Real time measurements of sulfur dioxide were made on board the R/V Discoverer from 11 October 1995 to 9 November 1995 during a transect from Seattle (USA) to Hobart, Tasmania (Australia) and from 15 November 1995 to 13 December 1995 in the Southern Ocean as part of the first IGAC Aerosol Characterization Experiment. Sulfur dioxide was measured using an HPLC/Fluorescence technique. SO2 was absorbed from air into an aqueous solution then derivatized to form a fluorescent isoindole. The isoindole was then analyzed by HPLC with fluorescence detection. The instrument has 10 minute time resolution and a 3-600 ppt gas calibration system. During the cruise the detection limit of the instrument was 2-4 ppt (3 sigma of the daily mean blank).
Background SO2 levels increased from 20 ppt at 20-30 N, to a maximum of 100 ppt at the Equator, then decreased again to 5-30 ppt in the Southern Ocean. In the Southern Ocean (40-50 S) levels were typically 5-10 ppt during late November and ranged from 10-30 ppt during December. Higher SO2 levels (200ppt-1ppb), associated with volcanic or anthropogenic emissions, were observed off Hawaii, New Zealand and Australia. A comparison of background SO2 with simultaneously measured DMS concentrations shows evidence of anticorrelated diurnal variations at both the equator and in the Southern Ocean. DMS/SO2 ratios increase dramatically with latitude indicating much higher DMS to SO2 conversion efficiencies at the equator.