Raman microspectroscopic analysis of size-resolved atmospheric aerosol particle samples collected with an ELPI: Soot, humic-like substances, and inorganic compounds

Raman microspectroscopy and mapping have been applied for the analysis of soot, humic-like substances (HULIS) and inorganic compounds in size resolved samples of air particulate matter collected with an electrical low pressure impactor (ELPI). Using several reference materials, we found that spectral parameters determined by curve fitting with five bands (G, D1-D4) enable a discrimination of soot and HULIS and provide information about the relative abundance and structural order of graphite-like carbon. In particular, the D1 band width exhibited a near-linear negative correlation with the ratio of apparent elemental carbon to total carbon in different types of soot. The ELPI samples of sub-micrometer atmospheric particles exhibited essentially the same Raman spectra and parameters as standard diesel soot. In winter samples this was also the case for larger particles with aerodynamic diameters up to 4 μm. Spring and autumn samples, however, exhibited increased D1 band widths and D3 band intensities, indicating a high prevalence of HULIS in the size range of 2-4 μm. In addition, various nitrates and sulfates (mostly NaNO3 and (NH4)2SO4; some NH4NO3, Ca(NO3)2, Na2SO4, and CaSO4) and small amounts of CaCO3 were detected. Different single- and multi-component spectra indicated the presence of externally and internally mixed particles. The relative abundance of different chemical components in different particle size ranges was quantified in mapping experiments (0-55% NaNO3, 1-15% (NH4)2SO4, 10-45% soot/HULIS, 30-60% highly fluorescent organics). Overall, the results of this study demonstrate that Raman microspectroscopy and mapping can provide qualitative and quantitative information about the composition of ELPI aerosol samples.