In August 1999 an intensive field campaign was conducted in Atlanta, Georgia, focusing on the characterization of urban particulate matter. During the study an aerosol time-of-flight mass spectrometer (ATOFMS) was used to measure continuously the aerodynamic size and chemical composition of individual particles in the fine fraction (0.2--2.5 ¿m) of the atmospheric aerosol. The inorganic and organic components of the particles were analyzed using laser desorption ionization time-of-flight mass spectrometry, generating positive and/or negative ion mass spectra. Here an overview of the ATOFMS results is presented with respect to the major species detected, including sodium, carbon (EC, OC, EC/OC), dust (Li, Na, Al, K, Ca, Fe), sulfate, nitrate, and ammonium. As described, many of the dust particle types have similar composition to those observed in ATOFMS coal source characterization studies. The ion signals, size distributions, and temporal evolution (30--60 min resolution) of the different particle types are described. The complexity of the Atlanta aerosol is shown here on an individual particle basis, demonstrating how single particle data obtained with ATOFMS can be used to gain unique insights into the mixing state of urban aerosols. The second paper in this two part series focuses on the scaling procedures used for converting the unscaled ATOFMS data presented in this paper into atmospherically representative number concentrations <Wenzel et al., 2002>.