Sulfur dioxide in the tropical marine boundary layer: dry deposition and heterogeneous oxidation observed during the Pacific Atmospheric Sulfur Experiment

• Published in: Journal of atmospheric chemistry Vol. 63; no. 1; pp. 13 - 32
• Main Authors: Faloona, Ian, Conley, Stephen A, Blomquist, Byron, Clarke, Antony D, Kapustin, Vladimir, Howell, Steven, Lenschow, Don H, Bandy, Alan R
• Format: Journal Article
• Language: English
• Published: Dordrecht Dordrecht : Springer Netherlands 01.05.2009; Springer Netherlands; Springer; Springer Nature B.V
• Subjects: Aerosols; Airborne particulates; Atmospheric aerosols; Atmospheric boundary layer; Atmospheric chemistry; Atmospheric Protection/Air Quality Control/Air Pollution; Atmospheric research; Atmospheric Sciences; Boundary layers; Chemical composition and interactions. Ionic interactions and processes; Convection, turbulence, diffusion. Boundary layer structure and dynamics; Dry deposition; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Exact sciences and technology; External geophysics; Ionization; Marine boundary layer; Marine environment; Meteorology; Oxidation; S(IV) oxidation; Sea salt aerosols; Sulfur; Sulfur cycle; Sulfur dioxide; Trade winds; Equatorial Pacific; Pacific Ocean; I, Pacific; Sulfur cycle; S(IV) oxidation; Dry deposition; Marine boundary layer; Sea salt aerosols; dimethyl sulfide; Atmospheric fallout; biogeochemical cycle; Marine atmosphere; aerosols; tropical zone; oxidation; Measurement campaign; Deposition rate; mass spectroscopy; Chemical reaction kinetics; Sulfur dioxide; Chemical ionization; Atmospheric chemistry; Atmospheric boundary layer; Aircraft observation; Shallow convection; Heterogeneous reaction
• ISSN: 0167-7764; 1573-0662
• DOI: 10.1007/s10874-010-9155-0

Research flights with the National Center for Atmospheric Research (NCAR) C-130 airborne laboratory were conducted over the equatorial ocean during the Pacific Atmospheric Sulfur Experiment (PASE). The focused, repetitive flight plans provided a unique opportunity to explore the principal pathways of sulfur processing in remote marine environments in close detail. Fast airborne measurements of SO₂ using the Drexel University APIMS (Atmospheric Pressure Ionization Mass Spectrometer) instrument further provided unprecedented insight into the complete budget of this important sulfur gas. In general, turbulent mixing in the marine boundary layer (MBL) continuously depletes SO₂ due to the shallow convection of the tropical trade wind regime by venting the gas into the buffer layer (BuL) above. However, on nearly one-third of the flights a net import of SO₂ into the MBL from the BuL was observed. Concurrent measurements of the DMS budget allowed for a heterogeneous S(IV) oxidation rate to be inferred from the SO₂ budget residual. The average heterogeneous loss rate was found to be 0.05 h⁻¹, which taken in conjunction with the observed aerosol surface area distributions and O₃ levels indicates that the supermicron aerosols maintain a near neutral pH. The average dry deposition velocity of SO₂ was found to be 0.4 cm s⁻¹, about 30% lower than predicted by standard parameterizations. The yield of SO₂ from DMS oxidation was found to be near unity. The mission averages indicate that approximately 57% of the SO₂ in the MBL is lost to aerosols, 27% is subject to dry deposition, 7% is mixed into the BuL, and 10% is oxidized by OH.