DIMETHYLSULFIDE, AEROSOLS, AND CONDENSATION NUCLEI OVER THE TROPICAL NORTHEASTERN ATLANTIC-OCEAN

Concentration of dimethylsulfide (DMS) in seawater, concentrations of DMS and sulfur dioxide (SO2) in the atmosphere, concentrations of methanesulfonate (MSA) and non-sea-salt sulfate (nss-SO42-) in size-segregated aerosols, and number concentration of condensation nuclei (CN) were measured during September and October 1991 in the northern tropical Atlantic Ocean in order to assess the role of DMS in CN production over this oceanic area. Radon-222 activity and aerosol ionic composition were used to distinguish air masses with oceanic, continental, and/or polluted characters. No obvious covariation appeared between DMS and its oxidation products (SO2, H2SO4, and MSA) over the whole period of the experiment. However, the division of data into subsets according to continental tracer information allowed us to show that SO2 and nss-SO4 concentrations correlated with DMS concentration in unpolluted air masses. MSA and nss-SO42- were found to be mainly concentrated in particles with diameters < of 0.6 mum. Daily mean nss-SO42- in the < 0.6-mum-diameter range and CN concentration were correlated (R = 0.91, n = 17, P < 0.001), which suggests that H2SO4 is an important CN precursor. Atmospheric DMS and CN number daily mean concentrations also correlated (R = 0.82, n = 21, P < 0.001). However, the CN population was strongly influenced by continental inputs less than 500 km downwind of Africa, whereas DMS seemed to be able to affect the CN number concentration at about 1500 km from this continent.