Reactive halogens play a key role in the oxidation capacity of the polar
troposphere. However, sources and mechanisms, particularly those involving
active iodine, are still poorly understood. In this paper, the photolysis of
an atmospherically relevant frozen iodate salt has been experimentally
studied using infrared (IR) spectroscopy. The samples were generated at low
temperatures in the presence of different amounts of water. The IR spectra
have confirmed that, under near-ultraviolet–visible (UV–Vis) radiation, iodate is efficiently
photolysed. The integrated IR absorption coefficient of the iodate anion on
the band at 750 cm
Atmospheric iodine compounds are present in the marine and polar boundary layers (Saiz-Lopez et al., 2012), where they play a relevant role in catalytic ozone destruction (Saiz-Lopez et al., 2007b; Read et al., 2008), and they could also be involved in new particle formation in the polar environment (Allan et al., 2015; Roscoe et al., 2015). Moreover, in the polar atmosphere, iodine has also been suggested as one of the possible sinks of gaseous elemental mercury (Calvert and Lindberg, 2004; Saiz-Lopez et al., 2008).
Although the concentration of atmospheric iodine is highly variable at different regions, ground- (Frieß et al., 2001; Saiz-Lopez et al., 2007b; Atkinson et al., 2012) and satellite-based instrumentation (Saiz-Lopez et al., 2007a; Schönhardt et al., 2008) measurements have confirmed remarkably high concentrations (up to 20 pptv) of IO in coastal Antarctica. Nevertheless, the sources and mechanisms of iodine emissions from ice remain poorly understood (Saiz-Lopez et al., 2015; Kim et al., 2016).
Apart from observations of gaseous iodine species, different studies have
conducted analysis of the iodine fraction in rainwater (Laniewski et al.,
1999) and aerosols (Baker et al., 2000). In all of them, iodine
concentrations are considerably enriched over seawater, and an appreciable
fraction of soluble iodine species like I
A recent study has suggested that IO
For the study of the photolysis of iodate salts, we have tested several
iodated compounds. Firstly, the photolysis of frozen solutions of KIO As mentioned above, it was not possible to monitor iodate signal in
the presence of high concentrations of water since the infrared iodate band
overlaps with water absorptions. The fact that the chosen salt has a cation
like NH As far as we know, there is no information in the literature of the
integrated value of the IR absorption coefficient of the iodate band, and in
consequence it was not possible to directly quantify the amount of iodate in
the samples. One of the possibilities to solve this problem is to use an
iodate salt for which the integrated absorption coefficient of the IR band of
the counter-ion was known, like ammonium iodate. This was the procedure that
we have followed, and more details of these calculations are given in the
next section. Moreover, ammonium iodate is expected to be an abundant iodate salt
in the atmosphere, since ammonium concentrations are high in some
environments, and it could be deposited into the ice as large fluxes of
iodinated compounds have been observed during glacial periods (Spolaor et al.,
2013); the presence of ammonium ions in ice samples is also expected.
Moreover, ammonium and iodinated compounds have been detected at the same
time in melting Arctic sea ice, implying that this salt could be
atmospherically relevant (Assmy et al., 2013). Note, however, that other salts
such as NaIO
Solid samples containing iodate anions were produced through the sudden
freezing of droplets of aqueous solutions of NH
Schematic view of the experimental setup.
UV–Vis spectra of the studied salts were obtained in water solution at different concentrations using both a UV–Vis Uvikon spectrophotometer 930 from Kontron Instruments and a double-beam spectrophotometer (Shimadzu UV-3600), equipped with quartz cuvettes of 10 mm size. The spectra resolution was fixed at 0.5 nm, from 190 to 500 nm.
In all experiments, a pulsed valve was filled with a solution 0.1 M of
ammonium iodate (Across Organics, for analysis). A slight He overpressure
behind the liquid solution filling the valve improved the performance. This
generation procedure does not lead to a uniform film, and the thickness of
the ice samples, which typically range from
Initially, deposition at low temperature (
Due to the requirements of the experimental setup, the ratio of
NH
To summarize the procedure to generate the samples, they were firstly generated by HQ deposition at 100, 140, 160, 200, 260 or 298 K, and after deposition at those temperatures, three different processes were carried out: (i) the samples at the deposition temperature were just irradiated; (ii) samples deposited at low temperatures were firstly annealed to around 170 K for some minutes to eliminate part of the water, then cooled down to a certain low temperature (from 100 to 140 K) and then irradiated; (iii) or samples were annealed to 240 K for around 10 min to dry them completely, then cooled down at a selected low temperature at which a certain amount of water from vapour phase was deposited and finally irradiated. A complete list of all the samples and deposition conditions (and the resulting rate constants of the photolysis process) is included in Table S1 in the Supplement.
Column densities of water, NH
We assumed that the observed photolysis of ammonium iodate samples should be
mainly due to the highest frequency photons (below 400 nm) emitted by the
solar lamp. The reason is that IO
UV–Vis absorption spectra from 190 to 400 nm for
KIO
In order to illustrate whether this irradiance power is characteristic of
environmental conditions, it could be compared with the average irradiance
(also below 400 nm) received at the Earth's surface, which has been
estimated as around 0.01 W cm
Positions (in cm
Note that, due to the limitations mentioned above and the use of the thermopile for the determination of the photon flux, there are limitations and uncertainties (thermopile is practically insensible to photon frequency), and other methodologies (as for example the use of chemical actiometers) could be more adequate to quantify this parameter.
Figure 3 shows IR spectra of different samples of solid ammonium iodate salt
(with a small water proportion) at 200 and 100 K including those of 4 and
2 : 1 H
Mid-IR transmission spectra of pure NH
After generation, all samples were irradiated for 3 to 5 h by a
1000 W Xenon Arc lamp. In all cases NH
Slopes obtained in the linear regression fit of the
representation of the natural logarithm of the integrated intensities (in
arbitrary units) of the
It is important to note that there was no evolution of the IR spectra of the samples observed in dark conditions (this fact was checked many times throughout the experimental measurements).
Typical UV–Vis spectra of common ammonium salts (i.e. NH
Natural logarithm of the integrated intensities (in
arbitrary units) of the
In addition to those changes at the 1430 and 740 cm
Evolution of the mid-IR transmission spectra of a pure
NH
The mechanism of iodate photolysis is largely unknown. In the study of
Spolaor et al. (2013), during the irradiation of IO
However, independently of the photolysis mechanism, the photolytic rate
constant,
According to Eq. (3), a representation of the natural logarithm of the
integrated band intensities of NH
The photolysis rate can be also estimated according to Eq. (4):
In order to estimate the absorption cross section of the ice mixtures, we
have recorded the UV–Vis spectra of different concentrations of
NH
Absorption cross section of the ammonium iodate solution (error bars in red).
These results are similar to those obtained for other iodate solutions (see
UV–Vis spectra in Fig. 2), and, in all cases, nearly null absorptions were
recorded above 300 nm, which is also in agreement with those of Saunders et
al. (2012) and Awtrey and Connick (1951), who found nearly null absorption
above 300 nm for NaIO
However, note that in Fig. 2 the glass window shows nearly null
transmission below 250 nm (where the cross section of iodate peaks). At
300 nm, the absorption cross section is around
In order to have a realistic estimation of the wavelength range relevant for
iodate photolysis, we have calculated an action spectrum entailing the
product of the IO
Nevertheless, we should also consider that these cross section values are
obtained for liquid solutions, so they could be somehow different from frozen
samples as in our case. Several studies have shown that the absorbance
spectrum of a species in ice could be estimated by red-shifting the solution
spectrum (e.g. Dubowski and Hoffmann, 2000). Moreover, simulations with
methyl peroxide in frozen water predict that absorption spectra are also
red-shifted at low temperatures (Epstein et al., 2012). According to these
previous studies, we evaluate the red shift of the liquid cross section
values of NH
Quantum yield estimation for the photolysis process studied as variation of the red shift of the cross section (from 250 to 400 nm) obtained for the ammonium iodate solution.
According to these results, the photoreactivity of the iodate salts should
be related to the low-temperature effect and to the fact that iodate solutions
or salts are frozen, in agreement with the results from Spolaor et
al. (2013). It is well known that different photochemical reactions are
greatly accelerated in frozen solution due to the concentration effect of
solutes in porous cavities or channels formed in the water ice network (see
e.g. Grannas et al., 2007; Kahan et al., 2010, and references therein). For
the case of NO
The increase of photolysis rates at low temperature can be caused by either a substantial change in the absorption cross section (due to a red shift in relation to solution) or an increase of the quantum yield of the process, or in fact by both factors at the same time. Our experiments do not allow discrimination of these factors which need to be further studied in subsequent experimental work. Instead, the integrated absorption cross section obtained in this work should be regarded as a lower limit. The reason is mainly the limitations associated with distributing the samples homogeneously during deposition, which could generate areas free of samples on the substrate. For these cases, the irradiance received by the samples could be lower than calculated (which assumes a homogeneous distribution of the sample), thereby leading to a higher calculated absorption cross section value than the one obtained in this work. Based on the dispersion of our results, we have estimated that this effect could account for an increase in this value by up to a factor of 2.
In addition, due to the characteristics of our experimental setup, our
results represent the photolysis of the iodate in the bulk. However, as in
the case of NO
We have incorporated the experimentally derived absorption cross section
value into an atmospheric model in order to assess the implications that this
process could have in polar atmospheric chemistry. Although high levels of
reactive iodine have been measured in coastal Antarctica, the emission
mechanism over ice still remains unclear. We use an atmospheric model (for
details see Saiz-Lopez et al., 2008) of the Antarctic boundary layer to
assess the potential of iodate photolysis to release reactive iodine to the
polar atmosphere. The model is initialized with typical concentrations of
atmospheric constituents in coastal Antarctica (Jones et al., 2008) for
October. An action spectrum considering the Antarctic photon flux has also been
calculated (see Sect. S5 in the Supplement), which shows approximately
6-times-lower intensity than that for the laboratory experiments. The reason
for such a difference is the smaller Antarctic sunlight photon flux. We
constrain the ice surface in the model with an average iodate concentration
at the ice surface of 19 nM, as recently measured over the Weddell Sea
(Atkinson et al., 2012). The model incorporates a two-stream radiation code to
compute the actinic flux at the surface for springtime Antarctic irradiation
conditions (Saiz-Lopez et al., 2008), and the iodate absorption cross
sections and quantum yield values estimated in this work. We assume that
there is an iodine atom unity conversion of iodate into reactive gas phase
following iodate photolysis. This assumption is based on previous studies on
the photolysis of nitrite and nitrate on ice, which pointed out that 1 : 1 nitrogen
atom conversion from inert to reacting species was necessary to model the
NO
We have explored the photolysis of ammonium iodate salt in frozen solutions.
The samples were generated by different deposition methods, and at different
temperatures and water concentrations, in order to obtain samples of
different morphologies. The samples were processed by simulated solar light
with an average light power of 0.19 W cm
Óscar Gálvez acknowledges financial support from the Ministerio de Ciencia e Innovación, “Ramón y Cajal” programme, and from the Ministerio de Economía y Competitividad, project “CGL2013-48415-C2-1-R”. M. Teresa Baeza-Romero and Mikel Sanz acknowledge financial support from the Ministerio de Economía y Competitividad, project “CGL2013-48415-C2-2”. Óscar Gálvez, M. Teresa Baeza-Romero and Mikel Sanz acknowledge financial support from the Spanish crowdfunding platform PRECIPITA from the FECYT foundation. Authors acknowledge the different reviewers (specially the helpful comments of referee no. 4) of this manuscript, who have contributed to noticeably improving this article. Edited by: M. Ammann Reviewed by: four anonymous referees