Global climate models (GCMs) and satellite retrieval algorithms require aerosol chemical composition as a function of size to calculate aerosol radiative effects and cloud condensation nuclei number. In most cases, the size distribution is prescribed in the model or algorithm because of the difficulty in representing the many processes controlling the size distribution (e.g., nucleation, preexisting size distribution, condensation, cloud processing, wet and dry deposition). Multiwavelength satellite retrievals can be used to derive aerosol size distributions but still require a large number of assumptions [King et al., 1999]. Field campaigns provide a direct measurement of aerosol properties that can be used to constrain, test, validate, and refine the assumptions used in GCMs and satellite retrieval algorithms. Although the field campaigns cover a very limited time and space, the data can be binned to provide regional aerosol properties based on location and air mass back trajectories.
 During February–March 1999, the Indian Ocean Experiment (INDOEX) Intensive Field Phase (IFP) quantified the atmospheric chemistry, aerosol properties, and radiation budget over the Indian Ocean [Ramanathan et al., 2001]. Measurements aboard the R/V Ronald H. Brown covered the Southern and Northern Indian Ocean, Arabian Sea, and Bay of Bengal (Figure 1). Reported here are aerosol number and volume size distributions measured aboard the ship summarized by air mass source regions. The regions were defined using air mass back trajectories calculated for the ship's position at six hour intervals with the hybrid single-particle Lagrangian integrated model HY-SPLIT 4 based on the National Centers for Environmental Prediction (NCEP) global wind fields [Draxler, 1992; http://www.noaa.gov/ready-bin/fnl.pl]. The regions conform to the meteorological regimes defined for the study area by W. P. Ball et al. (Bulk and size-segregated aerosol composition: Continental impacts during INDOEX 1999, submitted to Journal of Geophysical Research, 2001, hereinafter referred to as Ball et al., submitted manuscript, 2001). [Southern Hemisphere maritime extra-tropical (SHmX), Southern Hemisphere maritime equatorial (SHmE), Northern Hemisphere maritime equatorial (NHmE), Northern Hemisphere continental tropical (NHcT), Northern Hemisphere continental extra-tropical (NHcX), and Northern Hemisphere continental (NHc)] and the aerosol source regions defined by Quinn et al. .
 The particle size distributions reported here are in terms of geometric diameter at 55% relative humidity (RH). A major goal of our measurement program is to quantify the relationships between the chemical, physical, and optical properties of the aerosol [Quinn et al., 2002]. To reduce the uncertainties in these comparisons we attempt to measure all the aerosol properties at a common controlled RH (55%). The size distributions can be adjusted to other relative humidities with published growth factors [e.g., Swietlicki et al., 2000].