The remarkable effect of FeSO4 seed aerosols on secondary organic aerosol formation from photooxidation of α-pinene/NOx and toluene/NOx

To investigate the effects of Fe(II) and Fe(III) ions on secondary organic aerosol (SOA) formation, we conducted a series of photooxidation experiments with α-pinene and toluene in the presence of nitric oxides (NOx) with/without FeSO4 or Fe2(SO4)3 seed aerosols. The FeSO4 seed aerosols suppressed SOA formation, while Fe2(SO4)3 seed aerosols did not display a noticeable effect on SOA formation. We did not observe effects of FeSO4 and Fe2(SO4)3 seed aerosols on gas phase compounds, including ozone, NOx, and hydrocarbons (HCs). The negative effect of Fe(II)-containing seed aerosols on SOA formation due to the reduction of condensable compounds (CCs) generated from hydrocarbon oxidation is discussed. The mean molecular weight of CCs reduced by Fe(II) is tentatively estimated to be larger than 300, indicating a possibility that many of the CCs reduced by Fe(II) are oligomers. Reduction of oligomer precursors may interrupt the oligomerization of other aldehyde products. If Fe(II) regeneration from photoreduction of Fe(III) is considered, the estimated mean molecular weight of the CCs reduced would be smaller. However, the negligible effect of Fe(III)-containing seed aerosols on SOA formation indicates that Fe(III) photoreduction is negligible in our experiments.