Hydroxyl radicals are the most important oxidants in the atmosphere, playing a critical role in atmospheric chemistry. Oxygenated hydrocarbons like acetone are a major source of hydroxyl radicals in the drier upper troposphere. The role of the oceans in cycling acetone into or out of the troposphere is believed to be important but is currently not well established. A better understanding of the production and destruction processes that control acetone concentrations in the sea would ultimately allow scientists to estimate the global air-sea flux of acetone and its impact on tropospheric chemistry.
In this project, researchers at Chapman University will study the sea-air flux of acetone by addressing four short-term goals: (1) determining acetone at nanomolar levels in seawater with a method compatible with undergraduate student research capabilities; (2) determining acetone concentrations in surface seawater at coastal sites in Southern California over several diurnal periods in the dry and wet rainy season; (3) estimating the biological and photochemical production rates of acetone in seawater; and (4) estimating the photochemical and biological degradation rates for acetone in seawater.
The long term goals are to improve the overall understanding of the processing of acetone in surface seawaters and ultimately the flux of acetone across the air-sea interface. The proposed research builds on the PIs? extensive previous experience in the analysis of trace gases produced in the surface ocean and the photochemistry of CDOM in coastal waters and will use existing analytical facilities in the PIs? laboratories at Chapman.
This proposal will have significant broader impacts on the integrated research and educational experience of undergraduate students at Chapman University. Chapman is a small private liberal arts-based university located in a culturally diverse community in Southern California. In addition to providing significant research opportunities to undergraduate students, findings and methods from this study will be incorporated into the curriculum and laboratories of undergraduate courses taught by the PIs in Advanced Topics in Environmental Chemistry: Atmospheric Chemistry, Instrumental Analysis and Physical Chemistry, taken by 10-20 chemistry majors per year.
Oxygenated hydrocarbons like acetone are a source of hydroxyl radicals (important oxidants) to the atmosphere. The role of the oceans in cycling acetone into or out of the atmosphere is not clear. The aim of this research was to study the production and degradation of acetone in seawater from both a biological and photochemical perspective. This research was carried out by undergraduates, requiring a simple analytical method be used. A dinitrophenylhydrazine derivitization liquid chromatography technique was optimized to measure acetone in seawater. We originally only proposed studying acetone cycling, but were able to measure other oxygenated hydrocarbons (acetaldehyde, formaldehyde) and reactive species (hydrogen peroxide). Samples were taken from beach waters in Orange County, Southern California and adjacent salt marshes which drain into the coastal waters. Several studies were conducted at one beach site where water samples were taken hourly over 30 hour time periods to evaluate day and night-time levels. Earlier research showed that photochemical production of oxygenated hydrocarbons occurs when colored dissolved organic matter (CDOM) is exposed to sunlight in seawater. CDOM are large complex molecules found in natural water systems produced when plant materials like leaf litter decay. Optical properties of the samples were measured to characterize the CDOM they contained. We also carried out experiments in the laboratory to test how the reaction mechanisms (i.e. production processes) were occurring. The concentrations of formaldehyde, acetaldehyde and acetone we measured averaged 26 ± 24 nM, 9 ± 4 nM and 8 ± 3 nM respectively, where nM stands for nanomolar or 1 x 10-9 moles per liter of water. In the diel studies, cycling occurred between higher and lower concentrations over time but there was no clear photochemically dominated production i.e. concentrations did not increase during the day with sunlight levels. We did some experiments that showed that a second source of these compounds, other than photochemically from CDOM, was decaying plant material from kelp beds in the near-shore region. In general, photoproduction rates decreased as CDOM levels decreased but were very variable in beach waters with low CDOM. Apparent quantum yields (Θ; a measure of production efficiency where concentrations of a photochemically produced species are corrected to account for differences in CDOM and sunlight levels) increased rapidly for the beach waters with low CDOM levels. Θ increased with increasing spectral slope, a measure of how extensively CDOM has been aged and processed obtained from absorbance measurements. This is explained by enhanced production efficiency with increasing age and residence time of CDOM in coastal waters i.e. the more CDOM has been exposed to sunlight already, the more effectively it produces oxygenated hydrocarbons with more sunlight. Θ trends in the lab studies with molecular reaction probes suggested that the major production mechanisms are direct photolysis (CDOM absorbs sunlight and directly produces oxygenated hydrocarbons) and quenching by oxygen (CDOM absorbs sunlight, transfers this energy to oxygen by reacting with it and subsequent reactions indirectly produce the oxygenated hydrocarbons). A second system was developed to measure acetone degradation rates in seawater based on isotope dilution purge and trap gas chromatography mass spectroscopy. In isotope dilution analysis, a species with two or more naturally occurring stable isotopes is analyzed in the presence of a standard in which one of the isotopes has been enriched and concentration determined from the measured isotope ratio. For a degradation study, to decouple production and destruction processes, and measure an absolute rate water samples were spiked with D-6 labeled acetone and concentration changes in the D-6 labeled species measured as a function of time. Acetone degradation rates were measured at a single site multiple times over 6 months. We found that photochemical degradation for acetone from sunlight was insignificant relative to biological processes. We assessed biological degradation as the difference between seawater filtered through a 0.2 x 10-6m pore-size filter (removes microbes and microorganisms) and unfiltered seawater. Biological degradation rates for acetone were concentration dependent (first order) i.e. rates were higher when initial concentrations of acetone were higher. The average acetone degradation rate normalized to the initial concentration is 2.6 ± 0.5 day-1. This project resulted in 8 research presentations at regional and national scientific meetings, 3 manuscripts and a book chapter. It supported 2 chemistry faculty members at an institution with no graduate chemistry programs, senior research projects and research experiences for 8 undergraduate chemistry majors and summer research projects for 2 high school interns. The undergraduates have gone on to professional graduate studies in pharmacy and medical schools and Ph.D research programs in chemistry. Five undergraduate students presented their research at American Chemical Society and American Geophysical Union conferences. Three of the supported students have also co-authored publications in scientific research journals based on their work.
