Investigating Organic-Mineral Core-Shell Aerosols: A Study of Hydroxyl Radical Oxidation

Aerosols significantly influence air quality, climate, and health. Specifically, aerosols can influence climate directly though scattering/absorbing and indirectly in their capacity of cloud condensation nuclei. Aerosols exist in the atmosphere in different morphologies (e.g. core-shell) which can as a result can affect their scattering properties. This study explores the Mie scattering of an organically coated mineral aerosol when exposed to an atmospherically significant oxidant: Hydroxyl radical, OH∙.

Figure 1. Laser tweezer trap with annotated optical pathway.

Atmospheric oxidation by OH∙ can cause changes to an organic particle’s optical properties. Utilising a combination of Optical Tweezers and Mie Spectroscopy (figure 1) the significance of exposure to OH∙ has on both homogenous and coated aerosols can be studied. These techniques were used since they enable the manipulation and analysis of individual aerosol particles (~1 µm radii) in a controlled environment. OH∙ was generated in situ, due to its short lifetime, through the mechanism outlined in equation 1.

(eq.1)

From Mie theory we know that the movement of Mie resonance peaks indicate a change in the optical properties of the levitated particle. Said changes can be attributed to loss of organic material and/or shift in refractive index (indicating a chemical change in the organic sample).

Figure 2. Line graph illustrating the movement of a single peak from the recorded Mie spectra (peak shift) over time whilst the optically levitated particle is exposed to OH∙.

 

Results are measured in terms of the shifting of Mie resonance peaks and show that real urban atmospheric coatings react with OH∙, and that reaction is to a significant scale (-1.23 nm / 60 minutes) as seen in figure 2. Woodsmoke yielded no significant reaction (figure 2). Squalane thin films reacted significantly, presenting a peak shift of -5.73 nm / 60 minutes and was repeatable (figure 2). The homogenous squalane droplets also showed significant change. Overall, it has been determined that for the squalane and real urban organic samples form a core-shell morphology with silica. Furthermore, these thin films present a negative shift in peak position indicating a significant loss of material when exposed to OH∙. Therefore, it can be concluded that OH∙ results in alteration of the optical properties of organic thin films, and this observable behaviour can be considered to an order of magnitude that should be accounted for in atmospheric radiative forcing calculations, however future work is required to model this with detail.

This work was supported by the Natural Environment Research Council and the ARIES Doctoral Training Partnership [grant number NE/S007334/1], NERC grant NE/T00732X/1, with additional support from STFC's Central Laser Facility for access at the Research Complex at Harwell.