a typical experiment, the starting concentrations of O3 14 15 −3 low) were mixed in a glass vacuum line which was con- and H2 were ≈ 5 × 10 and ≈ 5–7 × 10 molec:cm . As nected directly to the reaction chamber by a PTFE piping. described previously (Bunkan et al., 2018), this scheme gen- Two capacitance manometers (10 and 100 Torr ranges) were erates not only OH radicals but also via, for example, Re- used to accurately measure pressures in the vacuum line. action (R3) HO2. HO2 is not expected to react with MSAM Crystalline MSAM melts at 363 K and has a boiling point but will influence the course of secondary reactions in this of approximately 453 K and an unknown vapour pressure system (e.g. by reacting with organic peroxy radicals) and (< 0:02Torr) at room temperature. Owing to its low vapour thus the end-product distribution, as described in detail in pressure, MSAM was eluted into the reaction chamber by Sect. 3.5. Simulations of the radical concentrations when flowing synthetic air (450 cm3 STP min−1, sccm) through a generating OH in this manner indicate that the HO2=OH ra- finger containing crystalline MSAM warmed to 333 K and tio is approximately 30, with individual concentrations of 11 −3 9 −3 subsequently through a cold trap at 298 K (to prevent con- ≈ 1 × 10 molec:cm HO2 and 3 × 10 molec:cm OH. densation downstream). This way we could ensure that the As an OH source, the photolysis of O3 in the presence of saturation vapour pressure of MSAM at 298 K was achieved. H2 has the advantage over other photochemical sources (e.g. In initial experiments without the trap we observed extra photolysis of H2O2, HONO, or CH3ONO) that neither H2 absorption features, which could be assigned to a dimer of nor O3 has strong absorption features in the infrared, result- MSAM (see below). ing in a less cluttered spectrum which simplifies retrieval of Gas-phase infrared spectra in the range of 4000–600 cm−1 concentration–time profiles of reactants and products. were recorded with a resolution of 2 cm−1 from 16 co- 2.3 Chemicals added interferograms (128 scans for the background) using a Fourier transform infrared (FTIR) spectrometer (Bruker Vec- A commercially available sample of methane sulfonamide tor 22) equipped with an external photoconductive mercury– (Alfa Aesar, > 98%) was used. O3 was generated by flow- cadmium–telluride (MCT) detector cooled to liquid nitrogen ing synthetic air (Westfalen) through a stainless-steel tube temperature. OPUS software was used to analyse and ma- that housed a low-pressure Hg lamp (PenRay) emitting at nipulate the IR spectra. Interferograms were phase-corrected 184.95 nm. Synthetic air (Westfalen, 99.999 %), H2 (West- (Mertz) and Boxcar apodized with a zero-filling factor of 4. falen, 99.999 %), CO2 (Westfalen 99.995 %), CO (West- Most of the products obtained (CO2, CO, HC.O/OH, HNO3, falen, 99.997 %), SO2 (Air Liquide, 1 ppmv in air), and and SO2) were identified and quantified from the IR refer- HC.O/OH (Sigma Aldrich) were obtained commercially. ence spectra of pure samples under similar experimental con- Anhydrous nitric acid was prepared by mixing KNO3 (Sigma ditions (700 Torr and 298.2 K, Fig. S1 in the Supplement). Aldrich, 99 %) and H2SO4 (Roth, 98 %) and condensing The low vapour pressure of MSAM precluded accurate HNO3 vapour into a liquid nitrogen trap. dosing into the chamber and thus generation of a calibra- tion spectrum. In order to calibrate the infrared absorption 2.4 Relative rate constant determination features of MSAM, we oxidized it in air and then con- ducted a sulfur and nitrogen balance of the products.