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Organic carbon (OC) and elemental carbon (EC) are operationally defined by the analysis methods, and different methods give in different results. The IMPROVE (Interagency Monitoring of Protected Visual Environments) and NIOSH (National Institute of Occupational Safety and Health) thermal evolution protocols present different operational definitions. These protocols are applied to 60 ambient and source samples from different environments using the same instrument to quantify differences in implemented protocols on the same instrument. The protocols are equivalent for total carbon sampled on quartz-fiber filters. NIOSH EC was typically less than half of IMPROVE EC. The primary difference is the allocation of carbon evolving at the NIOSH 850°C temperature in a helium atmosphere to the OC rather than EC fraction. Increasing light transmission and reflectance during this temperature step indicate that this …