Aircraft particle emissions contribute a modest, but growing, portion of the overall particle emissions budget. Characterizing aircraft particle emissions is required to improve aircraft combustor design and aircraft operating practices. Aircraft particle emissions are a complex mixture of soot and semi-volatile material, primarily inhabiting sizes smaller than 100 nm. New instruments are required to characterize aircraft particle emissions. We propose to build a new instrument for sensitive (>200 ng m-3 on a 1 Hz cycle) measurements of particle sulfate. The key instrument component will be a tunable infrared diode absorption spectrometer (TILDAS). Compared to existing sulfate measurement instruments, the TILDAS-sulfate instrument will be able to reject NO interferences, a key capability required for aircraft exhaust applications. Prior to reaching the TILDAS, gas phase SO2 will be removed using an acid gas denuder and particle sulfate will be converted to SO2 in a quartz oven. By running the TILDAS-sulfate in tandem with a commercial differential mobility analyzer, we anticipate obtaining size resolved sulfate mass loadings (10 size bins, from 10 nm to several hundred nm). Phase I tasks include evaluating acid gas denuder and SO2-to-sulfate technologies, determining the instrument detection limits, demonstrating instrument discrimination against NO and other interferences, and demonstrating the use of the instrument to characterize simulated aircraft exhaust gas.