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The effects of SO2 and NH3 on secondary organic aerosol formationhave rarely been investigated together, while the interactive effectsbetween inorganic and organic species under highly complex pollutionconditions remain uncertain. Here we studied the effects of SO2 andNH3 on secondary aerosol formation in the photooxidation system oftoluene∕NOx in the presence or absence of Al2O3 seed aerosolsin a 2 m3 smog chamber. The presence of SO2 increased new particleformation and particle growth significantly, regardless of whether NH3was present. Sulfate, organic aerosol, nitrate, and ammonium were allfound to increase linearly with increasing SO2 concentrations. Theincreases in these four species were more obvious under NH3-richconditions, and the generation of nitrate, ammonium, and organic aerosolincreased more significantly than sulfate with respect to SO2concentration, while sulfate was the most sensitive species underNH3-poor conditions. The synergistic effects between SO2 andNH3 in the heterogeneous process contributed greatly to secondaryaerosol formation. Specifically, the generation of NH4NO3 wasfound to be highly dependent on the surface area concentration of suspendedparticles, and increased most significantly with SO2 concentrationamong the four species under NH3-rich conditions. Meanwhile, theabsorbed NH3 might provide a liquid surface layer for the absorptionand subsequent reaction of SO2 and organic products and, therefore,enhance sulfate and secondary organic aerosol (SOA) formation. This effectmainly occurred in the heterogeneous process and resulted in asignificantly higher growth rate of seed aerosols compared to withoutNH3. By applying positive matrix factorisation (PMF) analysis to theAMS data, two factors were identified for the generated SOA. One factor,assigned to less-oxidised organic aerosol and some oligomers, increased withincreasing SO2 under NH3-poor conditions, mainly due to thewell-known acid catalytic effect of the acid products on SOA formation inthe heterogeneous process. The other factor, assigned to the highly oxidisedorganic component and some nitrogen-containing organics (NOC), increasedwith SO2 under a NH3-rich environment, with NOC (organonitratesand NOC with reduced N) contributing most of the increase

atmospheric chemistry and physics

Synergetic formation of secondary inorganic and organic aerosol: effect of SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and NH&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; on particle formation and growth