Abstract. Thermal–optical analysis (TOA) is a class of methods widely used 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 intercomparisons of these three analyzer models using two sets
of CSN quartz filter samples, all analyzed using the IMPROVE_A protocol with
reflectance charring correction. 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 a mean bias within
5 % for TC and OC and within 12 % for EC. Relatively
larger and diverse inter-model discrepancies (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 proved critical in bringing OC and
EC measurements by different TOA analyzer models into 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), leaving an insufficient dynamic range for the detection of
carbon pyrolysis and thus no optical charring correction. Differences in
instrument parameters and configuration, possibly 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.
Desert Research Institute cloud droplet videometer measurements in support of MASTEX