In February and March 1990, measurements of aerosol physicochemistry were made during five transects across the equator between 15¿N and 10¿S. Marked equatorial gradients in both the aerosol NH+4/SO=4 ratio and the strong surface water chlorophyll were associated with boundaries separating oligotrophic waters and regions of equatorial upwelling. Highest aerosol ammonium concentrations appeared to be unrelated to continental signatures but corresponded to regions of highest chlorophyll concentrations. Favorable aerosol chemistry, wind directions, and cruise tracks in conjunction with rapid aerosol sampling made it possible to estimate the flux of ammonia from the surface in these transition regions at about 10 μmol m-2 d-1 with possibly greater values in regions where higher chlorophyll concentrations exceeded about 0.25 mg m-3. Low NH4+/SO4= values in aerosol over oligotrophic regions with chlorophyll concentrations below 0.1 mg m-3 suggest the flux in these regions was about an order of magnitude lower. Aerosol sulfate concentrations were also generally elevated over the upwelling region but showed a less pronounced relationship to chlorophyll abundance, suggesting independent source mechanisms for ammonium and sulfate. Rapid variability in mass and number concentrations were evident in convective regions. Both depletion of larger aerosol (mass) through precipitation scavenging and an increase in the number of smaller aerosol in region of subsidence indicate the complex relationship amount CN, CCN, and aerosol mass in the remote marine boundary layer. ¿ American Geophysical Union 1993