Secondary organic aerosol formation from toluene photooxidation under various NOx conditions and particle acidity

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Cao, G. ; Jang, M. (2008)

Secondary organic aerosol (SOA) formation from photooxidation of toluene is studied using a 2 m<sup>3</sup> indoor Teflon film chamber under three different NO<sub>x</sub> conditions: low (&le;3 ppb), intermediate (90–105 ppb) and high (280–315 ppb). SOA experiments are conducted in the presence of either neutral or acidic sulfate seed aerosols under two different humidity levels (%RH 15–22 or 38–49). NO<sub>x</sub> concentrations in the chamber air affect not only SOA yields but also SOA growth described by the organic mass (OM) produced as a function of the toluene concentration consumed over the course of a single SOA experiment. The particle acidity effect on toluene SOA formation varies with NO<sub>x</sub> concentrations. For the low and the intermediate NO<sub>x</sub> experiments, SOA yields with acidic sulfate seed considerably increase by: 36%–115% at low %RH and 25–44% at high %RH compared to those with neutral seed. No significant particle acidity effect is observed for the high NO<sub>x</sub> experiments. The humidity effect on SOA formation is also different at the three NO<sub>x</sub> levels. For the low NO<sub>x</sub> experiments, SOA yields are 29%–34% lower at high %RH than those at low %RH in the presence of either neutral or acidic sulfate seed. For the intermediate NO<sub>x</sub> experiments, SOA yields at high %RH increase by 39% in the presence of neutral seed but slightly decrease by 7% in the presence of acidic sulfate seed compared to those at low %RH. For the high NO<sub>x</sub> experiments with a high NO fraction, no significant humidity effect on SOA yields is found with both neutral and acidic sulfate seeds.
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