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. Thermal–optical measurement techniques are widely used for the monitoring ofcarbonaceous aerosols. Although results of different thermal–opticalmeasurement techniques are comparable for total carbon, they can vary widelyfor values of elemental carbon especially in the presence of brown carbon.Charring of organic material during the inert heating phase ofthermal–optical measurements has been found to be a major confounder, but noliterature about investigations of structural changes during this process inatmospheric aerosols is available. In a recent study we investigated thesestructural changes for combustion aerosol standard (CAST) soot. Now we applythis approach to selected atmospheric aerosol filter samples and a subset ofeight washed filter samples with low loadings of water-soluble organic carbon(WSOC). To investigate structural changes, Raman spectra were obtained forsamples heated to the corresponding temperature levels and gas atmospheresof the EUSAAR2 and NIOSH870 protocols. The temperature levels where changesin the Raman spectra occurred (i.e., changes in structure) varied fordifferent samples. For the washed samples with low WSOC loadings and absenceof other water-soluble aerosol components such as inorganic salts, changesin structural ordering and darkening of the samples were not observed. Wewere able to show for the first time that the darkening of a sample(measured in terms of transmission laser signal) is not necessarily causedby an increase of structural ordering in the sample. Possibletransformations at lower temperatures could include a formation ofnon-graphitic light-absorbing intermediate organic carbon, a release of C−Hgroups or a decrease of carbonyl groups.

Investigation of structural changes of atmospheric aerosol samples during two thermal–optical measurement procedures (EUSAAR2, NIOSH870)