Airborne measurements of aerosol and cloud properties were made within the ITCZ as part of the Eastern Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System (EPIC) in September and October, 2001. Nine flights dedicated to cloud studies were made over a region from 8° - 12° N, 93° - 97° W and focused on characterizing out-of-cloud aerosol properties and in-cloud microphysics. Two of these research flights have been analyzed to evaluate the effect of anthropogenic cloud condensation nuclei (CCN) on the formation of clouds and precipitation development. The two cases show sharp contrasts in the cloud characteristics that can be linked to similar contrasts in aerosol properties.

During research flight 13, at 30 m above the ocean surface, the CCN concentrations (at 1% supersaturation) were less than 200 cm-3, CN concentrations were less than 600 cm-3, and the scattering and absorption coefficients were 3x10-6 m-1 and 2x10-8 m-1, respectively. These measurements are typical of clean, maritime air. The average, maximum droplet concentration of clouds in this area, at an altitude of 2500 m, was "100 cm-3 but concentrations of drizzle exceeded 200 l-1 at the same level.

Measurements during research flight 17, also at 30 m altitude, showed CCN concentrations greater than 500 cm-3, CN concentrations > 1500 cm-3, and scattering and absorption coefficients of 3x10-7 m-1 and 6x10-7 m-1, respectively. These aerosol properties are a clear signal of anthropogenic origin whose most likely source is Central America, 600-800 Km to the east of the research area. On this day the average maximum droplet concentration of clouds was >400 cm-3 but with very little drizzle or precipitation measured.