Synergetic formation of secondary inorganic and organic aerosol: effect of SO₂ and NH₃ on particle formation and growth

Abstract. The effects of SO₂ and NH₃ on secondary organic aerosol formation have rarely been investigated together, while the interactive effects between inorganic and organic species under highly complex pollution conditions remain uncertain. Here we studied the effects of SO₂ and NH₃ on secondary aerosol formation in the photooxidation system of toluene∕NO_x in the presence or absence of Al₂O₃ seed aerosols in a 2 m³ smog chamber. The presence of SO₂ increased new particle formation and particle growth significantly, regardless of whether NH₃ was present. Sulfate, organic aerosol, nitrate, and ammonium were all found to increase linearly with increasing SO₂ concentrations. The increases in these four species were more obvious under NH₃-rich conditions, and the generation of nitrate, ammonium, and organic aerosol increased more significantly than sulfate with respect to SO₂ concentration, while sulfate was the most sensitive species under NH₃-poor conditions. The synergistic effects between SO₂ and NH₃ in the heterogeneous process contributed greatly to secondary aerosol formation. Specifically, the generation of NH₄NO₃ was found to be highly dependent on the surface area concentration of suspended particles, and increased most significantly with SO₂ concentration among the four species under NH₃-rich conditions. Meanwhile, the absorbed NH₃ might provide a liquid surface layer for the absorption and subsequent reaction of SO₂ and organic products and, therefore, enhance sulfate and secondary organic aerosol (SOA) formation. This effect mainly occurred in the heterogeneous process and resulted in a significantly higher growth rate of seed aerosols compared to without NH₃. By applying positive matrix factorisation (PMF) analysis to the AMS data, two factors were identified for the generated SOA. One factor, assigned to less-oxidised organic aerosol and some oligomers, increased with increasing SO₂ under NH₃-poor conditions, mainly due to the well-known acid catalytic effect of the acid products on SOA formation in the heterogeneous process. The other factor, assigned to the highly oxidised organic component and some nitrogen-containing organics (NOC), increased with SO₂ under a NH₃-rich environment, with NOC (organonitrates and NOC with reduced N) contributing most of the increase.