We developed the Light Aircraft Aerosol Package (LAAP) to make measurements of aerosol properties up to ~4000 m asl. The LAAP reliably measured, with empirical and theoretical corrections, aerosol microphysical and chemical properties. During the Puerto Rico Dust Experiment (PRIDE), June--July 2000, we measured aerosol properties over the tropical North Atlantic. On the basis of aerosol filter samples and measured aerosol size distributions, we (1) modeled sea-salt aerosol concentration with altitude, characterizing the rapid decrease in sea-salt concentration with altitude; (2) found no evidence of sea salt above the marine boundary layer (MBL); and (3) observed mineral dust in the Saharan Air Layer (SAL) to exceed 170 ¿g m-3. Substantial variability in the vertical distribution and small differences in the size distribution of dust between the MBL and SAL suggested dust properties were more dependent on the separate transport histories than the interaction between the MBL and SAL near Puerto Rico. Relatively constant dust size distributions with altitude suggested either (1) no relofting of dust aerosols once they get into the deposition layer or (2) dry deposition removes dust particles independent of particle size. Dust size distributions measured at low (~25%) and high (~93%) relative humidity were indistinguishable. A simple model calculation suggests dust particles over the tropical North Atlantic during summer increase their geometric diameter <6% for a relative humidity change from 25 to 95%. Thus optical properties of those dust particles should not change for relative humidities <95%.