5. Comparison of Aerosol Carbon Instruments at a Remote, Background Site

Major problems still remain in the measurement of carbonaceous materials in ambient aerosols. Specifically, the use of speciation techniques and organic tracers to identify the sources of primary and secondary organic aerosols in the atmosphere is hampered by the ambiguities in aerosol organic carbon measurement. TVA is operating four speciation instruments at the Look Rock, TN, site as part of the VISTAS RPO focus site sampling. Instruments include an R&P Model 5400 Aerosol Carbon Analyzer and a Magee Scientific Aethalometer. For a period of about 5 weeks in early fall, 2003, TVA also operated a collocated Sunset Labs OC-EC analyzer at this site. Both carbon instruments were operated on a 1-hour sampling protocol, and the 5-min Aethalometer data were averaged over 1-hr periods for comparison.. Operation of the Model 5400 instrument used the normal protocol of dual collectors with analysis of 1-hr collected aerosol samples the following hour with two temperature thresholds, 275ºC for organic carbon and 750ºC for total carbon, while the second collector was used for sampling. Quantification of CO2 from combustion of the samples uses a Licor Model 800 sensor. The operation of the Sunset Labs OC-EC instrument uses a single in-line quartz collector with a 50-min sample collection period, followed by a 10-min analysis period with the temperature of the sample ramped at about 10ºC/sec to 650ºC, and about 6ºC/sec for the 650-850ºC ramps (organic and elemental). To deal with the adsorbed VOC problem, an in-line denuder is used with the OC-EC instrument, and the Model 5400 is operated with metal collectors, but no in-line VOC denuder. An optical BC-type signal is obtained from the OC-EC instrument as well as a TOT-like signal for elemental carbon.

A comparison of the data indicates that the organic carbon data are systematically higher than for the Model 5400 using a 275ºC temperature plateau. The optical EC data from the OCEC instrument agrees well with Aethalometer BC data, even though the BC levels at the Look Rock site were frequently at or below the Aethalometer limits of detection. Ambiguity still exists over the OC quantification with the instruments tested in this study, although there are preliminary indications that, when positive and negative artifact are accounted for, the difference between the Sunset Labs total carbon quantity and the Aethalometer BC value likely represents the current best estimate of organic carbon