Secondary organic aerosol (SOA), resulting from atmospheric photochemical reactions, is a major contributor to fine particulate matter (PM) in urban and rural areas. The objective of this study is to examine whether and how highly concentrated inorganic aerosols affect SOA formation. The experiments were conducted with irradiated m-xylene/NO_x/air mixtures in the presence of dry CaSO₄ and (NH₄)₂SO₄ seed aerosols in a 2 m³ smog chamber. The results indicate that SOAs are generated in m-xylene/NO_x/air mixtures upon ultraviolet light irradiation, and the SOA yields can be fit to a one-product model. The presence of CaSO₄ aerosols in the system was found to have no effect on SOA yields. However, the presence of (NH₄)₂SO₄ aerosols increased the semivolatile organic compound production by 36%, which may be attributed to the weak acidity caused by the interaction of (NH₄)₂SO₄ with water-contained organic material. The outcome of this work can help better understand the SOA formation mechanism, especially in some Chinese big cities experiencing serious PM pollution. Furthermore, the effect of (NH₄)₂SO₄ on SOA formation could be integrated into current air quality simulation models and help improve the accuracy of the model prediction results.

Key words: secondary organic aerosol, smog chamber, photooxidation reaction, aerosol yield, m-xylene, calcium sulfate, ammonium sulfate