Effect of high concentrations of inorganic seed aerosols on secondary organic aerosol formation in the m-xylene/NOx photooxidation system

High concentrations (>15 μm3 cm−3) of CaSO4, Ca(NO3)2 and (NH4)2SO4 were selected as surrogates of dry neutral, aqueous neutral and dry acidic inorganic seed aerosols, respectively, to study the effects of inorganic seeds on secondary organic aerosol (SOA) formation in irradiated m-xylene/NOx photooxidation systems. The results indicate that neither ozone formation nor SOA formation is significantly affected by the presence of neutral aerosols (both dry CaSO4 and aqueous Ca(NO3)2), even at elevated concentrations. The presence of high concentrations of (NH4)2SO4 aerosols (dry acidic) has no obvious effect on ozone formation, but it does enhance SOA generation and increase SOA yields. In addition, the effect of dry (NH4)2SO4 on SOA yield is found to be positively correlated with the (NH4)2SO4 surface concentration, and the effect is pronounced only when the surface concentration reaches a threshold value. Further, it is proposed that the SOA generation enhancement is achieved by particle-phase heterogeneous reactions induced and catalyzed by the acidity of dry (NH4)2SO4 seed aerosols.