A Potentially New Aerosol Particle Source Due to Photochemistry at the Ocean Surface Microlayer
Résumé
The sea surface microlayer extensively covers the Earthâs oceans and is host to numerous organic and biogenic compounds which also concentrate there. Many bulk and surface-bound organic materials, such as humic acids, are photosensitizers and, thus have the potential to trigger unique chemistry when irradiated by sunlight. It is well recognized that the exchange of gases and particles with the atmosphere are impacted by the presence of the sea surface microlayer, however, the exact mechanisms which accomplish this are not fully understood. Here, we present a laboratory study on VOC production and emission due to photochemical reactions occurring at the sea surface microlayer followed by secondary organic aerosol (SOA) generation. These data are valuable to the assessment of VOC and SOA atmospheric budgets and increase our fundamental understanding of their production.
Laboratory experiments were conducted in a custom-built Teflon reaction chamber with a pure or sea water reservoir containing nonanoic acid, a model surfactant proxy for a surface microlayer, with and without the presence of humic acids. Experiments were performed under irradiation of UV and visible light in a humidified low NOx and low ozone environment. Concentrations of VOCs were measured over time using a proton-transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). Characterization of organic and inorganic compounds in water and aerosol was performed using ion chromatography and liquid chromatography-high resolution-mass spectrometry (LC-HR-MS) utilizing a quadrupole-orbitrap detector. Aerosol size distribution and numbers were continually monitored. Physical and chemical characterization of SOA was investigated with scanning transmission X-ray microscopy coupled with near-edge X-ray absorption fine structure (STXM/NEXAFS) spectroscopy using the PolLux X-ray beamline at the Swiss Light Source.
Production of VOCs was observed while the chamber air and water were irradiated with UV light for the system, nonanoic acid and pure water (background levels of ozone and NOx). Gas phase products include alkenes, aldehydes and dienes, such as isoprene as observed by PTR-ToF-MS in two different ionization modes (H3O+ and NO+) and confirmed from independent experiments in a controlled Quartz reaction cell. Introduction of ozone into the chamber triggered new particle formation followed by condensational growth, likely due to the ozonolysis of present gas phase products having carbon double bonds to form lower volatility compounds. When a salt water system was used containing humic acid and the surfactant, we find that the VOC and SOA yield is further enhanced under irradiation. A spectroscopic signature of SOA produced in our chamber was acquired with STXM/NEXAFS characterized by oxygenated organic material dominated by the presence of the carboxyl and carbonyl functional groups. Secondary absorption peaks indicated a minor presence of hydroxyl functionality and carbon double bonding. This method allows for spectral comparison between the generated SOA and known SOA spectra from field and laboratory studies.
We suggest that light absorbing compounds, or photosensitizers, present at the interface trigger the production of a radical chemistry proceeding through an initiative step of hydrogen-abstraction on the surfactant, nonanoic acid. through hydrogen abstraction. Due to the high concentration of organic in the microlayer, unique chemistry follows involving self-reactions, which are unfavorable in the gas and aqueous phase. Ozonolysis of these products then stimulates SOA formation. These results underscore the significance of photon-induced chemistry at the ocean-atmosphere interface with the potential to significantly impact on VOCs and SOA over the oceans.