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I like to naively think of Earth's natural atmosphere as nitrogen, oxygen, argon, water, carbon dioxide and okay, some nitrogen and sulfur oxides, and again okay, a little hydrogen and methane from microorganisms. Everything else is pollution and shouldn't be there.

Just because I like to naively think that way doesn't meant it's so.

The excellent answers to Why are atmospheric BrO and ClO important to measure by satellite? talk about halogen oxides.

Are they a natural component of Earth's atmosphere, purely pollution, or a mixed bag?

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  • $\begingroup$ The answer depends very much on which part of the atmosphere (e.g., the troposphere or the stratosphere) in which you are interested. $\endgroup$ – David Hammen Feb 23 at 18:56
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Classifying the presence of halogen oxides in the atmosphere as either natural, pollution or mixed bag, would result in mixed bag.

Natural sources of halogen oxides include the ocean and volcanoes. In 1963, Duce et al,

showed that bromine, like chlorine, was lost from the sea salt particles

Volcanoes are another source of halogen oxides,

Halogens are emitted from volcanoes primarily as hydrogen halides (HCl, HF, HBr, and HI). Upon mixing with the atmosphere, chlorine and bromine species are partially converted to the halogen oxides OClO and BrO.

and

Traditionally, emissions of volcanoes were regarded to be mainly of importance for the atmospheric sulfur cycle, acid deposition, and stratospheric effects from explosive eruptions. This view changed recently, mainly because of the observation of very high concentrations of bromine oxide, BrO, in the tropospheric, nonexplosive plume of Soufrière Hills, Montserrat.

The Sun is a requirement for the ocean source of halogen oxides.

Salt lakes, such as the Great Salt Lake in Utah are also sources of halogen oxides.

Forest fires can be a source of fluorine.

Pollution sources of halogen oxides includes: steel production, phosphate fertilizer production, burning coal, the manufacture of glass and ceramics.

Bromine and chlorine are used in cleaning products which are another source. Fire fighting chemicals such as PFAS (PolyFluoroAlkyl Substances) are another source.

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    $\begingroup$ Thanks for the great answer! I'm curious how bromine and chlorine used in cleaning products end up as halogen oxides. Do they start as oxides and evaporate that way, or are they in organic compounds and reactions in the atmosphere convert them to oxides? $\endgroup$ – uhoh Feb 23 at 14:28
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As an expansion of Fred's anwer: Some of these oxides in question are well recognized and intentionally used. Thus

  • some of them are used to desinfect water replacing elementary chlorine gas. They and their derivatives are not very stable (e.g., $\ce{ClO2}$, and $\ce{HClO}$) because during their decomposition organic matter degrades. Take, for example, sodium hypochlorite found in many bleaches which in acidic media decomposes into chlorine gas, then with a stronger oxidizing action than chlorine gas itself. What is the typical application? John Doe vents the room where it is used, releasing it to the environment.

    Of course, the concentration they are used to desinfect water is kept low in swimming pools or drinking water. Note their low boiling point (e.g., $2\,^\circ\mathrm{C}$ for $\ce{ClO2}$ (reference)). Balance the low concentration per litre / per gallon with the volume of water engaged. (Water treatment, e.g. with UV light instead often may be an alternative, cf. an example.)

  • Let's not forget large amounts of wood-based paper is bleached; basically again chopping organic matter (e.g., lignine) into pieces. Many print-houses advertise that their paper used either is recycled, or is elemental chlorine free. But guess what is used instead -- often, again it is chlorine dioxide. The diagram below depicts the worldwide pulp-production, classifying bleach with chlorine gas ($\ce{Cl2}$), using chlorine oxide (ECT), or methods skipping chlorine altogether (TCF). So again, a large potential artificial source for $\ce{ClO2}$.

enter image description here

(image credit)

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  • $\begingroup$ Thank you for the answer and additional information! $\endgroup$ – uhoh Feb 24 at 2:52
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Bromine and chlorine are very reactive, so are always found as compounds. They are fairly common elements, mainly in the oceans but also in the Earth's crust. Both have industrial uses, and both damage the Earth's ozone layer. They are evaporated from the ocean by strong sunlight in the form of oxides (BrO and ClO), and in trace amounts are natural constituents of the atmosphere. Very small amounts of these atmospheric halogens are anthropogenic.

As strong sunlight is the motive force for evaporating these halogens from the sea (and also from salt evaporite deposits on land), most of the troublesome halogens come from tropical or subtropical seas, and a lesser amount from temperate seas. Measures have already been taken to reduce the anthropogenic component, but there seems to be nothing we can do about naturally produced halogen oxides.

As the Earth has managed quite well for billions of years despite this problem, which is responsible for the so-called ozone hole over Antarctica, which waxes and wanes, it doesn't seem like something we should worry about unduly, though of course our efforts to reduce the anthropogenic component should continue.

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    $\begingroup$ Could you add some sources/references, please? $\endgroup$ – gerrit Feb 23 at 9:53
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    $\begingroup$ Still no citations... $\endgroup$ – Will Feb 23 at 9:54
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    $\begingroup$ This answer is so very wrong. The Antarctic stratospheric ozone hole is predominantly man-made. There is a big source difference between tropospheric and stratospheric halogens. $\endgroup$ – David Hammen Feb 23 at 20:35

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