Comparison of IMPROVE and NIOSH carbon measurements

Organic carbon (OC) and elemental carbon (EC) are operationally defined by the analysis methods, and different methods give in different results. The IMPROVE Iinteragency Monitoring of protected Visual Environments) and NIOSH (National Institute of Occupational Safety and Wealth) thermal evolution p rotocols present different operational definitions. These protocols are app lied to 60 ambient and sonrce samples from different environments using the same instrument to quantify differences in implemented protocols on the sa me instrument. The protocols are equivalent for total carbon sampled on qua rtz-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 d egrees C temperature in a helium atmosphere to the OC rather than EC fracti on. increasing light transmission and reflectance during this temperature s tep indicate that this fraction should he classified as EC. When this porti on of NIOSH OC is added to NIOSH EC, the IMPROVE and NIOSH analyses are in good agreement. The most probable explanation is that mineral oxides in the complex particle mixture on the filter are supplying oxygen to neighboring carbon particles at this high temperature. This has been demonstrated by t he principle of the thermal manganese oxidation method that is also commonl y used to distinguish OC from EC, For both methods, the optical pyrolysis a djustment to the EC fractions was always higher for transmittance than for reflectance. This is a secondary cause of differences between the two metho ds, with transmittance resulting in a lower EC loading than reflectance. Th e difference was most pronounced for very black filters on which neither re flectance nor transmittance accurately detected further blackening due to p yrolysis.