This paper is the first of a series dedicated to an observational study of the aerosol indirect effect in marine stratocumulus clouds. The data were collected in 1997, during the second Aerosol Characterization Experiment (ACE-2) CLOUDYCOLUMN experiment, conducted over the eastern Atlantic Ocean, in the vicinity of the Canary Islands. Here we compare measurements made both on aircraft and at the surface, including condensation nuclei (CN) and accumulation mode aerosol concentrations, and aerosol size spectra. We also compare measured and predicted wet aerosol size spectra and examine statistics of vertical velocity within and below stratocumulus clouds. In general, aircraft and surface measurements of CN concentration, accumulation mode aerosol concentration and size spectra agree within expected uncertainties. However, a substantial disparity is documented in the comparison of predicted and observed wet aerosol spectra. We attribute this to either bias in the wet aerosol measurements, made with an FSSP-300, or to error in the sizing of dry aerosol particles. The analysis of vertical velocity indicates that the first and third moments of the vertical velocity frequency distribution do not change substantially between below-cloud and in-cloud flight segments; however, an increase in the second moment of the distribution across cloud base is documented. Overall, the results lend confidence to the use of surface site aerosol physical and chemical property data, as well as airborne measurements of vertical velocity, for modeling of the aerosol activation process, as described by Snider et al. <2003> (hereinafter referred to as part 2) in the special section. Here, and in the paper by Snider et al. <2003> (hereinafter referred to as part 2), we examine how sensitive the predictions are to the common assumption that the aerosol particles are compact spheres.