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Iodine is a crucial trace element that is engaged in a wide range of significant processes that take place within the Earth system. Iodine has been known to be important for human health for more than a century, and low iodine diets have been related to goitre, cretinism, and newborn deaths. Iodine has been found to have considerable effects on tropospheric photochemistry, which in turn affects climate by lowering ozone's radiative forcing and air quality by lowering extreme O3 concentrations in polluted areas. The majority of the iodine that naturally occurs in the water is in the form of aqueous iodide and iodate. The largest single supply of gaseous iodine to the atmosphere is thought to be the quick reaction of sea-surface iodide with O3. This release of iodine is thought to have increased significantly over the twentieth century, maybe by a factor of three, as a result of rising anthropogenic O3. However, there are significant obstacles to making accurate predictions about how iodine emissions and its biogeochemical cycle may change in the future or have changed in the past due to uncertainties in the marine iodine distribution and world cycle. Here, we summarize recent findings from our team and others that offer a novel viewpoint on the biogeochemical cycle of iodine on a global scale. We specifically contend that aqueous iodide concentrations in the surface ocean may shift significantly as a result of future climate-induced oceanographic changes, with implications for climate and air quality.

In spite of certain advanced discoveries related to iodine chemistry, still many unanswered questions remain there. Certain models have been proposed by different researchers as discussed previously. Certain sources of iodine and its impacts on atmosphere as well as human have been discussed but there may be more of different sources and widespread impacts. There are still significant uncertainties in the magnitude and impacts of the ocean- atmosphere cycling of iodine. However, it would appear from these calculations that iodine aerosol may play an important regional role in determining the aerosol load of the remote tropical ocean both in the present and in the preindustrial. Further observations in these regions would help us to constrain the magnitude of this role. The main objective of this work is to develop a global model to define the iodine chemistry in the atmosphere completely using certain programming with JAVA, C++ etc. and the results may be compared with the experimental data measurements done in the atmosphere.

Atmospheric Iodine

The chemistry of bromine and chlorine has received a lot of attention in the work on the stratosphere. Iodine, however, is thought to play a major role in the troposphere (Simpson et al., 2015). Iodinated organic molecules from the ocean were once assumed to be the main source of iodine in the atmosphere (Carpenter, 2003). Initially, methyl iodide (CH₃I) was thought to be the main source, but it was discovered that other organo-halogens (CH₂I2, CH₂IBr, and CH₂ICl) with higher photolysis rates would result in these compounds having a bigger flux despite their low concentrations (Chuck et al., 2005; Jones et al., 2010; Law and Sturges, 2006). Iodine has recently been discovered to be released into the atmosphere via inorganic halogen compounds (I₂ and HOI) (Carpenter et al., 2013).

Iodine species measurement is difficult because of their brief lives and low atmospheric concentrations, and as a result, there is a dearth of observational data. Measurements have been primarily on CH3I and organic chemicals for decades (Saiz-Lopez and von Glasow, 2012). Over the past ten years, there has been a rise in the amount of data available for both organic (such as CH3I and CH2IX, where X=Cl, Br, I) and inorganic (such as IO, OIO, I₂) species (Saiz-Lopez et al., 2012b). A global coverage attempt has also been made by satellites, however retrieval at tropospheric concentrations is challenging (Schonhardt et al., 2008).

Chemistry of Iodine and Its Destruction of Ozone (O3)

Even if there are some doubts, the primary iodine cycles that result in the degradation of O3 were initially emphasized by Chameides and Davis (1980), and these are reviewed (Atkinson et al., 2000, 2007; Sander et al., 2011).

The sections that follow now discuss iodine reactions. Starting with interactions with the HOx and NOx cycles. Iodine self reactions (IOx) are then taken into account The heterogeneous responses of iodine are then taken into consideration.

Figure 1– Simplified Inorganic Iodine Gas-Phase Chemistry Schematic.

Iodine in the Ocean

The two predominant iodine species in the ocean are aqueous iodide (I, reduced form) and iodate (IO3, oxidized form), with a combined concentration of typically 400–500 nM. Iodate is the preferred form of iodine from a thermodynamic standpoint (apart from in extremely oxygen-depleted waters), and it predominates above the marine mixed layer in oxygenated saltwater. Iodide's presence and distribution in the surface ocean are primarily controlled by processes of physical mixing and advection as well as its physiologically mediated interconversion with iodate. Up to 50% of iodine may be detected as iodide in the euphotic zone due to the reduction of iodate to iodide, which has been associated with primary productivity. Days to weeks have been reported for the iodate to iodide transition to occur in natural saltwater, but the mechanism is poorly understood, and it is unknown if the transformation occurs as an extracellular, cell surface, or dissimilatory reaction. Although iodate interactions with reduced sulfur species generated from cells during senescence and reduction by nitrate reductase enzymes have both been hypothesized, neither method has been proven to be a substantial pathway for conversion.

Once created, kinetically stable iodide progressively oxidizes back to iodate; however, the duration of this process is unknown, with estimates varying from six months to 40 years. In the past, mass balance methods have often been used to determine the rate of iodide oxidation. These estimations' oxidation rates for natural seawater were obtained from recent studies utilizing a radiotracer technique. Rates of chemical oxidation of iodide to iodate in seawater are too slow to account for the observed distribution of iodine species, and the process has been considered to be biologically mediated aside from processes unique to the sea-surface microlayer, such as oxidation by O3. The creation of ocean models of iodine transformations has been argued to be severely constrained by the ambiguity surrounding the rates and mechanisms involved. It has been suggested that bacterial nitrification may be related to the oxidation of iodide to iodate based on studies of the water column. Our latest research for laboratory cultures of ammonium-oxidizing bacteria has validated this. In the ammonium-oxidizing bacteria Nitrosomonas sp. (Nm51) and Nitrosoccocus oceani (Nc10) supplied with I and NH4+, we found a significant increase in iodate concentrations compared to media-only controls, showing that iodide oxidation to IO3 is linked to nitrification, and particularly ammonium oxidation. Nitrosomonas sp. had cell-normalized production rates of 15.69 (4.71) fmol IO₃ cell1 d1 while Nitrosococcus oceani had 14.35 (8.55) fmol IO₃ cell1 d1. Iodide oxidation to iodate may be common throughout the waters of the planet because nitrification is known to occur throughout the oceanic water column. As hypothesized by others, this mechanism offers a different or additional connectivity between the iodine and nitrogen cycles in the ocean.

Figure-2: Simplified Schematic of Iodine Cycling in the Surface Ocean

HOx Interactions

As atomic oxygen is regenerated after atomic iodine is photolysed, a "null cycle" is completed. Atomic iodine directly combines with O₃ to make IO, but this reaction does not directly deplete O₃.

I + O₃ → IO+O₂    (1.1)

IO −^h→^ν  I+O             (1.2)

O2 + O + M → O3 + M       (1.3)

The transfer of this unusual oxygen is made possible by secondary interactions involving IO and other trace gases, even if the majority of IO is photolysed to regenerate atomic iodine and oxygen (Ox). A catalytic cycle is created if processes can replenish atomic iodine without releasing oxygen, which results in the loss of oxygen and a consequent net drop in O₃. The IO reaction with the hydroperoxy radical making HOI, which is photolyzed to replenish atomic iodine without generating an Ox molecule, is a good illustration of this. Additionally, this alters the HO2:OH ratio, which reduces the formation of O₃ and alters the lives of species that depend on OH (such as CH4) (Saiz-Lopez et al., 2012b). When IO concentrations are lower ( 1 pmol mol-1), this IO reaction route through HO2 is the main loss pathway (Saiz-Lopez et al., 2012a).

I + O₃ → IO + O₂  (1.31)

IO + HO₂ → HOI + O₂        (1.32)

HOI −^h→^ν  I + OH                (1.33)

net : O₃ + HO₂ → OH + 2 O₂

Iodine Self Reactions and Higher Oxide Chemistry

Higher IO concentrations (>2 pmol mol-1, Saiz-Lopez et al. 2012b) can also cause IO to interact with itself, resulting in the creation of an additional efficient catalytic Ox loss cycle through the I+O2 product channel (Reaction 1.46). The alternative channel that produces I₂O2 has two possible outcomes: it can either form a "null cycle" by photolyzing back to OIO+I or thermally decomposing to IO+IO, or it can be physically lost (for example, by deposition or aerosol uptake) leading to a termination step.

I + O₃ → IO + O₂  (1.43)

IO + IO −M→ OIO + I         (1.44)

OIO −^h→^ν  I + O₂     (1.45)

net : 2 O₃ → 3 O₂

Iodine has been demonstrated to produce iodine oxide particles, principally through the condensation of higher iodine oxides (Burkholder et al., 2004; G'omez Mart'n et al., 2007; McFiggans et al., 2004). (IxOy). However, observations for specific higher iodine oxide reactions rate constants remain poorly characterized, and discussion regarding potential production pathways as outlined in recent studies continues (Saiz-Lopez et al., 2012b; Sommariva et al., 2012). The proposed pathways essentially include sequential development through addition of OIO, as in the continuation of Reactions 1.53, or sequential addition of O by reaction of I₂OX with O₃ to generate I₂OX+1 (Sommariva et al., 2012). The validity of these approaches has recently been challenged by new research, which contends that the synthesis of I₂O₄ is the crucial stage in the formation of iodine particles (Gomez Mart'n et al., 2013a).

Because of their potential to produce particles in the marine boundary layer, greater iodine oxide pathways have drawn a lot of attention in the literature. The marine atmosphere is the planet's most pristine environment, and it is extremely susceptible to changes in the types and quantities of these aerosol precursors (Quinn and Bates, 2011). The marine boundary layer may provide a new pathway for the formation of particles, such as a source of cloud condensation nuclei (CCN), which could have important climatic implications.

I + O3 → IO + O2	(1.49)
IO + IO → OIO + I	(1.50)
→ I2O2	(1.51)
IO + OIO → I2O3	(1.52)
OIO + OIO → I2O4	(1.53)
IxOy → aerosol	(1.54)

Studies have different "cut-off" points for iodine's oxidation to higher oxides. Some studies (Breider, 2010) or all processes past I₂O₂ (Ord'on ez et al., 2012) do not consider O₃ loss via I₂O₂ + O₃ oxidation. Due to a lack of knowledge regarding the reversibility of the greater oxide loss route, the choice of which reactions to add is quite unpredictable. One study in particular draws attention to this, showing that the included I₂O₂ + O₃ oxidation route accounts for the majority of the iodine-driven O₃ loss (Breider, 2010), thereby treating all higher iodine oxides as being totally stable. Literature pays a lot of attention to the stability of these compounds (Saiz-Lopez et al., 2012b), and some recent work has even explored the effects of iodine photolysis without higher oxides on the world as a whole (Saiz-Lopez et al., 2014).

Uncertainties in Iodine Chemistry

The chemical kinetics of iodine is subject to substantial uncertainty. Iodine's reaction mechanisms have been thoroughly discussed in a number of papers (Carpenter, 2003; Saiz-Lopez et al., 2012b,a; Simpson et al., 2015), and more recent work has used box-modeling to examine uncertainty within known reactions (Sommariva et al., 2012).

A substantial corpus of experimental and theoretical work on iodine has been evaluated by JPL (Sander et al., 2011) and IUPAC (Atkinson et al., 2000, 2006, 2007, 2008) compilations. However, for other processes, the experimental rate data are scant or nonexistent. For instance, it is difficult to quantify the ultimate chemical destiny of higher iodine oxides (I2OX, where X 2), despite the fact that these events have a considerable impact on the lifetime of IOx (Sommariva et al., 2012). I2OX is created through combination processes, however as further addressed by Saiz-Lopez et al., there are still uncertainties regarding their polymerization, photolytic characteristics, and ultimate fate (2012b). Recent research shows the sensitivity of iodine concentrations and, consequently, the atmospheric implications of this higher oxide chemistry while demonstrating that some of the major iodine species are effectively buffered to mechanism uncertainties (Sommariva et al., 2012).

Atmospheric Processing

Dry and Wet Deposition

Henry's law values and formation enthalpies for a certain species must be understood in order to use dry and wet deposition calculation methods. These values for iodine are known for the majority of species (Sander, 1999, 2015) or have been estimated by analogy with related most previously investigated bromine species (Saiz-Lopez et al., 2007b). There are theoretical calculations for enthalpies of production for less well characterized species, including I2OX (Kaltsoyannis and Plane, 2008).

Photolysis

Iodine forms labile species and has a low ionization energy. Short lifespan species result from this, including those with seconds (I2/HOI), minutes (CH2I2), hours (CH2ICl), and days (I2/HOI) (CH3I).

According to the most recent JPL collection, absorption cross-sections are advised for all major iodine species (I2, HOI, IO, OIO, INO, INO2, INO3, CH3I, CH2I2, CH2IBr, and CH2ICl) (Sander et al., 2011). With the exception of CH3I, temperature dependencies are not stated, and quantum yields are still subject to uncertainty. In the most recent JPL/IUPAC compilations, higher iodine oxide cross-sections (for IxOy where x and y are greater than 1) are not reported (Sander et al., 2011). I2O2 and I2O3 "likely" values have been published by a single study using information taken from a study of the IO spectrum (Spietz et al., 2005). According to Bloss et al. (2010), several modeling studies made the assumption that IxOy had cross sections analogous to INO3 based on estimations from Bloss et al. (2001) and G'omez Mart'n et al. (2005) as well as ab initio results (Kaltsoyannis and Plane, 2008).

OIO's cross-section size had been the subject of debate in the past because the species was only observed during the day (Carpenter, 2003). (Stutz et al., 2007). However, thanks to the work of G'omez Mart'n et al., issue is now resolved (2009). Photolysis has been demonstrated to occur between 500 and 650 nm, and JPL provides the suggested cross-section (Sander et al., 2011). The photolysis routes of INO3 were the subject of a second historical dispute that limited knowledge of the efficacy of iodine-initiated O3 destruction (Carpenter, 2003), but this issue has also been settled (Saiz-Lopez et al., 2012b).

Observational Constraints

Organic Iodine Species

Iodine species are difficult to observe because they are quickly destroyed by photolysis or oxidation and have a very short life span. Unsurprisingly, CH3I, the iodine species with the longest lifetime (6 days; Bell et al. 2002), is the most measured. Having said that, the expansion of model comparisons and the creation of worldwide organic halogen emission schemes have been made possible by the improved availability of observations (Ordonez et al., 2012). Using satellite chlorophyll-a as a proxy for concentration, Ordonez and colleagues (2012) used the association between observations of species and their sources (such as biotic, abiotic), and they employed scaling factors to account for greater emission in coastal regions versus open waters. Iodocarbon species (CH3I, CH2IBr, CH2ICl, and CH2I2) and bromocarbon species (CHBr3, CH2Br2, CH2BrCl, CHBrCl2, and CHBr2Cl) were both present in this work. Ziska et al. (2013) recently estimated fluxes of halogen species using the expanding dataset of ocean surface concentrations, although only CH3I was calculated for iodine.

Iodine's biogeochemical cycling in its marine and atmospheric compartments has traditionally been investigated separately and by various scientific communities. Because its biogeochemical cycles take place over a wide range of durations, from seconds for some atmospheric processes to up to millennia in the ocean, studying iodine on an Earth system scale is incredibly difficult. Global ocean iodine modeling is in its infancy, despite significant work being made in recent decades on building atmospheric models of iodine cycling. We still have several fundamental questions that severely restrict the representation of iodine biogeochemistry. One of these is how iodide present at the ocean's surface is quantitatively converted into iodine emissions to the atmosphere. Another one of these is the rates and controls of iodine cycling in the ocean, including in oxygen-depleted waters. Weak laboratory and field data on atmospheric HOI, which is thought to be the main carrier of iodine from the ocean to the atmosphere, as well as a dearth of observations of ocean iodine speciation in some regions, particularly in the subtropical gyres and in the Arctic, are major observational gaps that limit our fundamental understanding. At any particular site, the range of seasonal fluctuation in sea-surface iodide concentrations is likewise very weakly limited. We suggest that iodine should be taken into account from this perspective even though the tools to study iodine's role in the Earth system are not yet fully developed. This is because understanding the interactions and feedbacks between biogeochemical and physical processes in the ocean and ozone (and other oxidants) in the atmosphere, which have policy-relevant effects ranging from emissions of ozone precursors to climate change, ocean acidification, and stratospheric ozone, are crucial. The possible effect of ocean acidification on atmospheric iodine emissions over the next two to three decades has been noted here. Continuing ocean acidification, deoxygenation, and decreased productivity, as well as modifications to ocean circulation and vertical mixing, could result in additional substantial alterations over the ensuing decades. The management of tropospheric ozone levels via precursor (nitrogen oxides and hydrocarbons) emission control is affected by these fluctuations in iodine.

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