Reactive halogens and their measurements in the troposphere

Transcription

1 Indian Journal of Geo-Marine Sciences Vol.43(9),September 2014,pp General Article Reactive halogens and their measurements in the troposphere Lokesh Kumar Sahu Space and Atmospheric Sciences Division Physical Research Laboratory (PRL), Navrangpura, Ahmedabad , India [ lokesh@prl.res.in; l_okesh@yahoo.com] Received 2 April 2013; revised 4 July 2013 In the earth-ocean-atmosphere system, the halogenated species take part in the cycles of several key processes involving both gas and heterogeneous interactions. The atmospheric cycles of reactive halogens are very complex specifically for those emitted from natural sources. As far as their roles in the tropospheric chemistry, the halogen compounds particularly those containing bromine (Br) and chlorine (Cl) play key roles. The reaction rate constants of many trace gases with halogen radicals are faster than those with hydroxyl radicals (OH). Near the source regions, however, halogen radicals can greatly influence the oxidizing capacity of the troposphere due to their reactive behaviors. In the lower troposphere, particularly in the marine boundary layer (MBL) and polar boundary layer, the reactive halogen compounds cause substantial destruction of ozone. The in-situ observations are available only for very limited geographical regions mainly in the mid- and high- latitudes of the northern hemisphere. One of the reasons for the lack of studies could be the technological constraint owing to very reactive nature of halogens hence the uncertainty in detection and quantification. Nonetheless, it is imperative to study the photochemistry of halogens in global troposphere for the better understanding of chemistry-climate interactions. Many theoretical aspects related to photochemistry of halogenated species in the troposphere need to be verified by the observations. Present study highlighted recent scientific progress about the roles of reactive halogens and their measurements in the troposphere. In spite of greater scientific opportunities in atmospheric studies of halogens, study over Indian subcontinent and surrounding marine regions are almost nil. [Keywords: Halogens, Reaction, Radical, Tropical, India, Ozone, Photochemistry] Introduction Halogenated species in the earth s atmosphere has attracted a great deal of research interest with implications to both gas phase and heterogeneous interactions in the troposphere 1. Halogens are a series of nonmetal elements from group 17 IUPAC (International Union of Pure and Applied Chemistry) (formerly groups of VII or VIIA) in the periodic table comprising fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At) with atomic numbers of 9, 17, 35, 53, and 85, respectively. All halogens exist as diatomic molecule in the natural form with seven electrons in the outer shell. As a result, halogens are highly electronegative and their reactivity increases from astatine to fluorine. In addition to applications in industry and laboratory halogenated products are widely used in daily life. For example, fluorine-containing compounds have been used in making non-stick cookware, processing of uranium (U) nuclear fuel, extraction of aluminum (Al) metal, and manufacturing of low fraction plastics like Teflon. Similarly, chlorinated species have various applications in industry and daily life as well. Chlorinecontaining compounds are often used to make pesticides, antiseptics, disinfectants, hydrochloric acid (HCl), bleaching powder, etc. Most importantly, however, about 85% of medicines include chlorine. Bromine-containing compounds are added in petrol and diesel fuels as anti-knock to make engines vibration or shock free. Other applications of bromine include photographic films, tear gas, halogen lamp, medicines, fire extinguisher, soil fumigants, etc. Halogens particularly fluorine and chlorine were used quite extensively to produce chlorofluorocarbons (CFCs). Primarily CFCs were used as coolants in refrigerator, air conditioner, as propellant and solvents. CFCs are not important (chemically) in the troposphere, firstly because the intensity of ultra violet (UV) radiation which can break CFCs is weak. Secondly, CFCs do not contain carbon-hydrogen (C-H) bond and are not oxidized by the hydroxyl radicals (OH) in the troposphere. On the other hand, CFCs are important (radiatively) as they are the strong absorbers of the earth s outgoing infra

2 1616 INDIAN J MAR SCI VOL 43, NO.9, SEPTEMBER 2014 read (IR) radiation and hence act like greenhouse gases (GHGs) in the troposphere. In spite of their low concentrations, CFCs have pronounced greenhouse effect due to absorption in atmospheric window region (8000 nm nm). In the stratosphere, CFCs are photolyzed by the energetic ultra violet (UV) radiations and release reactive halogen radicals which are known to play an important role in anthropogenic ozone depletion chemistry 2. In the year 1987, the government agencies through the United Nations Environment Program (UNEP) agreed to limit the production and release of a variety of CFCs. Their applications are banned under Montreal protocol provisions due to detrimental impacts in the stratospheric ozone. So far we have discussed the applications and environmental impacts of halogenated species emitted from anthropogenic sources and their roles in the stratospheric ozone depletion and greenhouse effect in the troposphere. However, the earth/ocean system is a significant reservoir of many halogenated species which are more reactive in the atmosphere compared to those emitted from anthropogenic sources. Therefore, halogenated species emitted from natural processes play key roles in the tropospheric chemistry and hardly some species reach to the stratosphere. The objective of this article is to present an overview of halogen-containing compounds and their roles in the tropospheric chemistry. Sources of reactive halogens in the troposphere Major natural and anthropogenic sources of the global tropospheric reactive halogen compounds (RHCs) are listed as here 1. (a) Emissions from oceanic sources: sea salt and decomposition of algae. (b) Emissions from polar surface: fresh sea ice and frost flowers in Arctic and Antarctic. (c) Emissions from continental sources: natural and anthropogenic processes like volcanoes, biomass burning, decay of terrestrial vegetation, use of fossil fuels, industrial application, vehicular exhaust, dust plume, cooling towers, swimming pools, salt lakes, etc. A simplified representation of sources of halogen compounds and their reaction cycles in the troposphere is presented in Fig. 1. Global oceans cover about two third of the earth s surface and are the important source of halogens particularly those of chloride and bromide ions. Therefore, halogen chemistry plays a major role in the marine boundary layer (MBL) chemistry 3. One of the most important sources of chlorine is sea salt spray produced from the interaction of wind at the ocean surface. Fig. 1- A simplified representation of sources of halogen compounds and their reaction cycles in the troposphere. Mechanically, sea spray is produced by the bubble burst mechanism producing film and jet drops. The size of sea salt particles ranges from sub micrometer up to a few micrometers. In Fig. 2, however taken as an example, the concentrations of sea salts showed exponential dependency with the wind speed during the Atmospheric Brown Cloud-East Asian Regional Experiment 2005 (ABC-EAREX 2005). Based on many studies 1, the relation between the total concentration of sea salt and surface wind speed is empirically expressed by following exponential equation. c = d exp (eu 10 ) (1) where c is the concentration of aerosol, d and e are the empirical constants, and u 10 denotes the wind speed at 10 m altitude. As described here, there are three main steps which release inorganic halogens from sea salt. (1) HCl is released from sea salt particles by following acid displacement reaction. H 2 SO 4, g + 2NaCl aq 2HCl g + Na 2 SO 4, g (R1) (2) Oxidants (HOx= HO, HO 2 ) can convert I -, Br - and Cl - ions to Br 2, BrCl, ICl or IBr through following gas phase reaction. HCl + OH Cl + H 2 O (R2) (3) Oxidized nitrogen compounds can react with sea halide ions to release HBr, HCl, and other species like ClNO 2 and BrNO 2. N 2 O 5 + NaCl aq NaNO 3, aq + XNO 2 (R3) Species listed in above reactions (R1-R3) are just taken as examples can be generalized for other halogenated species. Sea salt aerosols were observed to be deficient in chlorine because of acid displacement reaction (R1) in the MBL. Concentration of sodium ion (Na + ) has been used as a reference to estimate the enrichment factor (EF) of halide ions (X - ) 4. EF X = ([X - ]/[Na + ]) air /([X - ]/[Na + ]) seawater (2) In other words, for the fresh sea aerosols the enrichment factor is 1.0. Acid displacement

3 SAHU et al: HALOGENS IN TROPOSPHERE 1617 reactions in the surface of sea salt particles can deplete halide ions resulting in lower values (<1.0) of EF. The depletion is often represented as deficit (deficit = 1- EF). Chlorine is highly enriched in sea salts than other halogens, the molar ratios of [Cl - ]/[Br - ] and [Br - ]/[I - ] in seawaters were estimated to be around 650 and 15000, respectively. On the other hand, these ratios were found to be very variable in snow or ice surfaces. It is worth to mention, that iodine containing compounds are mainly emitted from the coastal seawaters but not as sea salt in which molar ratio of [Cl - ]/[I - ] is of the order of Measurements of some halogen containing salts were conducted during the Indian Ocean Experiment (INDOEX) campaign over the Indian Ocean. In this study, sea salt particles were found to be depleted in bromide with a mean EF Br of around 0.5 in maritime air masses which were not affected by the transport of pollutants from continental sources. On the other hand, however in some samples only, the enrichments of Br (EF Br >1.0) were attributed to the transport of polluted air from cities like Mumbai and Kolkata. Higher values of EF Br were contributed by the traffic exhaust in absence of other significant continental sources of bromine in India. Irrespective of the origin of air masses, the values of EF Cl varied between 0.0 and 1.0 which suggest a general deficit of chloride at all locations over Indian Ocean 4. Na + ( g m -3 ) (a) r 2 = Wind speed (m s -1 ) Cl - ( g m -3 ) 5 (b) r 2 = Wind speed (m s -1 ) Fig. 2- Relations of sea salts with the magnitude of wind observed during the measurement campaign at Jeju Island, Korea, in spring The biological activities involving phytoplankton and seaweeds in the ocean also release many halogen compounds in the atmosphere. In the seawaters, organoiodine compounds are produced by the decomposition of algae 5. Emissions of bromocarbon from algae in the tropical oceans (20 N-20 S) account for about 75% of total oceanic emission 6. Halogen compounds released in the MBL region can be uplifted to the higher heights due to strong convection in the tropical regions. Frost flowers which grow on frozen surfaces are important source of halogens in the tropospshere. The levels of bromide in sea ice could be ~3 times higher than in the sea water. Frost flowers with high salinity on sea ice provide a large surface area for heterogeneous reactions and could be a major source of reactive bromine. The acid catalyzed reaction of HOBr with Br ions in acidic sea salt releases Br 2 or BrCl into the gas phase followed by their photo-dissociation into atomic Br. For example, each HOBr molecule provides two Br atoms into the gas phase in one complete cycle. This simplified heterogeneous autocatalytic mechanism can cause exponential increase of Br radicals in the troposphere. Autocatalytic mechanism releasing bromine in gas phase is also known as the bromine explosion 7,8. HOBr + Br - aq + H + Br 2 + H 2 O (R4) Br 2 + hν Br + Br (λ < 600 nm) (R5) The mechanism of bromine release in volcanic plumes is similar to that of bromine explosion. Halogens are mainly emitted as hydrogen halides (HX, where X =Cl, Br, I, F) in volcanoes. In the first step, the uptake of bromine occurs from gaseous HOBr and HBr into the particles followed by the acid catalysed reaction in the aqueous phase and then release of Br 2 back to the gas phase 9. HOBr g HOBr aq (R6) HBr g Br - + H + aq (R7) HOBr aq + Br - aq + H + aq Br 2,g +H 2 O (R8) Although volcanoes are important source of reactive halogens, but emissions fluxes are highly variable due to extremely sporadic numbers and magnitudes. Nonetheless, annual emission estimates of HCl (4.3 Tg yr -1 ), HF (0.5 Tg yr -1 ), HBr (5-15 Tg yr -1 ), and HI (0.5-2 Tg yr -1 ) from volcanoes might exceed all other sources except from the oceanic emissions 9. Anthropogenic activities like fossil fuel combustion, biomass burning, incineration, industry, some indoor applications, etc. also make significant contribution to the global budget of reactive halogens in the troposphere 10. For example, the global biomass burning accounts for about 20% of CH 3 Cl to its global budget. The concentrations of chlorine in petrol and diesel are usually very small, but the chlorine content is about 0.1% weight in coal. In many developing countries, halogen compounds like ethylene dibromide and ethylene dichloride are added as anti-knock in leaded gasoline. Emissions from coal combustion and incineration plants are among the major anthropogenic sources of HCl. Several industrial processes such as steel plant, pulp and paper manufacturing, etc. are known to be important sources of reactive halogens in the different regions of world. Wind-blown mineral (soil and dust) are important sources of halogens over the land.

4 1618 INDIAN J MAR SCI VOL 43, NO.9, SEPTEMBER 2014 In the troposphere, partitioning of halogen compounds takes place due to various complex processes such as, photochemistry, degradation of organic halogen compounds, oxidation of sea salt, etc. Typically, inorganic halogen species are defined as the sum of reactive halogen species (RHS= X, X 2, XY, XO, OXO, HOX, XNO 2, XNO 3, etc.) and halogen reservoir species (HX, XONO 2, etc.). Where X and Y denotes a halogen atom (Cl, Br and I), usually F is excluded as it is efficiently converted to non-reactive hydrofluoric acid (HF) in the troposphere. Importance of halogen chemistry in the troposphere The reaction rate constants of several trace gases with OH, Cl, and Br radicals are presented in Table 1 for the comparison. As can be noticed for many key species the reaction rates of key atmospheric species with halogen radicals are faster than their reactions with OH. In any case, the halogen radicals take part in complex chemical cycles involving both gas and heterogeneous reactions in the troposphere. One of the most important roles of halogen chemistry is the catalytic destruction of ozone in the lower troposphere. Halogen radicals formed by the photo-dissociation of X 2, XY, HOX, XONO 2, XNO 2, etc. react with ozone. Based on the field and model studies 11, the two reaction cycles known to cause catalytic destruction of ozone in the troposphere are given as here. Reaction cycle I XO + YO X + Y + O 2 (R9) X + O 3 XO + O 2 (R10) Y + O 3 YO + O 2 (R11) Net: O 3 + O 3 3O 2 (R12) Reaction cycle II XO + HO 2 HOX + O 2 HOX + hν X + OH X + O 3 XO + O 2 OH + CO H + CO 2 (+M) HO 2 Net: O 3 + CO + hν CO 2 + O 2 (R13) (R14) (R15) (R16) (R17) The reaction cycle (I) has been identified as the major cause for ozone destruction in the polar boundary layer. Whereas reaction cycle (II) dominates in lower halogen level regimes such as MBL over the oceanic regions. In following subsections we have presented a brief discussion about the roles of halogen radicals in different regions. A comprehensive discussion of their roles in various chemical cycles leading to environmental and climatic impacts is beyond the scope of this paper. Ozone depletion in the polar region During 1980s, the researchers observed almost complete destruction of ozone in the Arctic boundary layer during late winter and early spring seasons 12,13. These episodes lasting for a few days are known as ozone depletion events (ODEs). During the ODEs, the mixing ratios of ozone can drop from the background values of about 30 ppbv to below detection level of the instruments. ODEs are triggered by the photolysis of halogenated species, the same mechanism which is responsible for the destruction of ozone in the stratosphere. Destruction of ozone is driven primarily by the bromine radicals as the concentrations of filterable bromine (f-br) were observed to be enhanced during the ODEs. ODEs are quite often associated with low wind and stable atmospheric conditions. In the troposphere, the vertical profile of ozone depletion varies with the height of inversion layer. Inversion typically ranges from 10 m to 1000 m in the polar boundary layer and may limit the dispersion of reactive halogens. On the other hand, the elevated layers of ozone-poor air over Antarctica have been attributed to the upward transport of depleted air masses from the ice surface. A key depleting agent BrO is often located close to the surface but some evidences of BrO layers at higher altitude have also been observed. Halogen chemistry in the MBL In the MBL, the chemical reactions contributing to the loss of ozone in the absence of halogens can be summarized as follows. O 3 + hν O 2 + O( 1 D) (R18) O( 1 D) + H 2 O 2OH (R19) OH + O 3 HO 2 + O 2 (R20) HO 2 + O 3 OH + 2O 2 (R21) Reactions R18-R19 leading to destruction of ozone is conventional HOx chemistry. However, the observed loss of ozone in MBL could not be fully accounted by the photochemical reactions led by O( 1 D) and HOx in different regions. Recently, significant progress has been made in the field observation and model studies to understand the roles of halogen radicals in MBL. It has been highlighted that the activated halogens from seas salt particles play important roles in the MBL chemistry. Outflow of relatively long lived halogen compounds from continental sources can also be significant sources in the coastal MBL. Therefore, in many simulation based studies the halogen chemistry has been successfully invoked to account for the additional loss of ozone in the remote MBL. For example, the field observations over the tropical Indian Ocean and Atlantic Ocean have suggested that the reactive halogen species

5 SAHU et al: HALOGENS IN TROPOSPHERE 1619 released from the marine surface contribute to ozone destruction 14, 15. During the pre-indoex campaign, Feb- Mar 1995, the mixing ratio of ozone showed large diurnal variation of about 32% with respect to the mean ozone level over the tropical Indian Ocean 14. Photochemical box model calculation using HOx chemistry accounted only for about 12% of diurnal variation. On the other hand, the destruction of ozone due to halogen radicals accounted for about 22% of the variation. Similarly, the annual measurements of halogen oxides (IO and BrO) show significant diurnal and seasonal variations in the MBL at the Cape Verde observatory in the tropical Atlantic Ocean during October 2006 to October Box model calculations have shown that the observed diurnal variation of halogen radicals with maxima at daytime and minima at night play important role in controlling ozone variation. For a particular study at Cape Verde, the model underestimated the ozone loss by about 47% if halogens were excluded in the box model calculations. Implementation of oceanic halogen sources in chemistry and climate models is important to estimate the preindustrial levels and also the long term changes in tropospheric ozone 16. Iodine containing compounds react rapidly with ozone and contribute to new particle formation in the coastal MBL. The estimation of the loss of ozone due to halogen chemistry has been a major challenge due to lack of observations in the global MBL. In a different perspective, the laboratory studies have shown that the reactions with halogen oxide radicals, especially IO, are important in the oxidation of dimethylsulfide (DMS). Halogen chemistry in polluted region Typically, the conversion of inorganic halogens into gaseous halogens is considered to take place mainly in the oceanic and polar regions. However, recent studies have suggested that a substantial fraction of tropospheric chlorine over the continental locations may come from anthropogenic sources 17. In the polluted troposphere, the night-time chemistry of involving oxides of nitrogen converts inorganic chloride into reactive forms. The production of HNO 3 takes place mainly by heterogeneous reactions of N 2 O 5 under lower temperature and high NOx (=NO+NO 2 ) conditions during the night hours. N 2 O 5, g + H 2 O aq 2HNO 3, aq (R22) - Alternatively, nitryl chloride (ClNO 2 ) and NO 3 are produced by following reaction. N 2 O 5,g + Cl - - aq ClNO 2, g + NO 3 aq (R23) The ClNO 2 produced in above reaction accumulates in the night and later photodissociated in the presence of sunlight yielding NO 2 and reactive Cl radical. ClNO 2, g + hν NO 2, g + Cl g (R24) The production rate of Cl can be sufficient enough to affect the regional photochemistry leading to production of ozone. The mechanism is similar to that initiated by the oxidation of non-methane hydrocarbons (R-H) or volatile organic compounds (VOCs) by OH radicals in the presence of NOx and sunlight 18, 19. R-H + Cl HCl + R (R25) R + O 2 + M RO 2 + M (R26) RO 2 + NO NO 2 (R27) 2(NO 2 + hν + O 2 NO + O 3 ) (R28) Long-range transport of pollutants mainly NOx and its reservoir species can affect halogen activation and photochemistry of remote troposphere. For example, the Indian Ocean and Pacific Ocean are significant source of reactive hydrocarbons where halogen activation can impact the local photochemistry related to ozone in the MBL 20, 21. Oxidation of mercury (Hg) Mercury (Hg), emitted from both natural and anthropogenic sources, is a global environmental pollutant with significant adverse effects on ecosystems and human health. The major anthropogenic sources of mercury include coalfired plants, medical practices, fluorescent light, thermometer, switches, etc. In the atmosphere, the gaseous form of mercury (Hg 0 ) also known as elemental mercury accounts for about 70% of emission from the anthropogenic sources. The oxidized mercury, referred as, Reactive Gaseous Mercury (RGM), is rapidly scavenged from the atmosphere. The Hg(II) is a most common oxidation state, however, the mechanism of oxidation is not fully explained but believed to be caused by both gaseous and aqueous phase reactions in the atmosphere. The recent measurement and model studies have suggested that the bromine oxides in the sea salt can oxidize Hg 0. In several studies, following reactions mainly involving bromine radicals have been suggested for the oxidation of Hg 0. Hg 0 + Br HgBr (R30) HgBr + X HgBrX (R31) The oxidized mercury is soluble in water and can finally reach to deep sediments. The atmospheric deposition is the principal source of mercury in the ocean water. The increased level of mercury in the surface ocean water and accumulation in the marine biota is a cause of concern. Therefore, the deposition rate of Hg may be increased by the presence of halogen radicals 11.

6 1620 INDIAN J MAR SCI VOL 43, NO.9, SEPTEMBER 2014 Table 1 The reaction rate constants (k at 298 K) of some important hydrocarbons with OH, Cl, and Br radicals 30. Compound k OH k Cl k Br Methane Ethane Ethene Ethyne Propane Benzene k OH, Cl, Br [cm -3 molecule -1 s -1 ] Measurement methods of reactive halogens in troposphere Despite significant progress in understanding about the roles of halogen species in tropospheric chemistry their measurements remained very limited. As halogen radicals are short-lived species with low atmospheric abundance their accurate and reliable detection is quite challenging. Recently, various techniques which were mostly used for the laboratory experiments have been modified and successfully used to detect the halogen species in the troposphere 3. There is a growing demand for the observational data of reactive halogens which play important roles in tropospheric photochemistry. A brief summary of rather recent methods used to measure various halogen radicals or halogencontaining gases in the troposphere have been presented here. (a)mist Chamber Techniques: A tandem mist chamber technique has been used to sample inorganic chlorine gases viz. HCl (HCl, NOCl, ClNO 2, and ClNO 3 ) and Cl 2 (Cl 2 and HOCl) in ambient air 22. In this method, air samples pass through the chambers containing acidic (ph > 1) and alkaline solutions to collect HCl and Cl 2 species, respectively. Subsequently, the ion chromatography (IC) technique is used to analyze Cl - ions collected in the mist chambers. Although the filter or adsorbent based techniques are widely used for the analysis of gaseous halogens, but the data may be subjected to artifacts mainly caused by the phase transformations (gas to particle conversion). (b) Atmospheric Pressure Ionization Mass Spectrometry (API-MS): The atmospheric measurements of dihalogen compounds (X 2 and XY) in air can be achieved by the API-MS technique. In this method, the air sample is - introduced into the discharge region where O 2 ions transfer an electron to Cl 2, Br 2, and I 2 as they have higher electron affinities compared to O - 2. Finally, the selected molecular ions are detected by two stages of mass spectrometry (MS). This technique was used for the first time measurements of Br 2 and BrCl during the ODEs in the Arctic region 23. (c) Chemical Ionization Mass Spectrometry (CIMS): In recent years, ionizations based methods for the measurements of BrO, Br 2, Cl 2, ClNO 2, etc. have been developed and used by several researchers 24. The detection of X 2 in ambient air is based on its ionization using radioactive sources like 63 Ni to produce ions X 2 - ions 25. Several investigators have also utilized I - as a reagent for the simultaneous measurements of HOBr, BrO, Br 2, ClNO 2, and Cl 2. (d) Differential Optical Absorption Spectroscopy (DOAS): In recent years, the detection of halogen radicals using the DOAS has become very popular. DOAS technique utilizes UV and visible spectra of atmospheric species that have a banded absorption structure. Identification and quantification of trace gases are based on their narrow band (< 5 nm) optical absorption feature in the atmosphere. Typically, the light source in DOAS can be artificial lamp (active) or natural (passive) such as scattered sunlight. Active DOAS has been successfully used for the ground based measurements of ClO, BrO, IO, OIO, and I 2, while the passive DOAS system is used for balloon and satellite based measurements. For example, sseveral satellite based instruments namely the Global Ozone Monitoring Experiment (GOME), Scanning Imaging Absorption Spectrometer for Atmospheric CartograpHY (SCIAMACHY) and Ozone Monitoring Instrument (OMI) use DOAS method for tropopsheric observations of halogen radicals. From the measured raw data, slant column densities (SCD) of the absorbing trace gases are calculated. The calculation of the vertical column density (VCD, or vertically integrated concentration) using measured SCD need radiative transport modelling to derive air mass factor (AMF = SCD/VDC) 26. A modified system known as Multi-Axis DOAS (MAX-DOAS) utilizes scattered sunlight from several directions between zenith and horizon using a telescope. The vertical profiles of halogen oxides in the boundary layer can be retrieved from the MAX-DOAS as it has the capability to discriminate between tropospheric and stratospheric absorbers. The DOAS techniques for the field observations of RHCs have been widely used 27. (e) Cavity Ring-Down Spectroscopy (CRDS): This is one of the highly sensitive absorption spectroscopy techniques which is based on the Beer Lambert law. CRDS consists of a laser that is used to illuminate optical cavity having greater

7 SAHU et al: HALOGENS IN TROPOSPHERE 1621 reflectivity in which the intensity increases in the cavity due to constructive interference. Moment laser is turned off the intensity of light decays exponentially in the absence of absorbing species inside the cavity. If the cavity is filled with light absorbing species, the intensity of light decreases faster than that without absorption. Time required for the intensity to decay by 1/e of its initial value is called "ring down time". The calculated ring down time can be used to estimate the concentration of absorbing species in the cavity. However, to obtain the mixing ratios of trace gases accurate absorption cross sections at selected wavelengths are required. An open-path CRDS has been used for the measurements of iodine monoxide (IO) radicals in the field 28. (f) Resonance Fluorescence (RF): This technique combined with chemical conversion has been used for the measurements of halogen oxides. Nitric oxide (NO) is mixed to the to the air sample which converts halogen oxides to atomic halogen (XO + NO X + NO 2 ). The resulting halogen atoms are detected by the RF in the vacuum at 131 nm for Br and 119 nm for Cl. A modified sampling system with low internal pressure to minimize the effects of quenching and absorption by water vapor and oxygen is developed for the tropospheric measurements. Recently, a vacuum UV RF system has been developed to detect atomic iodine at around nm 29. Fluorescence-based techniques have been applied for the measurements of atomic iodine and IO radicals. The technique provides high sensitive and time resolved measurements. Summary Halogen radicals, however found in pptv levels, play important roles in the photochemistry of the troposphere. Several observation and model based studies have highlighted the importance of reactive halogen chemistry in the different regions of the troposphere. Halogen compounds are emitted mostly from the natural sources and take part in cycles of gas phase and heterogeneous reactions before being released in to the reactive forms. Relevance of halogen chemistry has been found to be significant in both remote and polluted environments. Measurements of ractive halogens have been reported mostly for the mid and high latitude regions, but their measurements in the tropical regions are rare. One of the most important impacts of halogen radical is the destruction of ozone in the boundary layer. On the other hand, halogens can also play an important role in the oxidations of DMS and mercury. Measurements of ozone and precursor gases have been conducted over different locations in India and surrounding marine regions under different projects. The major objectives of these programs were mainly to assess the role of transport from different sources regions and boundary layer dynamics. In this region, the studies investigating the roles of various competing photochemical processes related to tropospheric ozone in both continental and marine regions are rare. Therefore, it is essential that the roles of halogen radicals in photochemistry be investigated by the Indian researchers. There were several challenges for the accurate detection of halogens radical in air, but recent technological progress has made the task easier. The implementation of halogen in the chemistry and climate models is important to estimate the long term changes in tropospheric ozone and climate change. Acknowledgements Author thanks the members of Space and Atmospheric Sciences (SPA-SC) Division, Physical Research Laboratory (PRL), Ahmedabad, India for their constructive suggestions. References 1 von Glasow, R & Crutzen, P J, Tropospheric halogen chemistry, in: Treatise on Geochemistry, edited by H.D. Holland, K.K. Turekian ((Elsevier- Pergamon, Oxford) 2007, pp Molina, M. 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