Furthermore, the enrichment of trace metals in the submicron
particles received more attention due to their high toxicity. We
define Relative Enrichment Times (RETij) of trace metals as the ratio
of the mass concentration of trace element i in ash particles at the j
stage to that at first stage. The semi-volatile metals such as As, Cd
and Zn of PC-generated PM are heavily enriched in the submicron
particle ranges with RETij >10. But the RETij values of As, Cd and Zn
in CFB-generated PM are only one-fourth and one-tenth of that of
PC-generated PM [27].