Reactive Chlorine Compounds in the Atmosphere

CHAPTER 1 Reactive Bromine Compounds O.N.Singh 1 · P.Fabian 2 1 Department of Applied Physics, Institute of Technology, Banaras Hindu University, Varanasi- 221 005, India. E-mail: [email protected] 2 University of Munich, Lehrstuhl für Bioklimatologie und Immissionsforschung, Am Hochanger 13, D-85354 Freising-Weihenstephan, Germany. E-mail: [email protected] Bromine, a minor constituent in the Earth’s atmosphere – with its 50-fold higher efficiency of ozone destruction compared to chlorine – contributes significantly to the ozone hole formation and wintertime stratospheric ozone depletion over northern mid and high latitudes.In addition ozone episodes observed in the Arctic during polar sunrise are solely due to atmospheric bromine.CH3Br, CH2Br2 and CHBr3 are the major brominated gases in the atmosphere, of which CH3Br being most abundant, contributes about 50% and CH2Br2 around 7 to 10% of the total organic stratospheric bromine.Bromocarbons with shorter lifetimes like CHBr3 ,CH2BrCl, CHBr2Cl, CHBrCl2 and CH2BrI decompose before reaching the stratosphere, and are responsible for the ozone episodes.But for 3CHBr, which has also significant anthropogenic sources, all the aforementioned bromocarbons are mostly of marine origin.Halons (H-1211, H-1301, H-2402, H-1202) are solely anthropogenic and are far more stable.They decompose only after reaching the stratosphere.It is estimated that 39% of the stratospheric organic bromine (ª 7 pptv) loading is due to these halons.Increa- ses are being still registered in the atmospheric abundance of halons in spite of production restrictions.Though extensively investigated,the existing knowledge with regard to the production and degradation of atmospheric bromine gases, is not commensurate with its importance. Keywords: Bromine chemistry, bromocarbons, ozone depletion, ozone episode, ozone hole. 1Introduction . 2 2 Methyl Bromide . 3 2.1 Introduction . 3 2.2 Measurements . 4 2.3 Sources . 6 2.3.1 Oceanic Sources . 6 2.3.2 Agricultural and Allied Usage . 8 2.3.3 BiomassBurning . 10 2.3.4 Gasoline Additives and Other Sources . 12 2.4 Sinks . 13 2.4.1 Atmospheric Removal . 13 2.4.2 SurfaceDeposition . 14 2.4.3 Oceanic Removal . 15 2.5 Lifetime and Ozone Depletion Potential . 17 The Handbook of Environmental Chemistry Vol.4 Part E Reactive Halogen Compounds in the Atmosphere (ed.by P.Fabian and O.N.Singh) © Springer-Verlag Berlin Heidelberg 1999 2 O.N.Singh · P.Fabian 3 Other Bromocarbons in the Earth’s Atmosphere . 18 3.1 Introduction . 18 3.2 Sources and Measurements . 20 3.3 Arctic Surface Ozone Hole . 24 4 Tropospheric Impact . 31 5 Halons . 33 6 Stratospheric Bromine Budget and Chemistry . 34 7 Conclusions . 39 8 References . 40 1 Introduction Bromine is a minor constituent in the Earth’s atmosphere.But for methyl bromide (CH3Br) with an atmospheric mixing ratio of about 10 pptv (parts per tril- lion by volume), all other bromine bearing gases are only at a few pptv levels [1]. Around 7 pptv as bromine in the atmosphere comes from anthropogenic halons and 2 to 6 pptv from other naturally occurring bromine compounds [2].Despite this low abundance, the bromine species play a vital role in our atmosphere. The discovery of extraordinary ozone depletion in the Antarctic lower stratosphere [3, 4] and a little later in the Arctic lower troposphere [5, 6], popu- larly known as ozone hole and ozone episodes (also called surface ozone hole), have generated wide interest in the bromine chemistry, particularly in polar environments [7, 8].Bromine per atom is about 50-fold more efficient than chlorine in converting ozone to oxygen.20 to 25% of the austral springtime stratospheric ozone depletion over Antarctica, about one third of the wintertime stratospheric ozone depletion at northern mid and high latitudes, and almost 100% surface ozone destruction observed in the Arctic after polar sunrise are likely to be due to bromine.During the last two decades issues like sources, sinks, abundance and profiles, various chemical pathways and mecha- nisms etc.related to atmospheric bromine compounds and bromine have been vigorously addressed, and enormous amounts of monitoring and research work – experimental (observation and laboratory), theoretical and modelling – have been carried out and are still being continued. Like most chlorofluorocarbons (CFCs), bromocarbons as such have high global warming potentials (GWP).But the recent comprehensive studies taking into account their share of stratospheric ozone destruction,predict a net cooling or considerably reduced radiative forcing of the Earth’s surface due to further additions of bromocarbons.It may be pointed out that halons and methyl bromide with high GWP considered in the 1980s as double greenhouse gases,are now expected to slow down the pace of the anthropogenic greenhouse effect [9]. 1 Reactive Bromine Compounds 3 An attempt to review the current status of bromine species and related chemistry in our atmosphere is being made in this chapter. 2 Methyl Bromide 2.1 Introduction Methyl bromide, the largest reservoir of atmospheric bromine, is likely to con- tribute at least 50% of the organic bromine budget to the Earth’s atmosphere.It has both natural (ocean, wild fires) as well as anthropogenic (industrial production for agricultural and other kind of fumigation, biomass burning, gasoline additives) sources.Since halons, the other major bromine bearing substances, are being phased-out (under the regulations set out by the Montreal Protocol), methyl bromide has gained added importance.This is amply demonstrated by the amount of interest it has generated in the recent past – both in science and politics [1, 2, 9–35].WMO, in its report no.25 [12] states that if the global CH3Br abundance was to be reduced by 10%, the ozone layer protection would be achieved approximately equivalent to an advance of the CFC phase out schedule by three years; and in the UNEP,1992 report [16] it has been claimed that elimi- nation of anthropogenic methyl bromide (estimated atmospheric abundance about 3 pptv) will provide an ozone layer protection comparable to that of an advance of the CFC and CCl4 phase out schedule by 1.5–3 years. In accordance with the Montreal Protocol, its production for consumption in the developed countries has been capped at the 1991 level since the beginning of the year 1995 [1].The US Environmental Protection Agency (US EPA) has already announced its complete phase-out by the year 2001 in the US [1] and global controls were established in 1995 to phase it out by the year 2010 [34].Methyl bromide is also regulated domestically in a number of countries.The Netherlands phased out the use of methyl bromide for soil fumigation in 1992 because of ground water contamination concerns.Denmark and Nordic countries will ban agricultural use of methyl bromide by 1998, and Sweden is expected to follow a similar schedule.The European Union and Canada will cut agricultural use by 25% in 1998.A number of other countries are also contemplating regulatory action for methyl bromide use and production [34]. Since the atmospheric lifetime of CH3Br is now estimated to be around one year (best estimate 0.7 year) [2, 35a], cessation of its emissions should produce prompt results.The benefit of such action could be realised in a few years, whereas for the longer-lived CFCs, it would take until the year 2050 to reduce concentrations to what they were before the ozone hole developed.It will take centuries to reduce the atmospheric burden of the CFCs by a factor of 1,000; the atmospheric burden of anthropogenic CH3Br could be reduced by that much within less than a decade [17]. Two extended and thorough reviews, under the auspices of UNEP [16] and WMO [1] respectively, have covered almost all aspects of atmospheric methyl bromide and have addressed to the uncertainties in its sources, sinks, budget, 4 O.N.Singh · P.Fabian hemispheric distribution, lifetime and ozone depletion potential (ODP).At a cursory glance, these seem to present a complete and comprehensive descrip- tion leaving no scope or need for further review.But the large amount of data generated and results published in the meantime and the investigations cur- rently in progress, indicate that our understanding with regard to the various above mentioned uncertainties is rather sparse, and we find ourselves back to square number one. Nevertheless, these two well written articles [1, 16] will be treated as the basis and will be extensively referred to.Much remains unclear about the origins and the impact of CH3Br.Special sessions of two meetings: (i) Production and Fate of Atmospheric Organic Halogen in the Marine Environment,Spring Meeting of the AGU, Baltimore, Maryland 30 May-2 June 1995; and (ii) 1995 Methyl Bromide State of Science Workshop Monterey, California, 5–7 June 1995, were aimed to iron out some of the uncertainties [17].Later, during 13–16 November 1995, at Hradec Kralove in the WMO Meeting “Consultation of Experts on Reactive Halogen Compounds and their Possible Effect on Ozone” these aspects were again discussed [18]. 2.2 Measurements Methyl bromide is an ubiquitous component of our lower atmosphere.Over the past two decades, sporadic measurements of surface abundance of methyl bromide have been carried out,and only a few measurements from the free troposphere or stratosphere, using balloon or aircraft platforms, are available [1, 19, 32].A body of data from both hemispheres have been collected by several campaigns.The mean volume mixing ratios of methyl bromide in

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