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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Short summary
Three models of carbon analyzers were used in the past decade to measure carbonaceous particles from samples collected within the Chemical Speciation Network. This study compares results from these analyzer models to investigate impact on long-term data from instrument differences. Overall good agreement was found among the three models for total carbon, organic carbon, and elemental carbon, while reasons and implications of some notable differences in their subtractions are investigated.
Preprints
https://doi.org/10.5194/amt-2020-436
https://doi.org/10.5194/amt-2020-436

  30 Nov 2020

30 Nov 2020

Review status: this preprint is currently under review for the journal AMT.

Inter-Comparison of Thermal-Optical Carbon Measurements by Sunset and DRI Analyzers Using the IMPROVE_A Protocol

Xiaolu Zhang, Krystyna Trzepla, Warren White, Sean Raffuse, and Nicole Pauly Hyslop Xiaolu Zhang et al.
  • Air Quality Research Center, University of California, Davis, CA

Abstract. Thermal-Optical Analysis (TOA) is a class of methods widely used by long-term air quality monitoring networks for determining organic carbon (OC) and elemental carbon (EC) in atmospheric aerosols collected on filters. Results from TOA vary not only with differences in operating protocols for the analysis, but also with details of the instrumentation with which a given protocol is carried out. Three models of TOA carbon analyzers have been used for the IMPROVE_A protocol in the past decade within the Chemical Speciation Network (CSN). This study presents results from inter-comparisons of these three analyzer models using two sets of CSN quartz filter samples, all analyzed with the IMPROVE_A protocol. One comparison was between the Sunset Model 5L (Sunset) analyzers and the Desert Research Institute (DRI) Model 2015 (DRI-2015) analyzers, using 4073 CSN samples collected in 2017. The other comparison was between the Sunset and the DRI Model 2001 (DRI-2001) analyzers, using 303 CSN samples collected in 2007.

Both comparisons showed a high degree of inter-model consistency in total carbon (TC) and the major carbon fractions, OC and EC, with mean bias within 5 % for TC and OC, and within 12 % for EC. Relatively larger and diverse inter-model differences (mean biases of 5 %–140 %) were found for thermal subfractions of OC and EC (i.e. OC1-OC4 and EC1-EC3), with better agreement observed for subfractions with higher mass loadings and smaller within-model uncertainties. Optical charring correction was found to be critical in bringing OC and EC measurements by different TOA analyzer models into better agreement. Appreciable inter-model differences in EC between Sunset and DRI-2015 (mean bias ± SD of 21.7 % ± 12.2 %) remained for ~ 5 % of the 2017 CSN samples; examination of these analysis thermograms revealed that the optical measurement (i.e. filter reflectance and transmittance) saturated in the presence of strong absorbing materials on the filter (e.g. EC), with excessive absorption leaving insufficient dynamic range for detection of carbon pyrolysis, thus no optical charring correction. Differences in possible instrument parameters and configuration related to disagreement in OC and EC subfractions are also discussed.

Our results provide a basis for future studies of uncertainties associated with the TOA analyzer model transition in assessing long-term trends of CSN carbon data. Further investigations using these data are warranted focusing on the demonstrated inter-model differences in OC and EC subfractions. The within- and inter- model uncertainties are useful for model performance evaluation.

Xiaolu Zhang et al.

 
Status: open (until 25 Jan 2021)
Status: open (until 25 Jan 2021)
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Xiaolu Zhang et al.

Xiaolu Zhang et al.

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Short summary
Three models of carbon analyzers were used in the past decade to measure carbonaceous particles from samples collected within the Chemical Speciation Network. This study compares results from these analyzer models to investigate impact on long-term data from instrument differences. Overall good agreement was found among the three models for total carbon, organic carbon, and elemental carbon, while reasons and implications of some notable differences in their subtractions are investigated.
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