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

Abstract

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 SO 2 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 SO 2 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 SO 2 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 SO 2 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 O 3 levels indicates that the supermicron aerosols maintain a near neutral pH. The average dry deposition velocity of SO 2 was found to be 0.4 cm s −1 , about 30% lower than predicted by standard parameterizations. The yield of SO 2 from DMS oxidation was found to be near unity. The mission averages indicate that approximately 57% of the SO 2 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.

Atmospheric deposition of methanol over the Atlantic Ocean

“…Eddy covariance measurements of the very soluble acetone resulted in air-sea flux at times opposite in direction to the prediction from the two-layer model (27). In the case of the surface reactive sulfur dioxide, aircraft flux measurements yielded k a values ∼30% lower than expected (28). Thus, flux observation of another gas with predominantly airside control, such as methanol, has the potential to reduce the uncertainty in k a and ultimately improve flux estimations based on Eq.…”

Free troposphere as a major source of CCN for the equatorial pacific boundary layer: long-range transport and teleconnections

“…Trajectories in the lower FT have wind speeds of 8-14 m s −1 that can take ∼ 10 days to reach CI after leaving SA (South America). CI was selected to study the natural marine sulfur cycle (Bandy et al, 1996;Faloona et al, 2009;Bandy et al, 2012) because it is located in a region of elevated DMS production and because of its remote environment, as the closest upwind continental sources are located about 10 000 km to the east in the northern part of South America.…”

Atmospheric sulfur cycling in the southeastern Pacific – longitudinal distribution, vertical profile, and diel variability observed during VOCALS-REx

“…Sea-salt aerosols from wave breaking account for the majority of the coarse (supermicron) number as well as total aerosol mass, and tend to be more basic than fine SO 2− 4 aerosols because of the initial alkalinity of seawater (pH∼8.1) and its carbonate buffering capacity. Because the reaction rate between SO 2 and ozone (O 3 ) is greatly accelerated at high pH (Hoffmann, 1985), some authors postulated O 3 oxidation in seasalt aerosols to be a significant sink of SO 2 and source of SO 2− 4 mass (Sievering et al, 1991;Faloona et al, 2010). In addition to affecting atmospheric chemistry, SO 2− 4 aerosols are climatically important because they alter the global radiative balance directly by scattering light (Charlson et al, 1992) and indirectly by controlling the optical properties, areal extent, and lifetimes of clouds by acting as cloud condensation nuclei (CCN).…”