An FTIR spectrometer equipped with a mercury cadmium telluride (MCT) detector (Nicolet 6700, Thermo Fisher Scientific K.K.) was used. The filtered extract was placed into a fixed-path-length (1 mm) CaF2 liquid cell (SC-CaF2-1.0, GL Sciences Inc., Tokyo, Japan). The FTIR spectra of the extract in the 4000–1200 cm1 region were recorded by co-adding 32 scans at a resolution of 4 cm1. The background spectrum was obtained using toluene. The FTIR spectra of OA samples were measured using the same spectrometer and procedure. Background spectra were obtained using air (empty cell) and toluene.