The announcement Arianespace orbits two satellites – JCSAT-17 and GEO-KOMPSAT-2B – to support connectivity and environmental monitoring in Asia mentions GEO-KOMPSAT-2B which hosts the Geostationary Environmental Monitoring Spectrometer or GEMS UV/Vis (300-500 nm) imaging spectrometer for hyperspectral imaging of Earth's atmosphere.

The instrument's purpose is given as

Ozone profile and total-column or gross profile of other species. Tracked species: BrO, NO2, O3, OClO, SO2 and aerosol

and its primary mission objectives are listed as

  • BrO Mole Fraction
  • ClO Mole Fraction
  • H2O Mole Fraction
  • HC3Br Mole Fraction
  • HCHO Mole Fraction

with the following as other mission objectives

  • HCHO Total Column
  • NO Mole Fraction
  • NO2 Total Column
  • NO2 Mole Fraction
  • O3 Mole Fraction
  • O3 Total Column
  • SO2 Mole Fraction
  • SO2 Total Column

I think that "Mole fraction" is a relative concentration and "Total column" is absolute?

I see that some of the species listed in the instrument's purpose are not in its primary objectives, but oh well.

Question: Why exactly are atmospheric BrO and ClO so important to measure, and measure by satellite, and what is the difference between ClO in the "objectives" and OClO in the "purpose"?

  • 1
    $\begingroup$ A guess (I don't have to research at the moment): BrO and ClO play a role in ozone destruction (→ ozone hole). $\endgroup$
    – njuffa
    Commented Feb 22, 2020 at 6:00
  • 1
    $\begingroup$ As noted in my answer, the OSCAR entry on the GEMS instrument is severely flawed: GEMS will not measure BrO or ClO. Those flaws do not detract from the primary question regarding the importance of measure atmospheric BrO and ClO, or from the auxiliary question regarding molar fraction vs total column. $\endgroup$ Commented Feb 24, 2020 at 1:02

3 Answers 3


BrO and ClO significantly deplete ozone from the atmosphere.

Researchers at Harvard University state:

It is a remarkable fact that perhaps the most important observation coupling climate forcing to UV dosage levels at the surface at mid-latitudes is the observation of high (e.g. > 10 ppmv) water vapor and low temperatures (< 210 K) with the simultaneous determination of BrO and ClO in the lower stratosphere at mid-latitudes.

Similarly, Science magazine states:

Halogen-radical chemistry was responsible for approximately one-third of the photochemical removal of O3; reactions involving BrO account for one-half of this loss.


... where the chlorine and bromine thus released participate in the catalytic destruction of ozone.

Kinetic studies of the BrO plus ClO cross-reaction over the range T=246-314 K


pretext: Seeing this question on the list of network questions caught my interest to the site.

I agree with the arguments put forward in the answer by Fred, gravitating on the radical character of halogene oxides listed (e.g., BrO, ClO), where unpaired electrons contribute to reactivity to the specis. Some of them are created in situ under light radiation and hence may interfere in the constant generation and decomposition of ozone in the Chapman cycle.

To expand the picture, there is a large «family» of chlorine oxydes, for some of them the structural formulae are given below (each containing a dot already is a radical in a chemical sense) and individual details (chlorine oxides), and bromine dioxide in addition:

enter image description here

(image credit)

However, I would point out that the original listing on OSCAR about GEMS' mission as-such accidentally may contain errors as it lists HC3Br Mole Fraction. For one, there isn't such a compound as a stable one you could store in a bottle like copper sulfate. Indeed I speculate, in analogy to CFCs like methylene chloride and chloroform, their target is bromoform, $\ce{CHBr3}$ which has natural sources, and is relevant to atmospheric ozone, too.

side note: Similar to the putative chemical error here, some elder data entries in crystallographic databases erroneously state a symmetry in $P1$. While initially submitted by the authors as $P\bar{1}$, the bar sometimes was lost during the processing (mentioned, for example, by Hofman here), like the first SHG signal described by Franken et al.


Regarding the Geostationary Environmental Monitoring Spectrometer (GEMS) itself:

The instrument's purpose is given as

Ozone profile and total-column or gross profile of other species. Tracked species: BrO, NO2, O3, OClO, SO2 and aerosol

That is rather inaccurate and misleading. GEMS does not track BrO or OClO (or ClO). It does track ozone, but the target is tropospheric rather than stratospheric ozone. GEMS is a satellite instrument whose primary goal is tropospheric air quality monitoring rather than stratospheric chemistry.

I think that "Mole fraction" is a relative concentration and "Total column" is absolute?

Mole fraction is the relative concentration of some atmospheric component as a function of altitude. Converting this relative concentration to an absolute quantity (e.g., number of moles, or mass) and integrating from the bottom of the atmosphere to the top yields the "total column" quantity for that component.

Why are atmospheric BrO and ClO important to measure by satellite?

This is the question raised in the title of the question.

A large number of halogenated hydrocarbons indirectly contribute to ozone depletion. Those halogenated hydrocarbons by themselves are not ozone-depleting substances. But if those compounds reach the upper stratosphere, sunlight can split a halogen atom off of those components. Split-off fluorine atoms tend to bind rather quickly with hydrogen and form an extremely stable hydrogen fluoride molecule. Fluorine barely registers as an ozone-depleting substance because hydrogen fluoride is so stable. Stratospheric hydrogen fluoride eventually diffuses into the troposphere, where it gets dissolved by water and falls as rain.

Split-off chlorine and bromine atoms have a different fate. Hydrogen chloride and hydrogen bromide aren't nearly as stable as is hydrogen fluoride. Chlorine and bromine instead alternate between reservoirs such as HCl and hBr and atomic/oxide forms that catalytically deplete ozone. The key reason it is important to monitor these halogens is that most (about 80%) of the chlorine and almost half (40-50%) of the bromine in the stratosphere is anthropogenic.


Molina, Mario J., et al. "Antarctic stratospheric chemistry of chlorine nitrate, hydrogen chloride, and ice: Release of active chlorine." Science 238.4831 (1987): 1253-1257.

Poulet, Gilles, et al. "Role of the BrO+ HO2 reaction in the stratospheric chemistry of bromine." Geophysical research letters 19.23 (1992): 2305-2308.

Choi, Won Jun, et al. "Introducing the geostationary environment monitoring spectrometer." Journal of Applied Remote Sensing 12.4 (2018): 044005.

Kim, Jhoon, et al. "New era of air quality monitoring from space: Geostationary Environment Monitoring Spectrometer (GEMS)." Bulletin of the American Meteorological Society 2019 (2019): 00.

In addition to the above scientific articles, there are many web pages on stratospheric chemistry. For example, https://personal.ems.psu.edu/~brune/m532/meteo532_ch7_stratospheric_chemistry.htm

  • $\begingroup$ yikes! it's going to take me a few days just to download and/or skim these, but I'm looking forward to it, thanks! $\endgroup$
    – uhoh
    Commented Feb 24, 2020 at 2:41

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