Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean
Authors: Michael J Pilling
John M C Plane
Sarah J Moller
Alastair C Lewis
Lucy J Carpenter
James D Lee
James R Hopkins
Anoop S Mahajan
Luis Mendes
Bruno V E Faria
Katie A Read
Hilke Oetjen
James B McQuaid
Dwayne E Heard
Alfonso Saiz-Lopez
Mathew J Evans
Journal: Nature (2008)
Discipline: Multidisciplinary
Keywords: tropospheric ozone; climate perturbation; precursor emissions; destruction processes; tropical marine boundary layer
Affiliations: Department of Chemistry, University of York, Heslington, York YO10 5DD, UK.

Abstract:  Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation; the prediction of future trends in tropospheric ozone will require a full understanding of both its precursor emissions and its destruction processes. A large proportion of tropospheric ozone loss occurs in the tropical marine boundary layer and is thought to be driven primarily by high ozone photolysis rates in the presence of high concentrations of water vapour. A further reduction in the tropospheric ozone burden through bromine and iodine emitted from open-ocean marine sources has been postulated by numerical models, but thus far has not been verified by observations. Here we report eight months of spectroscopic measurements at the Cape Verde Observatory indicative of the ubiquitous daytime presence of bromine monoxide and iodine monoxide in the tropical marine boundary layer. A year-round data set of co-located in situ surface trace gas measurements made in conjunction with low-level aircraft observations shows that the mean daily observed ozone loss is approximately 50 per cent greater than that simulated by a global chemistry model using a classical photochemistry scheme that excludes halogen chemistry. We perform box model calculations that indicate that the observed halogen concentrations induce the extra ozone loss required for the models to match observations. Our results show that halogen chemistry has a significant and extensive influence on photochemical ozone loss in the tropical Atlantic Ocean boundary layer. The omission of halogen sources and their chemistry in atmospheric models may lead to significant errors in calculations of global ozone budgets, tropospheric oxidizing capacity and methane oxidation rates, both historically and in the future. 

Retracted: No

PubPeer comments:
 Link to paper: https: www.nature.com articles nature07035 The authors report several months of spectroscopic measurements of halogen oxide radicals (BrO and IO) using the LP-DOAS technique at the Cape Verde Atmospheric Observatory (Republic of Cabo Verde). Mean daytime maximum mixing ratios are in the pptv range: 1,4u00b10,8(1u03c3) pptv for IO and 2,5u00b11,1(1u03c3) pptv for BrO. Following further modelling work, these findings are claimed to support that halogen chemistry is a significant tropospheric ozone-depleting pathway in the North Atlantic Ocean. Supplementary Figure 3 shows two typical spectral fits obtained during the experimental campaign leading to retrievals of IO and BrO at Cabo Verde, respectively. ![file] However, **the** **BrO** **fit** is **duplicated** from a previous Science paper (by three of the same authors) reporting unprecedented LP-DOAS retrievals of IO and BrO at Halley Station (Antarctica). *(Saiz-Lopez, A., Mahajan, A.S., Salmon, R.A., Bauguitte, S.J.-., Jones, A.E., Roscoe, H.K. Plane, J.M.C. 2007,  Boundary layer halogens in coastal Antarctica , Science, vol. 317, no. 5836, pp. 348-351. https: science.sciencemag.org content 317 5836 348)* ![file] The BrO fit shown in the Nature paper extends over a wider wavelength range (320-340 nm vs 324-338 nm range in the Antarctica Science Paper). Complete match between both spectral fits is more obvious if, for instance, Nature plot is resized in order to get the same scaleu00b4s length than the fit found in the Science paper. ![file] Under the LP-DOAS retrieval algorithm, it is impossible to get the exactly same shape fit from two different raw spectra. It is also remarkable that the duplicated fits differ in the magnitude of the optical density (Y-axis): exactly one order smaller in the Nature paper than in the Science one (up to $10^{-4}$ vs up to $10^{-3}$), in agreement with the respective mixing ratios range retrieved from measurements at Cabo Verde (pptv units) and at Antarctica (up to the 10 pptv range). In concrete, claimed mixing ratio for the duplicated plot in Antarctica Science paper is (18,8 u00b1 1) pptv. In other words, in each paper the chosen optical density magnitude (for the same fit) suits the range of its respective retrieved mixing ratios. Concluding, since the duplication is clear, at least one of the two papers wrongly shows the figure as a typical spectral fit derived from its respective atmospheric measurements (carried out at different locations). Which campaign paper (Cabo Verde Nature or Antarctica Science), if any, does the duplicated plot really correspond to? Since the Science paper was published around one year before the Nature paper, it is primarily the authors of the latter who should clarify the duplication. Providing some labelled raw spectra (and not only the targeted fits obtained after spectral processing) would help in order to solve this spectroscopic inconsistency u201cNature vs Scienceu201d resulting from the duplication. , 
 The author of the above comment has queried the validity of the spectral fits from two papers and suggested that the fits are the exact same, with optical density exactly one order smaller in the Nature paper (Read et al., 2008) than in the Science paper (Saiz-Lopez et al., 2007), which show data from two different field campaigns from two different geographical locations (Cape Verde and Halley Bay, Antarctica), albeit with the same instrumental setup. This comment is unfortunately a good example of why only visual inspection, instead of analysing the data, can be misleading. Although the fits look similar with a factor difference (closer to 9 than 10 as mentioned by the author of the comment), it would be highly unexpected for this not to be the case since the spectral fitting follows the same procedure and the instruments are the same, leading to similar noise stray light effects. However, upon closer inspection, the differences between the spectra are clear. Below is a figure showing the actual ratio of the BrO fit from Halley Bay (Saiz-Lopez et al., 2007) to the BrO fit from Cape Verde (Read et al., 2008). It can be seen clearly that the difference is variable, and depends on the small differences of in noise in the spectra. We have attached the full window from the DOAS spectrum for the author of the comment to view rather than only the fitting window. Read, K.A., Mahajan, A.S., Carpenter, L.J., Evans, M.J., Faria, B.V.E., Heard, D.E., Hopkins, J.R., Lee, J.D., Moller, S.J., Lewis, A.C., Mendes, L.M., McQuaid, J.B., Oetjen, H., Saiz-Lopez, A., Pilling, M.J., Plane, J.M.C., 2008. Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean. Nature 453, 1232u20131235. https: doi.org 10.1038 nature07035 Saiz-Lopez, A., Mahajan, A.S., Salmon, R.A., Bauguitte, S.J.-B., Jones, A.E., Roscoe, H.K., Plane, J.M.C., 2007. Boundary Layer Halogens in Coastal Antarctica. Science (80-. ). 317, 348u2013351. https: doi.org 10.1126 science.1141408 ![file] , 
 Despite that the reported duplication would not need any further discussion (that is, that BrO spectral fits found in both Nature and Science papers are the same figure with a change in their y-axis of exactly one order of magnitude), some additional comments are provided in response to the authoru00b4s feedback. First, the plot provided by Dr. Mahajan is not in any case a proof of no duplication, for the following reasons: i)tThe plot is claimed to be the ratio between the optical densities (y-axis values) and not absolute ones. ii)tThe duplicated fits extend over the 324-338 nm (Science) and 320-340 nm (Nature) ranges, then there is no way to validate the claimed ratio values out of the 324-328 nm (that is, the range where the optical density for both fits is graphically available for pubpeer reviewers). iii)tAccording to the variability observed in the claimed ratios for 324-338 nm, differences between Science and Nature spectral fits should be detectable upon a visual inspection made under the available resolution level. That is not the case. ![file] Second, Dr. Mahajan claims in his feedback that the identity between fits *u201cwould be highly unexpected for this not to be the case since the spectral fitting follows the same procedure and the instruments are the same, leading to similar noise stray light effectsu201d* An explanation based on the DOAS algorithm could be provided in order to show that two different raw spectra cannot lead to such similar spectral fits, especially if we are comparing spectra from different experimental campaigns (different locations and light paths) with different atmospheric conditions (polar vs tropical), each of them leading to significantly different BrO mixing ratios (18,8 pptv for Science paper and an undetermined quantity up to pptv units for Nature paper). However, a graphical proof may be more helpful within this context. In fact, authoru00b4s own bibliography contradict his statement. One of his papers reported LP-DOAS measurements of BrO at the Britanny coast (France). A figure is provided showing the the fit procedure starting from a raw spectrum following the DOAS algorithm. The Breton spectral fit in the 324-338 nm region (leading to a BrO mixing ratio of 4,7 pptv) is clearly different than the other two, regardless if it is zoomed or resized. ![file] Finally, reasons why the authors still deny (if so) that the fits are duplicated in both Nature and Science papers are not scientifically funded so they are out of the scope of my comments. They should be rather a matter of concern for Nature and Science editors. , 
 Dear Tutufa Tenuigranosa, Apologies for the late reply on this comment. After your second comment, I went through the data in detail. The first comment was posted when I was at sea and had only the final figure data with me and hence the analysis was hastily done. After returning from the ship-based campaign, I went through the raw data and as you had indicated it did point to a mistake in the figure in Nature. We have since been in touch with the journal and sent them the correct data which has undergone another round of peer review - a corrigendum to the supplementary text has been suggested. We sincerely thank you for identifying this mistake and have taken steps to correct it. If you have any further queries, I welcome you to contact me directly. Best wishes, Anoop , 
 As a postscript to the reply above, I want to emphasise that the BrO spectrum in the Nature Supplementary Material was not a duplicate of that published in the Science paper. The spectrum that was published in Science was modified to produce a training spectrum while testing the DOAS retrieval software at the low levels of IO expected at a mid-ocean site. Unfortunately, the training spectrum was then inadvertently inserted into Figure S3 in the Nature supplementary material, alongside an IO spectrum from the Cape Verde campaign. ,