**4. Conclusion**

Effects of various inorganic seeds, including neutral inorganic seed CaSO4, acidic seed (NH4)2SO4, transition metal contained inorganic seeds FeSO4 and Fe2(SO4)3, and a mixture of (NH4)2SO4 and FeSO4, were examined during *m-*xylene or toluene photooxidation. Our results indicate that the presence of CaSO4 seed aerosols and Fe2(SO4)3 seed aerosols have no effect on photooxidation of aromatic hydrocarbons, while the presence of (NH4)2SO4 seed aerosols and FeSO4 seed aerosols have no effect on gas-phase reactions, but can significantly influence SOA generation and SOA yields. (NH4)2SO4 seed aerosols enhance SOA formation and increase SOA yield due to acid catalytic effect of (NH4)2SO4 seeds on particle-phase surface heterogeneous reactions. While FeSO4 seed aerosols suppress SOA formation and decrease SOA yield possibly due to the reduction of some oligomer precursor CCs. These results reveal that many inorganic seeds are not inert during photooxidation process and can significantly influence SOA formation. These observed effects can be incorporated into air quality models to improve their accuracy in predicting SOA and fine particle concentrations.
