SATPC0032319 Tab 04 4 SOW_Redacted.pdf

July 24, 2023

STATEMENT OF WORK

Project Name

“Formation of Secondary Organic Aerosols via Oxidation of Volatile Methylsiloxanes”

Project Summary

Volatile methylsiloxanes (VMS) and their oxidation products are persistent contaminants on board the International Space Station (ISS). Analysis of secondary organic aerosol (SOA) samples collected on board the ISS revealed the presence of silicon, carbon, oxygen, and hydrogen, indicating that oxidized VMS may be partially responsible for aerosol formation.

We propose to study the effect of sequential oxidations on the physicochemical properties of selected VMS species and model their potential for SOA formation. The most relevant oxidation mechanism is initiated when a VMS hydrogen atom is abstracted by a hydroxyl radical under low NOx conditions. The most commonly observed product of gas-phase VMS oxidation is a silanol, in which a VMS methyl group has been replaced by a hydroxyl (OH) group. For each candidate VMS, we define its sequential oxidation products as increasing numbers of OH substitutions.

We are focused on the following linear and cyclic VMS identified from analysis of return-to-ground charcoal air filters, air quality monitor measurements, and whole air grab samples.

Linear VMS:

- Trimethylsilanol (TMS)

- Hexamethyldisiloxane (L2)

- Octamethyltrisiloxane (L3)

- Decamethyltetrasiloxane (L4)

- Dodecamethylpentasiloxane (L5)

- Hexadecamethylheptasiloxane (L7)

Cyclic VMS:

- Hexamethylcyclotrisiloxane (D3)

- Octamethylcyclotetrasiloxane (D4)

- Decamethylcyclopentasiloxane (D5)

- Dodecamethylcyclohexasiloxane (D6)

- Tetradecamethylcycloheptasiloxane (D7)

- Hexadecamethylcyclooctasiloxane (D8)

- Octadecamethylcyclononasiloxane (D9)

Milestones

Project period of performance is 18 months from approval of funding award, with possible no-cost extension of 1-6 months for optional task.

Task 1: Formulas and structures of VMS oxidation products (1 month) Generate list of chemical formulas and structures for sequential OH substitutions of candidate VMS. Results required for Task 2.

Task 2: Physical properties of VMS oxidation products (6 months) Use group contribution method to estimate boiling point, saturation concentration (C*), and Henry’s Law constant for every VMS oxidation product. Results required for Task 3.

Task 3: Effect of oxidation on vapor pressure (1 month) Generate plots of log(C*) as a function of O/Si ratio and C/Si ratio to understand effect of oxidation on saturation concentration. Results can be compared to published volatility basis sets and may reveal overall trends as well as specific effects for linear or cyclic structures. Results required for Tasks 5 and 6.

Task 4: Model of SOA formation from oxidized VMS (6 months) Use estimated physical properties and existing kinetic data to model gas-particle partitioning of VMS oxidation products in the ISS atmosphere. Identify specific VMS with highest potential for SOA formation. Results required for Task 6.

Optional Task: Oxidation reaction kinetics (1-6 months) If kinetic information required for Task 4 is not available in the literature, electronic structure calculations and/or quantitative structure-reactivity relationships can be used to estimate rate constants.

Task 5: Conference presentation and model revisions (3 months) Paper submitted for presentation at the 2024 International Conference on Environmental Systems. Any insights gathered from the community may be incorporated into revisions of the Task 4 model.

Task 6: Final report (1 month) Comprehensive final report summarizing methods, results, discussion of findings, and conclusions.