Nanoparticle agglomerates play an essential role in the manufacturing of many nanomaterials and are commonly found in combustion products. Conventional aerosol instruments based on equivalent spheres are not directly applicable to the measurement of nanoparticle agglomerates. The increasing interest in real-time assessment of the structure of engineered nanoparticle agglomerates and the mass concentration of potentially hazardous agglomerates (e.g., diesel soot, welding fume) makes an instrument devoted to online structure and mass measurements for nanoparticle agglomerates highly desirable. A recently developed instrument, universal nanoparticle analyzer (UNPA), utilizes the close relation between agglomerate structure and unipolar charging properties and infers agglomerate structure from measurement of the average charge per agglomerate. It was used in this study to characterize in situ the structure of metal nanoparticle agglomerates generated by spark discharge, to study the effects of sintering on the structure of these agglomerates, and to make real-time assessment of their airborne mass concentration. The primary particles sizes measured by UNPA for the gold (Au), silver (Ag), and nickel (Ni) agglomerates are in reasonable agreement with the TEM (transmission electron microscopy) sizing results, d p = 7.9 ± 1.5, 11.8 ± 3.2, and 6.6 ± 1.0 nm, respectively. In addition, findings from the study of agglomerate structural change during sintering using the UNPA sensitivity coincide with results from TEM and mobility analyses. With regard to the mass concentration of silver agglomerates at room temperature, good agreement was found under our experimental conditions between results given by UNPA, the effective density, and the gravimetric measurement.