(a) THz absorption spectral on basis of different growing years of moxa wool. Vertical arrows indicate absorption peaks of moxa wool. Error bars have been labeled on each data. (b) PLS prediction model of moxa wool in different growing years based on terahertz absorption spectrum. Changes of characteristic absorption peak intensities on different growing years of moxa wool at (c) 4.22 THz, (d) 4.57 THz, (e) 5.09 THz, and (f) 6.28 THz.