Abstract
Abstract. Thermal–optical analysis (TOA) is a widely used technique that fractionates carbonaceous aerosol particles into organic and elemental carbon (OC and EC), or carbonate. Thermal sub-fractions of evolved OC and EC are also used for source identification and apportionment; thus, oven temperature accuracy during TOA analysis is essential. Evidence now indicates that the "actual" sample (filter) temperature and the temperature measured by the built-in oven thermocouple (or set-point temperature) can differ by as much as 50 °C. This difference can affect the OC–EC split point selection and consequently the OC and EC fraction and sub-fraction concentrations being reported, depending on the sample composition and in-use TOA method and instrument. The present study systematically investigates the influence of an oven temperature calibration procedure for TOA. A dual-optical carbon analyzer that simultaneously measures transmission and reflectance (TOT and TOR) is used, functioning under the conditions of both the National Institute of Occupational Safety and Health Method 5040 (NIOSH) and Interagency Monitoring of Protected Visual Environment (IMPROVE) protocols. The application of the oven calibration procedure to our dual-optics instrument significantly changed NIOSH 5040 carbon fractions (OC and EC) and the IMPROVE OC fraction. In addition, the well-known OC–EC split difference between NIOSH and IMPROVE methods is even further perturbed following the instrument calibration. Further study is needed to determine if the widespread application of this oven temperature calibration procedure will indeed improve accuracy and our ability to compare among carbonaceous aerosol studies that use TOA.
Highlights
The Interagency Monitoring of Protected Visual Environment (IMPROVE, outlined by Chow et al, 1993) and National Institute of Occupational Safety and Health Method 5040 (NIOSH, 1996) thermal–optical analysis (TOA) methods have been used widely for decades to quantify total carbon (TC), organic carbon (OC), and elemental carbon (EC) concentrations in ambient and combustion source samples
Note that this study examines only the influence of temperature calibration on the carbon results measured under different TOA protocols in the same sample and not the representativeness of the diffusion flame aerosol to the wide range of source and ambient samples
Given that the purpose of the present study was to perform an independent calibration of the dual-optics carbon analyzer that is different in design than a DRI instrument, the old IMPROVE temperature protocol was used in order to independently measure temperature biases for the Sunset dual-optical carbon analyzer under the IMPROVE temperature ramps
Summary
The Interagency Monitoring of Protected Visual Environment (IMPROVE, outlined by Chow et al, 1993) and National Institute of Occupational Safety and Health Method 5040 (NIOSH, 1996) thermal–optical analysis (TOA) methods have been used widely for decades to quantify total carbon (TC), organic carbon (OC), and elemental carbon (EC) concentrations in ambient and combustion source samples. NIOSH and IMPROVE carbon fractions have been determined traditionally with instrumentation developed by Sunset Laboratory (Tigard, OR, USA) and Desert Research Institute (DRI, Reno, NV, USA), respectively. Are there hardware design and configurational differences between these instruments, but the protocols differ operationally in temperature programming and optical monitoring, as described by Chow et al (2001) whereby different OC and EC values can be obtained for many sample types.
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