During PEM-Tropics the P-3B aircraft was instrumented specifically to study marine boundary layer photochemistry and dynamics, with a particularly strong focus on the oxidation of dymethel sulfide (DMS) via the hydroxyl radical (OH) to form gaseous sulfur dioxide (SO₂) and sulfate cloud condensation nuclei. The environment surrounding Christmas Island provides a unique "laboratory" in which the oxidation of DMS can be studied. The meteorological conditions around Christmas Island are relatively uniform providing repeatable experimental conditions, and Christmas island is located in the equatorial upwelling region of the Pacific ocean, a significant source of DMS. Christmas Island is also far removed from major sources of anthropogenic SO₂, and volcanic SO₂, and from anthropogenic ozone precursors, and therefore provides and ideal "background" environment for the study of oxidation of DMS.