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Black carbon (BC) is a primary particle matter component emitted as a result of incomplete combustion. Due to its light-absorbing nature, BC can exert both direct and indirect influence on global climate. There is no universal research or regulatory definition of BC; instead, a variety of instruments and analytical methods define BC operationally. This study used a constant, source-generated BC concentration in order to quantify the variation in BC measurements using 10 different instruments and methodologies. Instruments examined include a multiangle absorption photometer, a transmissometer, rack-mount, portable, and micro-aethalometers, a thermal-optical carbon analyzer (using IMPROVE, NIOSH 5040, and NIST-EPA methods), a single-particle soot photometer (SP2), and a three-wavelength photoacoustic soot spectrometer (PASS3). BC-bearing soot was generated using an ethylene-air diffusion flame, diluted to similar to 100 mu g/m(3), which was subjected to simultaneous analyses. Measured BC and elemental carbon (EC) concentrations ranged from 20.5 to 39.9 mu g/m(3). BC concentrations measured by aethalometers and transmissometer instruments varied by <15%. However, measurements of BC were as much as 52% greater than EC determined by thermal-optical techniques. Gravimetric data from the current study suggest that the thermal-optical measurements of EC likely underestimated the carbon in flame soot samples, indicating an inability to close a carbon mass balance.