Week 9: Atmospheric Sulfur Chemistry


Monday, 26th, Wednesday 28th, and Friday 31st October, 2008, lecture 1 (pdf), lecture 2 (pdf), lecture 3 (pdf)

Overhead sources are
Chemistry of the Natural Atmospheres, Peter Warneck, Academic Press 1999

Atmospheric Chemistry and Global Change, Brasseur, Orlando, Tyndall, Eds., Oxford University Press, 1999
Chemistry of the Upper and Lower Atmosphere, Finlayson-Pitts and Pitts, Academic Press, 2000

 

Mandatory Reading this week:

Chapter 10: Sulfur Compounds in the Atmosphere, from Warneck, AP 1999

 


  Points/Topics to remember from this week’s classes:

  • Most sulfur on Earth is locked up in rocks (ores) and as sulfate in the oceans. However, much sulfur transport happens through the atmosphere, and major changes to global and atmospheric sulfur cycling have occurred and keep changing as a result of anthropogenic activities.
  • Anthropogenic sulfur emissions are a result of fossil fuel combustion (largely high sulfur coal) and metal ore smelting, both of which result in SO2 emissions.
  • The atmospheric fate of SO2 has been studied in detail in the gas and aqueous phase. In the gas phase, oxidation starts with OH attack as the rate-limiting step:
    1. SO2 + ·OH  (+M)  →  HOSO2  (+M)
    2. HOSO2 + O2   →  SO3 + HO2·
    3. SO3 + H2O  →  H2SO4

H2SO4 is formed in reaction 3 exists in the gas phase up to very low ppt levels. It is (based on current knowledge) the most important species initializing nucleation. Once small H2SO4×H2O clusters have formed, they quickly take up NH3 (to form ammonium sulfate), and sometimes semivolatile organic compounds, only to grow and become cloud condensation nuclei (CCN). This process is central to cloud formation on Earth! Without SO2 in our atmosphere, cloud formation and processing, and therefore climate (!), would look different.

  • Due to its good water solubility, SO2 is easily taken up into the aqueous phase in tropospheric hydrometeors before oxidation via reaction 1. Its equilibrium concentration is governed by Henry’s Law, and, in this case, additionally by dissociation to HSO3– (bisulfite). At typical pH values for atmospheric waters between 3 (acid rain) and 6 (bicarbonate dominated rainwater) the dominant liquid phase oxidant of HSO3– is H2O2, followed by ozone:

4.    HSO3– + H2O2  →  HSO4– (bisulfate) + H2O 

5.    HSO3– + O3  →  HSO4–  + O2                  (note: these are the net reactions!)

Besides reactions 4 and 5, many other oxidants and pathways to (bi-)sulfate have been identified, some of which can play important roles locally or regionally. However, reactions 4 and 5 dominate globally and in the clean troposphere. The SO2 lifetime will depend on total water volume (highest in clouds) and oxidant availability.

·       The dominant natural sources of SO2 are the oxidation of reduced sulfur emissions, and direct SO2 emissions from volcanoes.  Reduced sulfur includes all species in which S exists in S(-II) form (sulfides), such as H2S (hydrogen sulfide), CH3SH (methyl sulfide), and (CH3)2S (dimethyl sulfide, DMS), or Carbonylsulfide (OCS or COS). H2S is dominated by volcanic and anoxic soil emissions, DMS by biogenic emissions from the world’s oceans, highest in nutrient-rich areas such as upwelling regions (e.g. around Antarctica). Reduced sulfur species with the exception of OCS have relatively short lifetimes (τ < 1 week) and are oxidized before reaching the free troposphere in significant amounts. OCS on the other hand was recognized in the 70s as the major natural source of sulfur to the stratosphere, explaining the occurrence of the Junge Layer (named after its discoverer, Dr. Junge) of thin stratospheric H2SO4 clouds. OCS is both emitted directly from the ocean, and produced in-situ from tropospheric CS2 and DMS oxidation. Its atmospheric budget is now relatively well understood, consisting of mostly natural sources and sinks.

·       The highest natural sulfur emission is DMS from the oceans. DMS oxidation starts with OH attack:

6.    DMS + ·OH   →  ·CH2SCH3 + H2O

7.    DMS + ·OH  (+M)  →  CH3S(OH)CH3  (+M)

8.    ·CH2SCH3 + O2 (+M) + NO  →  NO2  (+M)  + CH3SCH2O·  →  CH3S· + HCHO

The adduct formed in reaction 7 and the CH3S radical formed in reaction 8 are the precursors to the dominant reaction products of DMS oxidation, methyl sulfonic acid (MSA), and SO2 (see oxidation scheme). 

A widely acknowledged (and not yet refuted) hypothesis called CLAW (named after the authors’ last name initials), states that our climate is partially regulated by a negative feedback process: Oceanic DMS emission leads to SO2 formation and then CCN production which leads to cloud formation. Increased cloud formation from increased CCN production lowers the average annual radiance to the oceans, lowering biological activity, which is responsible for the DMS emissions. Lower DMS emissions lead to less CNN production and therefore less cloudy conditions, allowing a higher surface irradiance, etc. (Brainstorm about other factors influencing DMS emissions besides radiation needed for photosynthesis in the oceans!). A recent article discussing geo-engineering related to the CLAW hypothesis is here.

·       Atmospheric sulfur has been altered dramatically by anthropogenic activities, in particular ore smelting and coal burning. Northern Hemisphere sulfur emissions are still dominated by these emissions. Southern Hemisphere emissions are dominated by natural sources (larger oceanic area!). After filter installations in smelters and coal-fired power plants reduced SO2 emissions drastically in Europe in the 80ies, today’s anthropogenic SO2 emissions (in Europe) are more diffuse in nature (with the exception of those countries that do not have strict air quality regulations). Single, large sources that remain are “old” power plants in the western world, unregulated emissions from new coal-fired power plants in the ‘developing’ world (e.g. China), and railroad and ship-based diesel engines (e.g. satellite imagery shows higher cloud reflectance over ship-tracks as a result of increased CCN production from emitted SO2 leading to a redistribution of cloud water into smaller droplets).

 

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