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

Thermal-optical measurement techniques are widely used for the monitoring of carbonaceous aerosols. Although results of different thermal-optical measurement techniques are comparable for total carbon, they can vary widely for values of elemental carbon especially in the presence of brown carbon. Charring of organic material during the inert heating phase of thermal-optical measurements was found to be a major confounder, but no literature about investigations of structural changes during this process in atmospheric aerosols is available. In a recent study we investigated these structural changes for combustion aerosol standard soot (CAST). Now we apply this approach to selected atmospheric aerosol filter samples and a subset of eight washed filter samples with low WSOCs loadings. To investigate structural changes, Raman spectra were obtained for samples heated to the corresponding temperature levels and gas atmospheres of the EUSAAR2 and NIOSH870 protocols. The temperature levels where changes in the Raman spectra occurred (i.e. changes in structure) varied for different samples. For the washed samples with low WSOC loadings and absence of other water soluble aerosol components such as inorganic salts, changes in structural ordering and darkening of the samples were not observed. For all samples, ion chromatography, integrating sphere measurements (yielding black and brown carbon data) and thermal-optical analyses were performed. We were able to show for the first time that the darkening of a sample (measured in terms of transmission laser signal) is not necessarily caused by an increase of structural ordering in the sample. Therefore we suggest that the widely used term charring should be used carefully when the darkening of a sample during thermal-optical measurement procedures is interpreted.