<p></p><p><a>COS and CS<sub>2</sub> are sulfur compounds that are formed
in natural waters. These compounds are also volatile, which leads them move
into the atmosphere and serve as critical precursors to sulfate aerosols.
Sulfate aerosols are known to counteract global warming by reflecting solar
radiation. One major source of COS and CS<sub>2</sub> stems from the ocean. While
previous studies have linked COS and CS<sub>2</sub> formation in these waters to the
indirect photolysis of organic sulfur compounds, much of the chemistry behind
how this occurs remains unclear. This study examined this chemistry by
evaluating how different organic sulfur precursors, water quality constituents,
and temperature affected COS and CS<sub>2</sub> formation in natural waters.</a></p>
<p>In the first part of this thesis (chapters 2 and 3), nine natural
waters ranging in salinity were spiked with various organic sulfur precursors
(e.g. cysteine, cystine, dimethylsulfide (DMS) and methionine) exposed to
simulated sunlight over varying exposures. Other water quality conditions
including the presence of O<sub>2</sub>, CO and temperature were also varied. Results
indicated that COS and CS<sub>2</sub> formation increased up to 11× and 4×,
respectively, after 12 h of sunlight while diurnal cycling exhibited varied
effects. COS and CS<sub>2</sub> formation were also strongly affected by the DOC
concentration, organic sulfur precursor type, O<sub>2</sub> concentration, and
temperature while salinity differences and CO addition did not play a
significant role.</p>
<p>To then specifically evaluate the role of DOM in cleaner matrices,
COS and CS<sub>2</sub> formation was examined in synthetic waters (see chapters 4 and
5). In this case, synthetic waters were spiked with different types of DOM
isolates ranging from freshwater to ocean water along with either cysteine or
DMS and exposed to simulated sunlight for up to 4 h. Surprisingly, CS<sub>2</sub> was
not formed under any of the tested conditions, indicating that other water
quality constituents, aside from DOM, were responsible for its formation.
However, COS formation was observed. Interestingly, COS formation with cysteine
was fairly similar for all DOM types, but increasing DOM concentration actually
decreased formation. This is likely due to the dual role of DOM on
simultaneously forming and quenching the reactive intermediates (RIs).
Additional experiments with quenching agents to RIs (e.g. <sup>3</sup>DOM* and ·OH)
further indicated that ·OH was not involved in COS formation
with cysteine but <sup>3</sup>DOM* was involved. This result differed with DMS
in that ·OH and <sup>3</sup>DOM* were both found to be involved. In
addition, treating DOM isolates with sodium borohydride (NaBH<sub>4</sub>) to reduce
ketone/aldehydes to their corresponding alcohols increased COS formation, which
implied that the RIs formed by these functional groups in DOM were not
involved. The alcohols formed by this process were not likely to act as quenching
agents since they have been shown to low in reactivity. Since ketones are known
to form high-energy-triplet-states of DOM while quinones are known to form
low-energy-triplet-states of DOM, removing ketones from the system further
supported the role of low-energy-triplet-states on COS formation. This was
initially hypothesized by findings from the testes on DOM types. In the
end there are several major research contributions from this thesis. First,
cysteine and DMS have different mechanisms for forming COS. Second, adding O<sub>2</sub> decreased
COS formation, but it did not stop it completely, which suggests that further
research is required to evaluate the role of RI in the presence of O<sub>2</sub>. Lastly,
considering the low formation yields of COS and CS<sub>2</sub> formation from the
organic sulfur precursors tested in this study, it is believed that some other
organic sulfur precursors are missing which are likely to generate these
compounds to higher levels and this needs to be investigated in future
research. </p><br><p></p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8632286 |
| Date | 15 August 2019 |
| Creators | Mahsa Modiri-Gharehveran (6594389) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/INDIRECT_PHOTOCHEMICAL_FORMATION_OF_COS_AND_CS2_IN_NATURAL_WATERS_KINETICS_AND_REACTION_MECHANISMS/8632286 |
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