NASA is one of several U.S. stakeholders in the aircraft particle emissions community. FAA, EPA, SERDP, U.S. Air Force, airport operators, and engine manufacturers all have a stake in understanding aircraft particle emissions and all of these are potential consumers of the TILDAS-sulfate instrument and its data. In addition to aircraft exhaust characterization, sulfate aerosol makes an important contribution to global climate forcing. Moreover, sulfuric acid has been shown to be a key component in atmospheric nucleation events. Improved instruments are required for rapid, size- and composition resolved measurements of sulfate particles. Current instruments are inadequate for characterizing particles smaller than 50 nm, suffer severe interferences from other atmospheric gases, lack composition resolution, lack size resolution, or lack the required sensitivity. The primary application of the proposed TILDAS-sulfate instrument will be characterization of aircraft particle emissions. NASA has sponsored a number of major aircraft particle characterization tests, including the three APEX experiments and the AAFEX experiment. These measurement activities have contributed to a growing understanding of aircraft exhaust particles. However, despite substantial effort, many questions remain. The most important outstanding question is the relative contribution of soot and semi-volatile particles to the overall particle mass emissions loading. Depending on the engine, fuel, and operating conditions, semi-volatile mass loadings vary from almost zero to more than 100 mg kg-1. A second outstanding question is the conversion efficiency of SO2 to SO3. By directly measuring particle bound sulfate and with limited restrictions on the particle size that can be detected the TILDAS-sulfate instrument will help answer these two important outstanding questions faced by NASA.