**4. Conclusions**

The tar yield decreases as a function of increasing temperature from 26.18% at 500°C to 6.38% at 900°C. H2O and CO2 influence significantly the tar homogeneous transformations at 700–900°C, while the tar reforming effect of 15 vol.% H2O is significantly higher than that of 29 vol.% CO2. H2O and CO2 have obvious effects on the transformation of PAHs. H2O and CO2 not only directly affect the tar transformation on biochar but also indirectly influence the reforming of tar through changing the structure of biochar catalyst. The formation of additional oxygencontaining functional groups is strengthened with the concentration of H2O and CO2 increasing. During tar heterogeneous reforming over biochar, the transformation of small aromatic ring systems (3–5 fused rings) to larger aromatic ring systems (≥6 fused rings) in the biochar structure is promoted by the increasing concentration of H2O and CO2. The activation by H2O/CO2 of biochar impacted the biochar surface's morphology and distribution of metal species. Activation/gasification under a CO2 in an Ar atmosphere produced more micropores, while adoption under a H2O in an Ar atmosphere favored the formation of mesopores. With the existence of gasification agents, especially for H2O, the simultaneous creation of pore structures is necessary to maintain biochar's catalytic activity during tar reforming. H2O/ CO2 also indirectly affects tar destruction by influencing the biochar structure and distribution of AAEM catalysts, while the reaction is occurring to ensure enough active sites on the biochar surface to maintain its catalytic activity. The activation and/or activity-maintaining effects of H2O/CO2 can notably enhance the in-situ reforming of both large and small aromatic ring systems present in biomass tar.
