Abstract
Chlorine radicals can function as a strong atmospheric oxidant1,2,3, particularly in polar regions, where levels of hydroxyl radicals are low. In the atmosphere, chlorine radicals expedite the degradation of methane4,5,6 and tropospheric ozone4,7, and the oxidation of mercury to more toxic forms3. Here we present direct measurements of molecular chlorine levels in the Arctic marine boundary layer in Barrow, Alaska, collected in the spring of 2009 over a six-week period using chemical ionization mass spectrometry. We report high levels of molecular chlorine, of up to 400 pptv. Concentrations peaked in the early morning and late afternoon, and fell to near-zero levels at night. Average daytime molecular chlorine levels were correlated with ozone concentrations, suggesting that sunlight and ozone are required for molecular chlorine formation. Using a time-dependent box model, we estimate that the chlorine radicals produced from the photolysis of molecular chlorine oxidized more methane than hydroxyl radicals, on average, and enhanced the abundance of short-lived peroxy radicals. Elevated hydroperoxyl radical levels, in turn, promoted the formation of hypobromous acid, which catalyses mercury oxidation and the breakdown of tropospheric ozone. We therefore suggest that molecular chlorine exerts a significant effect on the atmospheric chemistry of the Arctic.
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Acknowledgements
This work is part of the international multidisciplinary OASIS program and is financially supported by NSF grants ATM-0807702, ARC-0806437 and ARC-0732556. We thank the OASIS campaign organizers and the National Center for Atmospheric Research shipping department for logistical support. We also thank J. Fast and A. Stohl for making the FLEXPART-WRF code public (http://transport.nilu.no/flexpart). The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under the sponsorship of the National Science Foundation.
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H.J.B., P.B.S., F.M.F. and J.J.O. planned and organized the project. J.L., D.J.T. and L.G.H. conducted measurements of halogen species by CIMS at Barrow. J.L. analysed and modelled the data, performed laboratory tests and wrote the manuscript. L.G.H. revised the manuscript. Z.L. and Y.W. calculated FLEXPART back trajectories. C.A.C. and R.S.H. performed HO2 and RO2 measurements. P.B.S. and C.R.S. performed ClOx measurements. A.J.W. performed O3 and NO measurements. S.R.H. and K.U. performed photolysis rate measurements. H.J.B. measured chloride concentrations in the snow. E.C.A., D.R. and A.F. performed VOC measurements. R.L.M. performed OH measurements. J.N.S. measured aerosol size distribution. R.M.S performed ozonesonde measurements. All authors reviewed and commented on the paper.
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Liao, J., Huey, L., Liu, Z. et al. High levels of molecular chlorine in the Arctic atmosphere. Nature Geosci 7, 91–94 (2014). https://doi.org/10.1038/ngeo2046
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DOI: https://doi.org/10.1038/ngeo2046
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