Modellierung der Multiphasenchemie von Dimethylsulfid in ECHAM-HAMMOZ

<p>Die Emission von Dimethylsulfid (DMS) aus den Ozeanen in die Atmosphäre ist die größte natürliche Schwefelquelle. Daher ist die Oxidation von DMS der wichtigste Faktor für die Bildung natürlicher Sulfatpartikel und den natürlichen Strahlungshaushalt der Erde. Die Bedeutung der Multiphasenchemie für die DMS-Oxidation ist unbestritten, insbesondere für die Bildung von Methansulfonsäure (MSA). MSA kann die Bildung neuer natürlicher Partikel wirksam unterdrücken. Dies wird jedoch in den aktuellen Klimachemiemodellen vernachlässigt. Oft wird die DMS-Oxidation durch drei Reaktionen vereinfacht, obwohl bewiesen ist, dass viel mehr Reaktionen wichtig sind, um die Multiphasenchemie von DMS genau darzustellen.<br />In dieser Studie wurde die im Klimachemiemodell ECHAM-HAMMOZ implementierte Gasphasen-DMS-Chemie erweitert. Darüber hinaus wurde erstmals die Chemie der MSA-Bildung in deliqueszenten Aerosolpartikeln durch die Implementierung eines reaktiven Aufnahmekoeffizienten realisiert. Es wurden erste Simulationen für das Jahr 2017 durchgeführt, um den neuen Mechanismus zu testen. Es wurden Sensitivitätsstudien durchgeführt, indem der Aufnahmekoeffizient variiert wurde, um die entsprechenden Auswirkungen auf die MSA-Bildung zu untersuchen. Darüber hinaus wurde eine Simulation mit der alten Parametrisierung, wie sie im Aerosolmodul HAM enthalten ist, durchgeführt. Aus den verschiedenen Schemata wurden starke Unterschiede hinsichtlich der Ausbeute an MSA, SO<sub>2</sub> und H<sub>2</sub>SO<sub>4</sub> und der anschließenden Bildung von Sulfat simuliert. Es konnte gezeigt werden, dass die Berücksichtigung der reaktiven Aufnahme zu einem besseren Vergleich der simulierten mit den Feldmessungen für MSA führt. <br />Insgesamt weisen die Simulationen auf die Bedeutung einer detaillierteren DMS-Oxidation hin, da die Bildung von Zwischenprodukten zu starken Veränderungen bei der Simulation des arktischen Strahlungsantriebs führt. Darüber hinaus treiben multiphasenchemische Prozesse die Bildung von MSA in der Gasphase an und beeinflussen die simulierte Sulfatkonzentration, insbesondere im südlichen Ozean.</p>

Exploring DMS oxidation and implications for global aerosol radiative forcing

Aerosol indirect radiative forcing (IRF), which characterizes how aerosols alter cloud formation and properties, is very sensitive to the preindustrial (PI) aerosol burden. Dimethyl sulfide (DMS), emitted from the ocean, is a dominant natural precursor of non-sea-salt sulfate in the PI and pristine present-day (PD) atmospheres. Here we revisit the atmospheric oxidation chemistry of DMS, particularly under pristine conditions, and its impact on aerosol IRF. Based on previous laboratory studies, we expand the simplified DMS oxidation scheme used in the Community Atmospheric Model version 6 with chemistry (CAM6-chem) to capture the OH-addition pathway as well as the H-abstraction pathway and the associated isomerization branch. These additional oxidation channels of DMS produce several stable intermediate compounds, e.g., methanesulfonic acid (MSA) and hydroperoxymethyl thioformate (HPMTF), delay the formation of sulfate, and hence, alter the spatial distribution of sulfate aerosol and radiative impacts. The expanded scheme improves the agreement between modeled and observed concentrations of DMS, MSA, HPMTF, and sulfate over most marine regions based on the NASA Atmospheric Tomography (ATom), the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA), and the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) measurements. We find that the global HPMTF burden, as well as the burden of sulfate produced from DMS oxidation are relatively insensitive to the assumed isomerization rate, but the burden of HPMTF is very sensitive to a potential additional cloud loss. We find that global sulfate burden under PI and PD emissions increase to 412 Gg-S (+29 %) and 582 Gg-S (+8.8 %), respectively, compared to the standard simplified DMS oxidation scheme. The resulting annual mean global PD direct radiative effect of DMS-derived sulfate alone is −0.11 W m−2. The enhanced PI sulfate produced via the gas-phase chemistry updates alone dampens the aerosol IRF as anticipated (−2.2 W m−2 in standard versus −1.7 W m−2 with updated gas-phase chemistry). However, high clouds in the tropics and low clouds in the Southern Ocean appear particularly sensitive to the additional aqueous-phase pathways, counteracting this change (−2.3 W m−2). This study confirms the sensitivity of aerosol IRF to the PI aerosol loading, as well as the need to better understand the processes controlling aerosol formation in the PI atmosphere and the cloud response to these changes.

Structural and Mechanistic Insights Into Dimethylsulfoxide Formation Through Dimethylsulfide Oxidation

Dimethylsulfide (DMS) and dimethylsulfoxide (DMSO) are widespread in marine environment, and are important participants in the global sulfur cycle. Microbiol oxidation of DMS to DMSO represents a major sink of DMS in marine surface waters. The SAR11 clade and the marine Roseobacter clade (MRC) are the most abundant heterotrophic bacteria in the ocean surface seawater. It has been reported that trimethylamine monooxygenase (Tmm, EC 1.14.13.148) from both MRC and SAR11 bacteria likely oxidizes DMS to generate DMSO. However, the structural basis of DMS oxidation has not been explained. Here, we characterized a Tmm homolog from the SAR11 bacterium Pelagibacter sp. HTCC7211 (Tmm7211). Tmm7211 exhibits DMS oxidation activity in vitro. We further solved the crystal structures of Tmm7211 and Tmm7211 soaked with DMS, and proposed the catalytic mechanism of Tmm7211, which comprises a reductive half-reaction and an oxidative half-reaction. FAD and NADPH molecules are essential for the catalysis of Tmm7211. In the reductive half-reaction, FAD is reduced by NADPH. In the oxidative half-reaction, the reduced FAD reacts with O2 to form the C4a-(hydro)peroxyflavin. The binding of DMS may repel the nicotinamide ring of NADP+, and make NADP+ generate a conformational change, shutting off the substrate entrance and exposing the active C4a-(hydro)peroxyflavin to DMS to complete the oxidation of DMS. The proposed catalytic mechanism of Tmm7211 may be widely adopted by MRC and SAR11 bacteria. This study provides important insight into the conversion of DMS into DMSO in marine bacteria, leading to a better understanding of the global sulfur cycle.

Secondary aerosol formation from dimethyl sulfide – improved mechanistic understanding based on smog chamber experiments and modelling

Dimethyl sulfide (DMS) is the dominant biogenic sulfur compound in the ambient marine atmosphere. Low-volatility acids from DMS oxidation promote the formation and growth of sulfur aerosols and ultimately alter cloud properties and Earth's climate. We studied the OH-initiated oxidation of DMS in the Aarhus University Research on Aerosol (AURA) smog chamber and the marine boundary layer (MBL) with the aerosol dynamics and gas- and particle-phase chemistry kinetic multilayer model ADCHAM. Our work involved the development of a revised and comprehensive multiphase DMS oxidation mechanism, capable of both reproducing smog chamber and atmospheric relevant conditions. The secondary aerosol mass yield in the AURA chamber was found to have a strong dependence on the reaction of methyl sulfinic acid (MSIA) and OH, causing a 82.8 % increase in the total PM at low relative humidity (RH), while the autoxidation of the intermediate radical CH3SCH2OO forming hydroperoxymethyl thioformate (HPMTF) proved important at high temperature and RH, decreasing the total PM by 55.8 %. The observations and modelling strongly support the finding that a liquid water film existed on the Teflon surface of the chamber bag, which enhanced the wall loss of water-soluble intermediates and oxidants dimethyl sulfoxide (DMSO), MSIA, HPMTF, SO2, methanesulfonic acid (MSA), sulfuric acid (SA) and H2O2. The effect caused a 64.8 % and 91.7 % decrease in the secondary aerosol mass yield obtained at both dry (0 % RH–12 % RH) and humid (50 % RH–80 % RH) conditions, respectively. Model runs reproducing the ambient marine atmosphere indicate that OH comprises a strong sink of DMS in the MBL (accounting for 31.1 % of the total sink flux of DMS) although less important than the combined effect of halogen species Cl and BrO (accounting for 24.3 % and 38.7 %, respectively). Cloudy conditions promote the production of SO42- particular mass (PM) from SO2 accumulated in the gas phase, while cloud-free periods facilitate MSA formation in the deliquesced particles. The exclusion of aqueous-phase chemistry lowers the DMS sink as no halogens are activated in the sea spray particles and underestimates the secondary aerosol mass yield by neglecting SO42- and MSA PM production in the particle phase. Overall, this study demonstrated that the current DMS oxidation mechanisms reported in literature are inadequate in reproducing the results obtained in the AURA chamber, whereas the revised chemistry captured the formation, growth and chemical composition of the formed aerosol particles well. Furthermore, we emphasize the importance of OH-initiated oxidation of DMS in the ambient marine atmosphere during conditions with low sea spray emissions.

Secondary aerosol formation from dimethyl sulfide – improved mechanistic understanding based on smog chamber experiments and modelling

Dimethyl sulfide (DMS) is the dominant biogenic sulphur compound in the ambient atmosphere. Low volatile acids from DMS oxidation promote the formation and growth of sulphur aerosols, and ultimately alter cloud properties and Earth's climate. We studied the OH-initiated oxidation of DMS in the Aarhus University research on aerosols (AURA) smog chamber and the marine boundary layer (MBL) with the aerosol dynamics, gas- and particle-phase chemistry kinetic multilayer model ADCHAM. Our work involved the development of a revised and comprehensive multiphase DMS oxidation mechanism, both capable of reproducing smog chamber and atmospheric relevant conditions. The secondary aerosol mass yield in the AURA chamber was found to have a strong dependence on the reaction of methyl sulfinic acid (MSIA) and OH at low relative humidity (RH), while the autoxidation of the intermediate radical CH3SCH2OO forming hydroperoxymethyl thioformate (HPMTF) proved important at high RH. The observations and modelling strongly support that a liquid water film existed on the Teflon surface of the chamber bag, which enhanced the wall loss of water soluble intermediates and oxidants DMSO, MSIA, HPMTF, SO2, MSA, SA and H2O2. The effect caused a decrease in the secondary aerosol mass yield obtained at both dry (0–12 % RH) and humid (50–80 % RH) conditions. Model runs reproducing the ambient marine atmosphere indicate that OH comprise a strong sink of DMS in the MBL, although less important than halogen species Cl and BrO. Cloudy conditions promote the production of SO42− particular mass (PM) from SO2 accumulated in the gas-phase, while cloud-free periods facilitate MSA formation in the deliquesced particles. The exclusion of aqueous-phase chemistry lowers the DMS sink as no halogens are activated in the sea spray particles, and underestimates the secondary aerosol mass yield by neglecting SO42− and MSA PM production in the particle phase. Overall, this study demonstrated that the current DMS oxidation mechanisms reported in literature are inadequate in reproducing the results obtained in the AURA chamber, whereas the revised chemistry captured the formation, growth and chemical composition of the formed aerosol particles well. Furthermore, we emphasise the importance of OH-initiated oxidation of DMS in the ambient marine atmosphere during conditions with low sea spray emissions.

Aircraft observations of a new DMS oxidation product over the North Atlantic using a HR-ToF-CIMS

<p>Marine ecosystems are an important component of the climate feedback system. One of the main pathways for ocean-climate interaction is through the oxidation of DMS (dimethyl sulphide), a gas released from phytoplankton in the sea surface. DMS derived products are known to be important in marine cloud formation and the Earth’s radiation budget. Aerosol-Cloud interactions currently represent the largest uncertainty in climate modelling (Boucher et al., 2013). Our research focuses on airborne measurements using real-time high resolution instruments to identify and quantify trace oceanic biogenic gases on board the FAAM research aircraft. Here we present aircraft measurements made over the North Atlantic ocean using a HR-ToF-CIMS, across three seasons during the most recent ACSIS/ARNA campaigns. Here we report some of the first observations of an alternative DMS oxidation product, hydperoxymethyl thioformate (HPMFT) using chemical ionisation mass spectrometry with iodide reagent ion. Observations of this novel species have never been reported in the atmosphere but laboratory studies suggest that the main oxidation route of DMS occurs through this species, in certain environments (Berndt et al., 2019). This has potentially significant climate implications, none of which are currently represented in global climate models. The fate of this newly measured species once in the atmosphere is uncertain but is likely to alter our understanding of the marine sulphur cycle. These observations along with laboratory and modelling studies will aid in being able to understand the role of HPMFT in the ocean-climate feedback system.</p><p><strong>References </strong></p><p>T. Berndt, W. Scholz, B. Mentler, L. Fischer, E. H. Hoffmann, A. Tilgner, N. Hyttinen, N. L. Prisle, A. Hansel, and H. Herrmann, The Journal of Physical Chemistry Letters 2019 10 (21), 6478-6483,DOI: 10.1021/acs.jpclett.9b02567</p><p>Boucher O, Randall D, Artaxo P, Bretherton C, Feingold G, Forster P, Kerminen VM, Kondo Y, Liao H, Lohmann U, Rasch P, Satheesh S, Sherwood S, Stevens B, Zhang X. In: Clouds and aerosols. Cambridge: Cambridge University Press; 2013. United Kingdom and new york, NY, USA, book section Chapter 7, pp 571–658. https://doi.org/10.1017/CBO9781107415324.016.</p><p> </p><p> </p><p> </p>

Towards an operational CAPRAM multiphase halogen and DMS chemistry treatment in the chemistry transport model COSMO-MUSCAT

<p>Oceans are the general emitter of dimethyl sulfide (DMS), the major natural sulfur source, and halides and cover approximately 70 % of Earth’s surface. The main DMS oxidation products are SO<sub>2</sub>, H<sub>2</sub>SO<sub>4</sub> and methyl sulfonic acid (MSA). Hence, DMS is very important for formation of non-sea salt sulfate (nss‑SO<sub>4</sub><sup>2-</sup>) aerosols and secondary particulate matter and thus global climate. Reactive halogen compounds, activated by multiphase chemistry processes, are known to effectively deplete ozone, oxidise VOCs (especially DMS under marine conditions) and remove NOx from the atmosphere by conversion into particulate nitrate. Despite many previous model studies, a detailed representation of the multiphase chemistry occurring in aqueous aerosols and cloud droplets in CTMs is still missing.</p><p>To develop a detailed representation, a manual reduction of near-explicit multiphase chemistry mechanisms by means of detailed box model studies has been performed. The mechanism has been developed from the near-explicit DMS and halogen multiphase chemistry mechanism, CAPRAM DM1.0 and CAPRAM HM3. The reduced mechanism is evaluated by process model studies. Comparisons of simulations performed with the explicit and reduced mechanism reveals that the deviations are below 5 % for key inorganic and organic air pollutants and oxidants under pristine ocean and polluted coastal conditions, respectively.</p><p>Subsequently, the reduced mechanism has been implemented into the chemical transport model COSMO-MUSCAT and tested by 2D-simulations. Simulations are performed for two different meteorological scenarios mimicking unstable and stable weather conditions over the pristine ocean. The simulations demonstrate that the modelled concentrations of important halogen compounds such as HCl and BrO agree with ambient measurements demonstrating the applicability of the mechanism for tropospheric modelling investigations.</p><p>The 2D studies with the reduced mechanism are carried out to examine the oxidation pathways of DMS in a cloudy marine atmosphere in detail. They have shown that clouds have both a direct and an indirect photochemical effect on the multiphase processing of DMS and its oxidation products. The direct photochemical effect is related to in-cloud chemistry that leads to high DMSO oxidation rates and subsequently an enhanced formation of methane sulfonic acid compared to aerosol chemistry. The indirect photochemical effect is characterised by cloud shading, particularly in the case of stratiform clouds. The lower photolysis rates below the clouds affects strongly the activation of Br atoms and lowers the formation of BrO radicals. The corresponding DMS oxidation flux is particularly lowered under thick optical clouds. Besides, high updraft velocities lead to a strong vertical mixing of DMS into the free troposphere predominately under convective conditions. Furthermore, clouds reduce the photolysis of hypohalogeneous acids (HOX, X=Cl, Br, I) resulting in higher HOX-driven sulfite oxidation rates in aqueous aerosol particles below clouds.</p>

The potential role of methanesulfonic acid (MSA) in aerosol formation and growth and the associated radiative forcings

Atmospheric marine aerosol particles impact Earth's albedo and climate. These particles can be primary or secondary and come from a variety of sources, including sea salt, dissolved organic matter, volatile organic compounds, and sulfur-containing compounds. Dimethylsulfide (DMS) marine emissions contribute greatly to the global biogenic sulfur budget, and its oxidation products can contribute to aerosol mass, specifically as sulfuric acid and methanesulfonic acid (MSA). Further, sulfuric acid is a known nucleating compound, and MSA may be able to participate in nucleation when bases are available. As DMS emissions, and thus MSA and sulfuric acid from DMS oxidation, may have changed since pre-industrial times and may change in a warming climate, it is important to characterize and constrain the climate impacts of both species. Currently, global models that simulate aerosol size distributions include contributions of sulfate and sulfuric acid from DMS oxidation, but to our knowledge, global models typically neglect the impact of MSA on size distributions. In this study, we use the GEOS-Chem-TOMAS (GC-TOMAS) global aerosol microphysics model to determine the impact on aerosol size distributions and subsequent aerosol radiative effects from including MSA in the size-resolved portion of the model. The effective equilibrium vapor pressure of MSA is currently uncertain, and we use the Extended Aerosol Inorganics Model (E-AIM) to build a parameterization for GC-TOMAS of MSA's effective volatility as a function of temperature, relative humidity, and available gas-phase bases, allowing MSA to condense as an ideally nonvolatile or semivolatile species or too volatile to condense. We also present two limiting cases for MSA's volatility, assuming that MSA is always ideally nonvolatile (irreversible condensation) or that MSA is always ideally semivolatile (quasi-equilibrium condensation but still irreversible condensation). We further present simulations in which MSA participates in binary and ternary nucleation with the same efficacy as sulfuric acid whenever MSA is treated as ideally nonvolatile. When using the volatility parameterization described above (both with and without nucleation), including MSA in the model changes the global annual averages at 900 hPa of submicron aerosol mass by 1.2 %, N3 (number concentration of particles greater than 3 nm in diameter) by −3.9 % (non-nucleating) or 112.5 % (nucleating), N80 by 0.8 % (non-nucleating) or 2.1 % (nucleating), the cloud-albedo aerosol indirect effect (AIE) by −8.6 mW m−2 (non-nucleating) or −26 mW m−2 (nucleating), and the direct radiative effect (DRE) by −15 mW m−2 (non-nucleating) or −14 mW m−2 (nucleating). The sulfate and sulfuric acid from DMS oxidation produces 4–6 times more submicron mass than MSA does, leading to an ∼10 times stronger cooling effect in the DRE. But the changes in N80 are comparable between the contributions from MSA and from DMS-derived sulfate/sulfuric acid, leading to comparable changes in the cloud-albedo AIE. Model–measurement comparisons with the Heintzenberg et al. (2000) dataset over the Southern Ocean indicate that the default model has a missing source or sources of ultrafine particles: the cases in which MSA participates in nucleation (thus increasing ultrafine number) most closely match the Heintzenberg distributions, but we cannot conclude nucleation from MSA is the correct reason for improvement. Model–measurement comparisons with particle-phase MSA observed with a customized Aerodyne high-resolution time-of-flight aerosol mass spectrometer (AMS) from the ATom campaign show that cases with the MSA volatility parameterizations (both with and without nucleation) tend to fit the measurements the best (as this is the first use of MSA measurements from ATom, we provide a detailed description of these measurements and their calibration). However, no one model sensitivity case shows the best model–measurement agreement for both Heintzenberg and the ATom campaigns. As there are uncertainties in both MSA's behavior (nucleation and condensation) and the DMS emissions inventory, further studies on both fronts are needed to better constrain MSA's past, current, and future impacts upon the global aerosol size distribution and radiative forcing.