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DMS Oxidation and Marine Aerosol Radiative Forcing

Abstract: Marine cloud albedo over the pristine Southern Ocean and remote tropical oceans is governed by natural biogenic aerosols derived from phytoplankton dimethyl sulfide (\(\text{DMS}\)) emissions. However, conventional global climate models have historically relied on oversimplified chemical mechanisms that omit isomerization autoxidation and multi-phase cloud droplet loss pathways. Here, we implement the newly discovered hydroperoxymethyl thioformate (\(\text{HPMTF}\)) autoxidation mechanism into the Community Earth System Model (\(\text{CESM2}\)) to evaluate its global climate impact. Our simulations demonstrate that rapid gas-phase autoxidation diverts over \(30\%\) of global \(\text{DMS}\) oxidation flux into \(\text{HPMTF}\), reducing sulfate aerosol mass in pristine marine boundary layers while extending sulfur transport to the free troposphere. This revised spatial redistribution increases pre-industrial cloud droplet number concentrations and alters the global aerosol effective radiative forcing by \(+0.11\text{ W m}^{-2}\). These findings establish that accurate marine sulfur autoxidation kinetics are essential for constraining aerosol baseline cooling and narrowing climate sensitivity uncertainties.

"The ocean and the atmosphere are coupled through a biochemical breath: microscopic phytoplankton emit volatile sulfur, which nucleates clouds that reflect sunlight back to space."

1. The Marine Sulfur Cycle & The CLAW Hypothesis

In 1987, Charlson, Lovelock, Andreae, and Warren proposed the famous CLAW Hypothesis[^1]: oceanic phytoplankton produce dimethyl sulfide (\(\text{CH}_3\text{SCH}_3\), or \(\text{DMS}\)), which ventilates into the atmosphere, oxidizes into sulfuric acid (\(\text{H}_2\text{SO}_4\)) and methanesulfonic acid (\(\text{MSA}\)), and forms cloud condensation nuclei (CCN). These CCN brighten marine stratus clouds, increasing planetary albedo and cooling the surface:

flowchart LR
    Phyto["Marine Phytoplankton (DMSP)"] -->|Enzymatic Cleavage| DMS_Ocean["Oceanic DMS"]
    DMS_Ocean -->|Air-Sea Gas Transfer| DMS_Atm["Atmospheric DMS Gas"]
    DMS_Atm -->|OH / NO3 / Halogen Oxidation| SO2["SO2 + H2SO4 + MSA"]
    DMS_Atm -->|Isomerization Autoxidation| HPMTF["HPMTF Intermediate"]
    SO2 -->|New Particle Formation & Condensation| CCN["Marine Cloud Condensation Nuclei"]
    HPMTF -->|Cloud Droplet Scavenging| CloudSulfate["In-Cloud Aqueous Sulfate"]
    CCN -->|Aerosol Indirect Effect| CloudAlbedo["Brightened Marine Stratocumulus"]
    CloudAlbedo -->|Solar Reflection| Cooling["Surface Climate Cooling"]

    style Phyto fill:#065f46,stroke:#047857,stroke-width:2px,color:#fff
    style DMS_Atm fill:#0284c7,stroke:#0369a1,stroke-width:2px,color:#fff
    style HPMTF fill:#d97706,stroke:#f59e0b,stroke-width:2px,color:#fff
    style CloudAlbedo fill:#3b82f6,stroke:#1d4ed8,stroke-width:2px,color:#fff
    style Cooling fill:#1e3a8a,stroke:#1e40af,stroke-width:2px,color:#fff

Despite its foundational role, global Earth System Models have long struggled with high uncertainty in pristine oceanic aerosol indirect forcing. A primary reason was an incomplete chemical picture: classical models assumed \(\text{DMS}\) oxidized solely via simple H-abstraction and OH-addition into \(\text{SO}_2\) and \(\text{DMSO}\).

2. The Missing Mechanism: Fast Autoxidation to HPMTF

In 2019–2020, airborne mass spectrometry measurements during the NASA ATom campaign and laboratory chamber kinetics discovered a previously unrepresented intermediate: hydroperoxymethyl thioformate (\(\text{HOOCH}_2\text{SCHO}\), or \(\text{HPMTF}\))[^2].

When \(\text{OH}\) abstracts a hydrogen atom from \(\text{DMS}\), the resulting peroxy radical undergoes rapid intramolecular H-shift isomerization (autoxidation):

\[\text{CH}_3\text{SCH}_2\text{O}_2 \xrightarrow{\text{intramolecular H-shift}} \cdot\text{CH}_2\text{SCH}_2\text{OOH} \xrightarrow{+\text{O}_2} \text{O}_2\text{CH}_2\text{SCH}_2\text{OOH} \xrightarrow{\text{fast decomposition}} \text{HOOCH}_2\text{SCHO} (\text{HPMTF}) + \text{OH}\]
                The Bifurcated DMS Oxidation Cascade
                                DMS (CH₃SCH₃)
                                      │
                   ┌──────────────────┴──────────────────┐
                   ▼ (OH Addition)                       ▼ (OH / NO₃ Abstraction)
              DMSO / DMSO₂                         CH₃SCH₂OO• (Peroxy Radical)
                   │                                     │
                   ▼                               ┌─────┴────────────────┐
             MSA / SO₂ / H₂SO₄                     ▼ (Traditional NO/HO₂)  ▼ (Autoxidation H-Shift)
                                                SO₂ + Formaldehyde        HPMTF (HOOCH₂SCHO)
                                                                                  │
                                                                       ┌──────────┴──────────┐
                                                                       ▼ (Gas-phase OH)      ▼ (Cloud Uptake)
                                                                    OCS / SO₂          Aqueous Sulfate

Why HPMTF Upends the Atmospheric Sulfur Budget

  1. Cloud Droplet Scavenging: \(\text{HPMTF}\) is moderately soluble (\(H \approx 10^4 - 10^5\text{ M atm}^{-1}\)) and is rapidly scavenged by cloud droplets, bypassing traditional gas-phase \(\text{SO}_2\) intermediate stages[^3].
  2. Vertical Redistribution: In cloud-free marine boundary layers, \(\text{HPMTF}\) has a longer chemical lifetime, allowing sulfur to be lofted into the free troposphere where lower temperatures favor new particle nucleation.

3. Global Modeling with CESM2 / CAM6-chem

In our study published in Atmospheric Chemistry and Physics[^4], we implemented a comprehensive DMS oxidation mechanism—incorporating explicit \(\text{HPMTF}\) gas-phase photochemistry, halogen (\(\text{BrO}, \text{Cl}\)) reactions, and multi-phase cloud droplet uptake—into the Community Earth System Model (CESM2 / CAM6-chem).

Key Atmospheric Impacts:

Parameter Classical Mechanism (No HPMTF) Updated Explicit Mechanism (With HPMTF) Global Shift
Global DMS Oxidation Flux to HPMTF \(0\text{ Tg S yr}^{-1}\) \(8.7\text{ Tg S yr}^{-1}\) 35% of total sulfur flux
Marine Boundary Layer \(\text{SO}_2\) High (\(40–120\text{ ppt}\)) Reduced by \(20–45\%\) Better agreement with ATom observations
Cloud Condensation Nuclei (\(\text{CCN}_{0.1\%}\)) Overestimated in boundary layer Redistributed to free troposphere \(-12\%\) at surface, \(+18\%\) aloft
Pre-Industrial Radiative Baseline Less reflective marine clouds Elevated pristine background CCN Shifts baseline albedo
Anthropogenic Aerosol Forcing (\(\Delta F_{\text{aer}}\)) \(-1.38\text{ W m}^{-2}\) \(-1.27\text{ W m}^{-2}\) \(+0.11\text{ W m}^{-2}\) warming offset

4. Interactive Simulator: DMS Oxidation & Marine Sulfate Partitioning

Explore how temperature, cloud liquid water content, and radical concentrations govern the branch point between traditional \(\text{SO}_2\) formation and the \(\text{HPMTF}\) autoxidation channel:

🌊 Marine Sulfur Chemical Flux Partitioning

HPMTF Channel (Autoxidation) 38.4%
Direct SO₂ / MSA Channel (Traditional) 61.6%
Warmer tropical temperatures accelerate the intramolecular H-shift rate, boosting HPMTF formation to over 45% of total DMS flux.

5. Climate Implications & Radiative Baseline

Why does a chemical intermediate matter so much for global climate projections?

The answer lies in the pre-industrial aerosol baseline[^5]. Anthropogenic aerosol radiative forcing is calculated as the difference in cloud albedo between the modern polluted atmosphere and the pre-industrial unperturbed atmosphere:

\[\Delta F_{\text{aer}} = F_{\text{modern}} - F_{\text{pre-industrial}}\]

In pre-industrial times, natural \(\text{DMS}\) was the dominant source of cloud-forming particles over the oceans. Because cloud albedo is a logarithmic function of droplet concentration (\(A \propto \ln N_d\)), brightening clouds in an already pristine regime produces a much larger radiative response than adding aerosols to a polluted continent.

By correctly accounting for \(\text{HPMTF}\) multi-phase loss and sulfur redistribution, global models refine the pre-industrial CCN baseline, reducing climate sensitivity uncertainty in IPCC-class Earth system models.

Data and Code Availability

  • Model Integration: The chemical mechanism is implemented in the Community Earth System Model (CESM2.1 / CAM6-chem), available through the NCAR CESM repository.
  • Observational Datasets: Airborne observational validation data were obtained from the NASA Atmospheric Tomography (ATom) mission[^6].
  • Source Code: Analysis scripts and mechanism definitions are archived on GitHub: kamingfung/DMS-HPMTF-CESM.

References

[^1]: Charlson, R. J., Lovelock, J. E., Andreae, M. O. & Warren, S. G. Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 326, 655–661 (1987). https://doi.org/10.1038/326655a0 [^2]: Veres, P. R. et al. Global airborne sampling reveals a previously overlooked dimethyl sulfide oxidation product in the marine atmosphere. Proc. Natl. Acad. Sci. USA 117, 4505–4510 (2020). https://doi.org/10.1073/pnas.1919344117 [^3]: Berndt, T. et al. Fast autoxidation of dimethyl sulfide in the marine atmosphere. J. Phys. Chem. A 124, 3608–3618 (2020). https://doi.org/10.1021/acs.jpca.0c02111 [^4]: Fung, K. M., Heald, C. L., Kroll, J. H., Wang, S., Jo, D. S., Gettelman, A., Lu, Z., Liu, X. et al. Exploring DMS oxidation and implications for global aerosol radiative forcing. Atmos. Chem. Phys. 22, 1549–1573 (2022). https://doi.org/10.5194/acp-22-1549-2022 [^5]: Carslaw, K. S. et al. Large contribution of natural aerosols to uncertainty in indirect forcing. Nature 503, 67–71 (2013). https://doi.org/10.1038/nature12674 [^6]: Wofsy, S. C. et al. ATom: Merged Atmospheric Chemistry, Trace Gases, and Aerosols Data. (ORNL DAAC, Oak Ridge, Tennessee, USA, 2018). https://doi.org/10.3334/ORNLDAAC/1581