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

摘要

The effects of SO and NH 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 SO andNH on secondary aerosol formation in the photooxidation system oftoluene∕NO in the presence or absence of AlO seed aerosolsin a 2 m smog chamber. The presence of SO increased new particleformation and particle growth significantly, regardless of whether NHwas present. Sulfate, organic aerosol, nitrate, and ammonium were allfound to increase linearly with increasing SO concentrations. Theincreases in these four species were more obvious under NH-richconditions, and the generation of nitrate, ammonium, and organic aerosolincreased more significantly than sulfate with respect to SOconcentration, while sulfate was the most sensitive species underNH-poor conditions. The synergistic effects between SO andNH in the heterogeneous process contributed greatly to secondaryaerosol formation. Specifically, the generation of NHNO wasfound to be highly dependent on the surface area concentration of suspendedparticles, and increased most significantly with SO concentrationamong the four species under NH-rich conditions. Meanwhile, theabsorbed NH might provide a liquid surface layer for the absorptionand subsequent reaction of SO 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 withoutNH. 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 SO under NH-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 SO under a NH-rich environment, with NOC (organonitratesand NOC with reduced N) contributing most of the increase.