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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 18, 1643-1652, 2018
https://doi.org/10.5194/acp-18-1643-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
06 Feb 2018
Effect of relative humidity on the composition of secondary organic aerosol from the oxidation of toluene
Mallory L. Hinks1, Julia Montoya-Aguilera1, Lucas Ellison1, Peng Lin2, Alexander Laskin2, Julia Laskin2, Manabu Shiraiwa1, Donald Dabdub3, and Sergey A. Nizkorodov1 1Department of Chemistry, University of California Irvine, Irvine, CA 92697, USA
2Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA
3Department of Mechanical and Aerospace Engineering, University of California Irvine, Irvine, CA 92697, USA
Abstract. The effect of relative humidity (RH) on the chemical composition of secondary organic aerosol (SOA) formed from low-NOx toluene oxidation in the absence of seed particles was investigated. SOA samples were prepared in an aerosol smog chamber at < 2 % RH and 75 % RH, collected on Teflon filters, and analyzed with nanospray desorption electrospray ionization high-resolution mass spectrometry (nano-DESI–HRMS). Measurements revealed a significant reduction in the fraction of oligomers present in the SOA generated at 75 % RH compared to SOA generated under dry conditions. In a separate set of experiments, the particle mass concentrations were measured with a scanning mobility particle sizer (SMPS) at RHs ranging from < 2 to 90 %. It was found that the particle mass loading decreased by nearly an order of magnitude when RH increased from < 2 to 75–90 % for low-NOx toluene SOA. The volatility distributions of the SOA compounds, estimated from the distribution of molecular formulas using the molecular corridor approach, confirmed that low-NOx toluene SOA became more volatile on average under high-RH conditions. In contrast, the effect of RH on SOA mass loading was found to be much smaller for high-NOx toluene SOA. The observed increase in the oligomer fraction and particle mass loading under dry conditions were attributed to the enhancement of condensation reactions, which produce water and oligomers from smaller compounds in low-NOx toluene SOA. The reduction in the fraction of oligomeric compounds under humid conditions is predicted to partly counteract the previously observed enhancement in the toluene SOA yield driven by the aerosol liquid water chemistry in deliquesced inorganic seed particles.

Citation: Hinks, M. L., Montoya-Aguilera, J., Ellison, L., Lin, P., Laskin, A., Laskin, J., Shiraiwa, M., Dabdub, D., and Nizkorodov, S. A.: Effect of relative humidity on the composition of secondary organic aerosol from the oxidation of toluene, Atmos. Chem. Phys., 18, 1643-1652, https://doi.org/10.5194/acp-18-1643-2018, 2018.
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Short summary
We have observed a strong effect of relative humidity on the composition of particulate matter produced from the oxidation of toluene in clean air. At higher relative humidity, there was a significant reduction in the fraction of high-molecular-weight compounds present in the particles. The amount of particulate matter also decreased at higher relative humidity. The main implication of this study is that water vapor participates in the photooxidation of toluene in a complicated way.
We have observed a strong effect of relative humidity on the composition of particulate matter...
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