The SOA spectra
contain many broad peaks, and fewer sharp peak features
compared to single-component calibration standards, indicating
that the SOA is composed of a mixture of many compounds.
Syringol SOA contains fine structure peaks in the carbonyl region
that can be resolved into two main absorptions peaking at
1735 cm1 and 1696 cm1. Guaiacol and phenol SOA have broader
absorbance bands in the carbonyl region, with phenol SOA exhibiting
the lowest intensity absorbance in this region. Hardwood and
softwood SOA mixtures contain broader C]O absorbances that
most closely resemble absorbances in guaiacol SOA (Fig. 3). This is
potentially due to the combination of starting phenolic compounds
in the hardwood and softwood SOA samples (with guaiacol present
in both cases) producing compounds containing a broad range of
carbonyl classes