Indian Journal of Geo-Marine Sciences
Vol.43(9),September 2014,pp. 1615-1622
General Article
Reactive halogens and their measurements in the troposphere
Lokesh Kumar Sahu
Space and Atmospheric Sciences Division
Physical Research Laboratory (PRL), Navrangpura, Ahmedabad - 380009, India
[E-mail: [email protected]; [email protected]]
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 troposphere1. 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
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-12000 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 chemistry2. 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 here1.
(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)
chemistry3. 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 studies1,
the relation between the total concentration of sea
salt and surface wind speed is empirically
expressed by following exponential equation.
c = d exp (eu10)
(1)
where c is the concentration of aerosol, d and e
are the empirical constants, and u10 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.
(R1)
H2SO4, g + 2NaClaq → 2HClg + Na2SO4, g
(2) Oxidants (HOx= HO, HO2) can convert I-, Brand Cl- ions to Br2, BrCl, ICl or IBr through
following gas phase reaction.
HCl + OH → Cl + H2O
(R2)
(3) Oxidized nitrogen compounds can react with
sea halide ions to release HBr, HCl, and other
species like ClNO2 and BrNO2.
N2O5 + NaClaq → NaNO3, aq + XNO2
(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.
EFX = ([X-]/[Na+])air/([X-]/[Na+])seawater (2)
In other words, for the fresh sea aerosols the
enrichment factor is 1.0. Acid displacement
SAHU et al: HALOGENS IN TROPOSPHERE
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 106.
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 EFBr 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 (EFBr
>1.0) were attributed to the transport of polluted
air from cities like Mumbai and Kolkata. Higher
values of EFBr 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 EFCl varied
between 0.0 and 1.0 which suggest a general
deficit of chloride at all locations over Indian
Ocean 4.
5
(a) r2=0.57
(b) r2=0.49
4
Cl- (g m-3)
1.5
1.0
+
-3
Na (g m )
2.0
0.5
3
2
1
0.0
0
0
5
10 15 20 25 30
-1
Wind speed (m s )
0
5
10 15 20 25 30
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 2005.
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
algae5. Emissions of bromocarbon from algae in
the tropical oceans (20°N-20°S) account for about
75% of total oceanic emission6. 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.
1617
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 Br2 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
explosion7,8.
HOBr + Br-aq + H+ → Br2 + H2O
(R4)
Br2 + 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 Br2 back to the
gas phase9.
(R6)
HOBrg → HOBraq
HBrg → Br- + H+aq
(R7)
HOBraq + Br-aq + H+aq → Br2,g+H2O
(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 troposphere10. For
example, the global biomass burning accounts for
about 20% of CH3Cl 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.
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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, X2, XY, XO, OXO, HOX, XNO2,
XNO3, etc.) and halogen reservoir species (HX,
XONO2, 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 X2, XY, HOX, XONO2,
XNO2, etc. react with ozone. Based on the field
and model studies11, 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 + O2
(R9)
X + O3 → XO + O2
(R10)
(R11)
Y + O3 → YO + O2
Net: O3 + O3 → 3O2
(R12)
Reaction cycle II
XO + HO2 → HOX + O2
HOX + hν → X + OH
X + O3 → XO + O2
OH + CO → H + CO2 (+M) → HO2
Net: O3 + CO + hν → CO2 + O2
(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 seasons12,13. 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.
O3 + hν → O2 + O(1D)
(R18)
(R19)
O(1D) + H2O → 2OH
OH + O3 → HO2 + O2
(R20)
HO2 + O3 → OH + 2O2
(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(1D) 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
SAHU et al: HALOGENS IN TROPOSPHERE
released from the marine surface contribute to
ozone destruction14, 15.
During the pre-INDOEX campaign, FebMar 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 200715. 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 ozone16. 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 sources17. In the polluted
troposphere, the night-time chemistry of involving
oxides of nitrogen converts inorganic chloride
into reactive forms. The production of HNO3
takes place mainly by heterogeneous reactions of
N2O5 under lower temperature and high NOx
(=NO+NO2) conditions during the night hours.
N2O5, g + H2Oaq → 2HNO3, aq
(R22)
Alternatively, nitryl chloride (ClNO2) and NO3are produced by following reaction.
N2O5,g + Cl-aq → ClNO2, g + NO3-aq
(R23)
The ClNO2 produced in above reaction
accumulates in the night and later photo-
1619
dissociated in the presence of sunlight yielding
NO2 and reactive Cl radical.
(R24)
ClNO2, g+ hν → NO2, g + Clg
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 sunlight18, 19.
R-H + Cl → HCl + R
(R25)
R + O2 + M → RO2 + M
(R26)
RO2 + NO → NO2
(R27)
2(NO2 + hν + O2 → NO + O3)
(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 MBL20, 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 (Hg0) 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
Hg0. In several studies, following reactions
mainly involving bromine radicals have been
suggested for the oxidation of Hg0.
(R30)
Hg0 + Br → HgBr
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 radicals11.
INDIAN J MAR SCI VOL 43, NO.9, SEPTEMBER 2014
1620
Table 1 The reaction rate constants (k at 298 K) of some
important hydrocarbons with OH, Cl, and Br radicals30.
Compound
Methane
Ethane
Ethene
Ethyne
Propane
Benzene
kOH
6.40 × 10−15
2.40 × 10−13
9.00 × 10−12
9.00 × 10−13
1.10 × 10−12
1.23 × 10−12
kOH, Cl, Br
kCl
kBr
1.00 × 10−13
6.52 × 10−26
−11
5.90 × 10
2.32 × 10−21
−10
1.10 × 10
1.09 × 10−12
5.20 × 10−11
1.54 × 10−13
−10
1.40 × 10
1.31× 10−18
−12
4.00 × 10
-[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 troposphere3.
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,
ClNO2, and ClNO3) and Cl2∗ (Cl2 and HOCl) in
ambient air22. In this method, air samples pass
through the chambers containing acidic (pH > 1)
and alkaline solutions to collect HCl∗ and Cl2∗
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 (X2 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 O2ions transfer an electron to Cl2, Br2, and I2 as they
have higher electron affinities compared to O2-.
Finally, the selected molecular ions are detected
by two stages of mass spectrometry (MS). This
technique was used for the first time
measurements of Br2 and BrCl during the ODEs
in the Arctic region23.
(c) Chemical Ionization Mass Spectrometry
(CIMS): In recent years, ionizations based
methods for the measurements of BrO, Br2, Cl2,
ClNO2, etc. have been developed and used by
several researchers24. The detection of X2 in
ambient air is based on its ionization using
radioactive sources like 63Ni to produce ions X2ions25. Several investigators have also utilized Ias a reagent for the simultaneous measurements of
HOBr, BrO, Br2, ClNO2, and Cl2.
(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
I2, 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 used27.
(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
SAHU et al: HALOGENS IN TROPOSPHERE
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 field28.
(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 + NO2). 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 178–184 nm29.
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
1621
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.
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