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. On many of the flights precipitation was seen to form in very short time periods, often in less than 30 minutes of formation. As giant nuclei (diameter > 2 mm) can lead to the rapid formation of precipitation because of their role in producing large cloud droplets that rapidly coalesce, the aerosol measurements from 13 flights were analyzed to determine the concentration of such nuclei. The size distributions of large aerosols were measured with the FSSP-100, (2- 47 um). Total condensation nuclei (CN) and the size distributions of aerosol particles between 0.1 - 3 um were measured with a TSI Model 3010 and passive cavity aerosol spectrometer probe (PCASP), respectively. The flight legs covered the area 0° - 12° N, 93° - 97° W. The measurements were analyzed for flight legs near the surface (30 m) and when winds were from the west. These conditions constrain the data to aerosols on natural origin since the flight legs were more than 500 km west of Central America,

Large particle concentrations ranged from 100 - 300 l-1 and increased with wind speeds up to 12 ms-1. Above this wind speed, however, the concentrations decreased such that the overall shape of concentration vs. wind speed is parabolic with the peak near 12 ms-1. The concentration of CN is negatively correlated with the large particles, i.e. the CN concentrations decrease with wind speed to a minimum at 12 ms-1 before increasing at higher velocities.

The presence of large aerosols will be discussed within the context of the possible source and sinks that lead to the observed concentrations. A one dimensional cloud model is used to illustrate the effect of these nuclei on cloud formation and the subsequent production of drizzle and precipitation.