Organic carbon (OC) and elemental carbon (EC) are operationally de ned 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 de nitions. 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-ber lters. 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 re ectance during this temperature step indicate that this fraction should be classi ed as EC. When this portion 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 lter are supplying oxygen to neighboring carbon particles at this high temperature. This has been demonstrated by the principle of the thermal manganese oxidation method that is also commonly used to distinguish OC from EC. For both methods, the optical pyrolysis adjustment to the EC fractions was always higher for transmittance than for re ectance. This is a secondary cause of differences between the two methods, with transmittance resulting in a lower EC loading than re ectance. The difference was most pronounced for very black lters on which neither re ectance nor transmittance accurately detected further blackening due to pyrolysis.

Organic carbon (OC) and elemental carbon (EC) are operationally de ned 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 de nitions. 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-ber lters. 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 re ectance during this temperature step indicate that this fraction should be classi ed as EC. When this portion 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 lter are supplying oxygen to neighboring carbon particles at this high temperature. This has been demonstrated by the principle of the thermal manganese oxidation method that is also commonly used to distinguish OC from EC. For both methods, the optical pyrolysis adjustment to the EC fractions was always higher for transmittance than for re ectance. This is a secondary cause of differences between the two methods, with transmittance resulting in a lower EC loading than re ectance. The difference was most pronounced for very black lters on which neither re ectance nor transmittance accurately detected further blackening due to pyrolysis.

The Interagency Monitoring of Protected Visual Environments (IMPROVE) particle monitoring network consists of approximately 160 sites at which fine particulate matter (PM2.5) mass and major species concentrations and course particulate matter (PM10) mass concentrations are determined by analysis of 24-hr duration sampling conducted on a 1-day-in-3 schedule A simple algorithm to estimate light extinction from the measured species concentrations was incorporated in the 1999 Regional Haze Rule as the basis for the haze metric used to track haze trends. A revised algorithm was developed that is more consistent with the recent atmospheric aerosol literature and reduces bias for high and low light extinction extremes. The revised algorithm differs from the original algorithm in having a term for estimating sea salt light scattering from Cl(-) ion data, using 1.8 instead of 1.4 for the mean ratio of organic mass to measured organic carbon, using site-specific Rayleigh scattering based on site elevation and mean temperature, employing a split component extinction efficiency associated with large and small size mode sulfate, nitrate and organic mass species, and adding a term for nitrogen dioxide (NO2) absorption for sites with NO2 concentration information. Light scattering estimates using the original and the revised algorithms are compared with nephelometer measurements at 21 IMPROVE monitoring sites. The revised algorithm reduces the underprediction of high haze periods and the overprediction of low haze periods compared with the performance of the original algorithm. This is most apparent at the hazier monitoring sites in the eastern United States. For each site, the PM10 composition for days selected as the best 20% and the worst 20% haze condition days are nearly identical regardless of whether the basis of selection was light scattering from the original or revised algorithms, or from nephelometer-measured light scattering.

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