Ozone oxidation of sulfur in sea‐salt aerosol particles during the Azores Marine Aerosol and Gas Exchange experiment

Abstract

Sea‐salt aerosol particles in the lowest tens of meters above the ocean are, typically, more than three‐fourths water on a volume basis. Calculations herein indicate that aqueous‐phase conversion of sulfur dioxide dissolved in the water associated with sea‐salt particles (sea‐salt aerosol water) supported the production of 2–8 nmol m⁻³ of nonsea‐salt sulfate (nssSO₄⁼) during the Marine Aerosol and Gas Exchange (MAGE) experiment intensives. This production is based on ozone oxidation of dissolved SO₂ in sea‐salt aerosol water and accounts for sulfur gas and ozone mass transfer limitations as a function of sea‐salt particle size. Measurements showed that 1–15 nmol m⁻³ of nssSO₄⁼ was actually present in the sea‐salt particle mode except for four enhanced concentration cases due to continental sulfur input. The range in predicted, as well as observed, nssSO₄⁼ was primarily due to variability in sea‐salt aerosol water volume. The nssSO₄⁼ produced by ozone oxidation of sulfur dioxide, being in the sea‐salt particle mode with observed volume geometric median diameter of 3.5–5 μm, is dry deposited at a fairly rapid rate. The remainder, being large‐particle cloud condensation nuclei, may contribute little to cloud albedo over the global oceans. The two papers following this one, Kim et al. (this issue) on aerosol size distribution and water content and Parfai et al. (this issue) on compositional variations of sea‐salt‐mode aerosol particles observed by electron microscopy, complement and support results presented here. All three present results of the MAGE experiment and precede a forthcoming special issue of the Journal of Geophysical Research‐Atmospheres on MAGE (B. Huebert, guest editor).