Sulfur dioxide over the western North Atlantic Ocean during GCE/CASE/WATOX

Sulfur dioxide (SO₂) was measured with a pulsed fluorescence analyzer and filter pack system from the instrumented NOAA King Air aircraft. The measurements were part of the GCE/CASE/WATOX project in July 1988 and included a series of eight flights of 4‐hour duration each. During the first four flights the atmosphere was sampled over the Atlantic Ocean within 250 km east of Newport News, Virginia; during the remaining four flights the atmosphere in the vicinity of Bermuda was sampled. SO₂ data were collected at two elevations: about 750 mbar in the free troposphere (FT), and in the lower part of the boundary layer (BL) at about 1000 mbar. Average BL concentrations were almost 10 times greater near the east coast than near Bermuda (500±800 pptv versus 60±40 pptv). FT concentrations near the east coast also were greater than near Bermuda but by a lesser factor (110±50 pptv versus 70±40 pptv). The data from the continuous analyzer compared favorably with the data obtained from filter samples collected concurrently. SO₂ data from this study are somewhat lower (factor of 0.6‐0.9) than previous measurements at both sites. Average concentrations of SO₂ measured near Bermuda (60 pptv) were in the range of those observed in the remote marine atmosphere, although higher SO₂ concentrations (up to 450 pptv) were also detected on occasion. The SO₂ removal rate over the western Atlantic Ocean, near the east coast, is estimated empirically at 0.07±0.03 hour⁻¹. Analyses of the known removal mechanisms suggested that the main removal mechanism is dry deposition.