Influence of ferrous and ferric ions in the aqueous phase on SOA formation in flow reactor experiments

Aqueous-phase chemistry of fog and cloud waters plays an important role in the formation and aging of secondary organic aerosols (e.g. Ervens et al.,2011; Herrmann et al., 2015). Transitions metal ions driving Fenton chemistry in aqueous aerosols are one of the main sources of OH radicals besides direct uptake from the gas-phase (Ervens et al., 2003). The most abundant transition metal in aqueous aerosols is iron released from natural sources like sea salt spray and mineral dust or anthropogenic emissions. The generation of OH radicals by Fenton chemistry might drive or inhibit atmospheric new particle formation in cloud and fog water, or above salt lakes (e.g. Kamilli et al., 2015; Daumit et al., 2016), thus, having a direct impact on the climate system.

A first set of experiments has been carried out in a flow reactor investigating the influence of ferric and ferrous iron on new particle formation under dark and humid conditions (RH > 70%). α-Pinene was used as an organic precursor molecule for secondary organic aerosol (SOA) formation by dark ozonolysis. Droplets of FeSO₄, FeCl₃ and (NH₄)₂SO₄ (as control) were produced by nebulizing solutions of varying concentrations between 0.1 µM and 30 mM using a custom-built atomizer. Particle size distributions were measured using a scanning mobility particle size spectrometer (SMPS, Grimm Aerosoltechnik).

First results show a significant decrease in geometric mean diameter of the produced particle population with increasing FeCl₃ concentration. This effect neither occurs when nebulizing FeSO₄ nor (NH₄)₂SO₄. These results imply that Fe³⁺ might inhibit growth of SOA under dark conditions.

More data analysis is ongoing and further experiments are planned to better understand the influence of iron on aqueous-phase SOA.

 

 

References

Daumit, K. E., Carrasquillo, A. J., Sugrue, R. A., & Kroll, J. H. (2016). Effects of condensed-phase oxidants on secondary organic aerosol formation. The Journal of Physical Chemistry A, 120(9), 1386-1394.

Ervens, B., George, C., Williams, J. E., Buxton, G. V., Salmon, G. A., Bydder, M., ... & Herrmann, H. (2003). CAPRAM 2.4 (MODAC mechanism): An extended and condensed tropospheric aqueous phase mechanism and its application. Journal of Geophysical Research: Atmospheres, 108(D14).

 Ervens, B., Turpin, B. J., & Weber, R. J. (2011). Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): a review of laboratory, field and model studies. Atmospheric Chemistry and Physics, 11(21), 11069-11102.

Herrmann, H., Schaefer, T., Tilgner, A., Styler, S. A., Weller, C., Teich, M., & Otto, T. (2015). Tropospheric aqueous-phase chemistry: kinetics, mechanisms, and its coupling to a changing gas phase. Chemical reviews, 115(10), 4259-4334.

Kamilli, K. A., Ofner, J., Lendl, B., Schmitt-Kopplin, P., & Held, A. (2015). New particle formation above a simulated salt lake in aerosol chamber experiments. Environmental Chemistry, 12(4), 489-503.