We have developed an instrument, Universal NanoParticle Analyzer (UNPA), for quasi-online measurement of gas-borne nanoparticle agglomerates. The UNPA utilizes a Differential Mobility Analyzer (DMA), a Condensation Particle Counter (CPC), and a Nanoparticle Surface Area Monitor (NSAM, including a unipolar charger combined with an electrometer), to determine the primary particle size and measure the number, surface area, and volume distributions of loose nanoparticle agglomerates. By loose agglomerates we refer to those that can be modeled as clusters of spherical primary particles with open structures and fractal dimensions less than two. The key parameter measured is the UNPA sensitivity, which is defined as the current measured by the NSAM divided by the number concentration measured by the CPC. Our experimental data and theoretical model have shown that the UNPA sensitivity S depends on the particle morphology. The sensitivity S is larger for loose agglomerates than for spheres at a fixed mobility diameter, and S for partly sintered agglomerates is between those for loose agglomerates and spheres. Thus the measured value of S gives a direct indication of the particle morphology. From UNPA sensitivities, the primary particle size is determined using a fitting procedure. Using the model of Lall and Friedlander (2006) for loose agglomerates, the number of primary particles in agglomerates can be computed. Then the surface area and volume of agglomerates can be obtained. Operated under the scanning mode, the UNPA can provide the number, surface area and volume distributions of loose agglomerates in the range of 50 to several hundred nanometers in several minutes.