Aerosol dynamics in the equatorial Pacific Marine boundary layer: Microphysics, diurnal cycles and entrainment

During July-August of 1994 we measured the size-resolved physicochemical properties of aerosol particles at Christmas Island in the equatorial Pacific. In spite of rapid diurnal conversion of dimethylsulfide (DMS) to sulfur dioxide (SO2) we found no evidence for new particle production in the marine boundary layer (MBL) and more than 95% of all particles were consistently larger than 0.02 mu m diameter, indicating an aged aerosol. The submicrometer aerosol number (size-distribution) was bimodal with peaks near 0.05 mu m and 0.2 mu m particle diameter (D-p) and had a cloud-processed intermode minimum at about 0.09 mu m that varied in phase with diurnal changes in ozone concentration This suggests that the number distribution for condensation nuclei (CN) arid cloud condensation nuclei (CCN) was maintained by a quasi-equilibrium between entrainment (estimated to be 0.6 +/- 0.2 cm s(-1)) from sources aloft and processes in the MBL. This implies a replenishment timescale for nuclei of about 2 and 4 days for this region. The stability of the distribution and the 0.09 mu m cloud processed minima suggests trade wind cumulus supersaturations near 0.35% and updrafts near 1 m s(-1).