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

Abstract

Abstract. Thermal–optical measurement techniques are widely used for the monitoring of\ncarbonaceous aerosols. Although results of different thermal–optical\nmeasurement techniques are comparable for total carbon, they can vary widely\nfor values of elemental carbon especially in the presence of brown carbon.\nCharring of organic material during the inert heating phase of\nthermal–optical measurements has been found to be a major confounder, but no\nliterature about investigations of structural changes during this process in\natmospheric aerosols is available. In a recent study we investigated these\nstructural changes for combustion aerosol standard (CAST) soot. Now we apply\nthis approach to selected atmospheric aerosol filter samples and a subset of\neight washed filter samples with low loadings of water-soluble organic carbon\n(WSOC). To investigate structural changes, Raman spectra were obtained for\nsamples heated to the corresponding temperature levels and gas atmospheres\nof the EUSAAR2 and NIOSH870 protocols. The temperature levels where changes\nin the Raman spectra occurred (i.e., changes in structure) varied for\ndifferent samples. For the washed samples with low WSOC loadings and absence\nof other water-soluble aerosol components such as inorganic salts, changes\nin structural ordering and darkening of the samples were not observed. We\nwere able to show for the first time that the darkening of a sample\n(measured in terms of transmission laser signal) is not necessarily caused\nby an increase of structural ordering in the sample. Possible\ntransformations at lower temperatures could include a formation of\nnon-graphitic light-absorbing intermediate organic carbon, a release of C−H \ngroups or a decrease of carbonyl groups.