Deposition and transformation rates of sulphur oxides during atmospheric transport over the Atlantic

Deposition and transformation rates are basic parameters for the determination of dispersion and deposition of aerosols. On the basis of an air trajectory model and appropriate aerosol measurements at the Faroes and the British Isles the concentration difference resulting from about 1 000 kilometres transport over water is used to determine an eddy deposition rate for sulphur dioxide and sulphate which corresponds to a deposition velocity of 2 cm/sec ±50 % and 0.4 cm/sec ±50 %, respectively. The transformation rate for sulphur dioxide to sulphate is found to be close to the eddy deposition rate for sulphate with a half life of 60 hours ± 20 %. The clean Atlantic air contains 0.8 μg/m³ ± 50 % of sulphate and 0.2 μg/m³ ± 50 % of sulphur dioxide. These concentration levels are two to three times less than that previously assumed for global tropospheric averages used in estimating the global sulphur cycle, indicating that the natural production of sulphur at the free sea surface has previously been overestimated. Electron microscopic determination of the aerosol particle morphology and chemistry provides a unique and direct verification of the models used.