| Leeds Atmospheric Chemistry - John Plane's Group - Laboratory work on Iodine particles|

Laboratory studies of iodine oxide particles (IOPs): Gas to particle conversion of iodine oxides

Iodine atoms are released in the troposphere by photolysis of molecular iodine ¹ and a variety of volatile iodocarbons ², which are emitted by the marine biosphere. Their atmospheric fate is primarily to react with O₃, forming iodine monoxide ³:

I + O₃ --> IO + O₂ (1)

Reactions that convert IO back to iodine atoms without the formation of O(³P) can lead to a catalytic O₃ loss ⁴. An example of this type of reactions is the disproportionation of IO:

IO + IO	--> I₂ + O₂	ΔH⁰_Γ = -170 kj mol^-1	(2a)
	--> 2I + O₂	ΔH⁰_Γ = -18 kj mol^-1	(2b)
	--> I + OIO	ΔH⁰_Γ = -48 kj mol^-1	(2c)
	--> I₂O₂	ΔH⁰_Γ = -108 kj mol^-1	(2d)

The ozone destroying potential of iodine depends on the branching of reaction (2) and on the atmospheric fate of the iodine oxides OIO and I₂O₂. Iodine atoms are regenerated by channels (2b) and (2c). At daytime, the photolysis of I₂ followed by reactions (1), (2b), and (2c) produces a chain reaction destroying O3. Under tropospheric conditions, OIO and the asymmetric dimer IOIO are produced with branching ratios of ~ 40% and 60%, respectively 5. However, theoretical calculations on the dimer indicate that IOIO is relatively unstable (Figure 1) and would dissociate to I+OIO ⁶.

The atmospheric fate of OIO is currently under discussion. Gas to particle conversion has been proposed as a potentially important sink for OIO. Iodine condensable vapours are linked to new particle formation in the coastal marine boundary layer (MBL) at low tide and during the day ⁷. The photolysis of mixtures of O₃ and different precursors of atomic iodine in laboratory studies has demonstrated the occurrence of iodine-driven rapid particle production ⁸. Analysis of laboratory made iodine oxide particles (IOP) by transmission electron microscopy (TEM) with X-ray elemental analysis (XRD) shows that they have the empirical formula I₂O₅ ⁶. The major question is how this happens - is it through formation of I₂O₅ in the gas phase, followed by polymerization into ultra-fine aerosol, or by condensation of various I_xO_y to form amorphous iodine oxides ⁵, which subsequently rearrange to I₂O₅ in the solid phase?

The recombination of OIO with itself:

OIO + OIO

--> I₂O₄

ΔH₀ = -91 kj mol^-1

(3)

has been proposed as a route to particle formation ⁷, ⁸, However, quantum chemistry calculations predict that the OIO dimer is also rather unstable, with a lifetime of ~ 0.1 s in the MBL ⁶. There is also evidence that the recombination of IO with OIO is fast, proceeding close to the high pressure limit (~10^-10cm³s^-1) at 1 atm ⁵:

OIO + IO

--> I₂O₃

ΔH₀ = -120 kj mol^-1

(4)

Formation of either I₂O₃ or I₂O₄ still leaves open the question of whether I₂O₅ can form in the gas phase. The most likely route is through exothermic reactions with O₃(Figure 1) ⁶.

Using a kinetics time of flight mass spectrometry technique ⁹, we wish to determine the thermal stability of the higher iodine oxides, I_xO_y (x = 1-2, y = 2-5) and to understand at a fundamental level their tendency to condense from the gas phase into particles. An important reason for understanding the detailed mechanism of IOP formation is to be able to predict the formation of ultra-fine aerosol from iodine emissions over the open ocean, where emission levels are expected to be much lower than in coastal regions. This may result in a slow, though still significant production of Aitken nuclei, which are more difficult to observe compared with the dramatic particle bursts in coastal locations ⁷. If the particles that form under these conditions are largely composed of I₂O₅, which is hygroscopic, they should be much better nuclei than I₂O₄.

On the other hand, both iodine-catalysed ozone depletion and IOP formation will depend on the photochemical stability of OIO. The radical has strong absorption bands between 480 and 620 nm (Figure 2) ¹⁰, where photolysis to yield I + O₂ is possible ¹¹. Although two recent studies report upper limits to the I atom quantum yield of < 0.05 (560-580 nm) and < 0.24 (532 nm) ^12,13, these upper limits need to be refined and measurements made below 532 nm (photolysis to O + IO is possible below ~480 nm). The photochemistry of OIO will be studied using an improved cavity-ring-down based technique that we have developed recently ¹².

Figure 2 Absorption cross section of OIO ⁵

Figure 3 The time of flight (TOF) spectrometer: a) Side view, b) top view

Laboratory studies of iodine oxide particles (IOPs): Formation and growth of particles

Using a photo-chemical flow reactor, we have studied the formation and growth of particles from the photo-oxidation of molecular iodine (I₂) with ozone (O₃). These particles were trapped for subsequent analysis of shape using transmission electron microscopy (TEM) and composition by energy dispersive x-ray (EDX) spectroscopy (see Figure 4). Particles were found to be non-spherical or fractal-like aggregates, composed of smaller particles and quantitative EDX analysis confirmed particle composition as I₂O₅ [Saunders and Plane, 2005].

Figure 4 Electron micrographs showing: (a) iodine oxide particles (dark features) captured on a 'holey-carbon' film (light-coloured spherical features), imaged at low magnification (scale bar = 1 micrometer); (b) a single particle aggregate at higher magnification. Panel (c) shows a representative EDX spectrum from an iodine oxide nanoparticle - Figure taken with permission from Saunders and Plane, 2005.

We also studied the effects of varying initial I₂ concentration and I₂ photolysis rate on the particle size distributions which were measured using a scanning mobility particle sizer (SMPS) consisting of a condensation particle counter (CPC) and differential mobility analyser (DMA). These instruments allow for the collection of time-resolved particle size data which was used for modelling the particle growth within a second reactor system - see modelling of iodine particles. Generated particles were also passed through a multi-pass optics cell allowing for the determination of a wavelength-resolved optical extinction spectrum for the IOPs [Saunders and Plane, 2006a].

Further studies include:

Growth behaviour in water, sulphuric acid, and organics vapours, using a Scanning Mobility Particle Spectrometer (SMPS) system

Figure 5 Schematic of our experimental set-up to study the growth behaviour of IOPs in H₂O and H₂SO₄ vapours

Deliquescence / efflorescence behaviour of I₂O₅/HIO₃ crystals using a cold-stage/microscope system

Figure 6 Cold-stage-microscope system developed to study the deliquescence and efflorescence properties of I₂O₅ / HIO₃.

Ice nucleation potential of insoluble iodine-containing species using the cold-stage / microscope system.
Water activity, viscosity and spectral properties of relevant compounds such as I₂O₅ and HIO₃.
Quantitative study of the recycling of I₂ from the hydrolysis of I₂O₄ and from HIO₃ / I₂O₅ with organics such as oxalic acid and malonic acid

Laboratory study of glassy behaviour of Iodate (IO₃^-) solution droplets using a Raman Microscope

Recent studies have shown that atmospheric aerosol may exist in a glassy phase at low relative humidity (RH)^14,15. According to Kumar et al., 2010, iodate solution droplets do not show efflorescence behaviour at low RH which is shown in Figure 7. This indicates that they remain non-crystalline and become highly viscous at higher concentrations. These results suggested that iodate (IO₃^-) solution droplets may form glass in the atmosphere.

Figure 7 A series of microscope images from the efflorescence experiments with droplets formed from crystals of (NH₄)₂SO₄ (top), I₂O₅ (middle), and HIO₃ (bottom).

To verify these results, we are studying the glassy behaviour of iodate (IO₃^-) solution droplets using Raman microscope system. The experimental set-up is shown in Figure 8.

Figure 8 Raman microscope system and RH and temperature control chamber to study the glassy behaviour of iodate (IO₃^-) solution droplets.

References

1. Saiz-Lopez, A. ; Plane, J. M. C. Geophys. Res. Lett. 2004, 31, L04112.
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11. Ashworth, S. H.; Allan, B. J.; Plane, J. M. C. Geophys. Res. Lett.,2002, 29, art. no.-1456.
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13. Tucceri, M. E.; Holscher, D.; Rodriguez, A.; Dillon, T. J.; Crowley, J. N. Phys. Chem. Chem. Phys., 2006, 8, 834-846.
14. Murray, B. J., Wilson, T. W., Dobbie, S., Cui, Z. Q., Al-Jumur, S., Mohler, O., Schnaiter, M., Wagner, R., Benz, S., Niemand, M., Saathoff, H., Ebert, V., Wagner, S., and Karcher, B. Nature Geoscience, 2010, 3, 233-237.

15. Murray, B. J. Atmos. Chem. Phys., 8, 5423–5433, 2008a

Saunders, R.W., and J.M.C. Plane (2005) Formation pathways and composition of iodine oxide ultrafine particles. Environ. Chem. 2, 299.
Saunders, R.W., and J.M.C. Plane (2006a) Inorganic aerosol formation and growth in the Earth's lower and upper atmosphere. J. Phys. IV France, 139, 243.
Saunders, R.W. et al. (2010). Studies of the Formation and Growth of Aerosol from Molecular Iodine Precursor. Zeitschrift für Physikalische Chemie: Vol. 224, No. 7-8, pp. 1095-1117.

Kumar, R., et al. (2010) Physical properties of iodate solutions and the deliquescence of crystalline I₂O₅ and HIO₃, Atmos. Chem. Phys., pp 12251-12260, Copernicus Publications, 10.

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