Nitrogen and sulphur over the Western Atlantic Ocean

This paper reports new surface and aircraft measurements of sulphur and nitrogen species made during WATOX-85 at Lewes, Delaware and Bermuda. Concentrations of most species measured in this portion of the western Atlantic Ocean atmosphere were higher than those found in remote marine environments, showing clearly the influence of anthropogenic emissions from North America. The experiment was designed such that measurements were made following cold frontal passages in conditions of strong, dry westerly flow, to ensure that measurements at Bermuda were in air masses that had earlier crossed the east coast in the region of Lewes. Boundary-layer SO2 concentrations decreased by a factor of 20 between the east coast and Bermuda, while sulphate was the same at both locations. First-order decay distances for SO2 and total S were 340 and 620 km, respectively, under these conditions. The decay distance for total S is substantially shorter than previously determined, indicating that SO2 in particular is removed in near-coastal environments more quickly than previously supposed. Boundary-layer NOx and HNO3 concentrations decreased by close to an order of magnitude between the east coast and Bermuda, whereas for NO3− the decrease was a factor of two. Corresponding first-order decay distances of NO'x and total N were 500 and 550km, respectively.