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Halogen chemistry in Troposphere

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					 Halogen Chemistry

  in the troposphere

           EAS 6410


Xiaolu Zhang, Bo Yao, Jin Liao
                       Introduction

Halogens: very reactive radicals

   Play an important role in stratosphere chemistry

   CFCs          Cl, ClO           Ozone depletion   (Molina and Rowland, 1974)



                                                             Why
Tropospheric Halogens
                                                           important

    Influence the oxidation power of the atmosphere


          Direct way: O3, OH, NOx ( NO + NO2 )


          Indirect way: Cl + RH ( e.g. CH4 )
         Main reaction mechanisms
Formation of halogen radicals

     Photolysis of 1) dihalogens (X2 or XY)
                    2) inorganic species ( HOX, XONO2, XNO2)
                    3) organic halogen precursors


     O3 + X            XO + O2
                                               No O3 depletion
     XO + hv           X + O3



     Salt deposits / Sea salt aerosol

         Heterogeneous processes
         Main reaction mechanisms
O3 destruction paths


     O3 + X            XO + O2

    XO + HO2           HOX + O2

    HOX + hv           X + OH

    OH + CO, O3 or VOC            HO2 + products


    Net reaction : 2O3 → 3O2
        Main reaction mechanisms
O3 destruction paths
                                                            BrO + ClO
                                                       4 times faster than
                       X + O3 → XO + O2                     BrO + BrO
   Halogen
     oxide             Y + O3 → YO + O2
     cross
   reactions           XO + YO → X + Y + O2
                                          ( X, Y = Cl, Br, I )

                       Net reaction : 2O3 → 3O2
          Main reaction mechanisms
Sinks of Halogens

    Reactions with RH    Cl + RH → HCl + R




    Reactions with NOx

                                 hv
      XO + NO → XONO                   XO + NOx

                                 H2O
      XO + NO2 → XONO2                 HOX + HNO3 (Deposition)
                  Additional sources
  Volcanoes
                    Stratosphere


                             ~0.01%


                       Up to
                    hundreds Tg
                       of HCl
 Large Eruption




precipitation       Troposphere
        Sources of reactive halogens

Industry and fossil fuel burning



       Industrial CHCl3: 62 Gg (Cl) a-1 (Aucott et al, 1999)


                  Pulp and paper manufacturing

                  Water treatment

       Fossil fuel burning: 4.6 Tg (Cl) a-1 in 1990


       Swimming pools and cooling towers: ~1 Tg (Cl) a-1
         Sources of reactive halogens
Biomass Burning and dust plumes



    Biomass burning --- a source of Methylhalides

    Inefficient combustion:

               CH3OH + HCl                CH3Cl + H2O

    Global production in the late 1990s     CH3Cl 450 Gg (Cl) a-1   25%
       ( Andreae & Merlet, 2001)            CH3Br 24 Gg (Br) a-1    20%

                                            CH3I    12 Gg (I) a-1

    Dust as an important reactive surface
         Sources of reactive halogens
Organohalogen compound
                                       Main Sources


          Concentration   Lifetime       Ocean

CH3Cl       ~ 630 ppt       ~ 1yr        Terrestrial plants
CH2Cl2      ~ 32 ppt       83 days
                                         Fungi
CH3Br      20 - 40 ppt    1 - 2 yrs

CHBr3                       Days         Biomass burning

 CH3I       1 - 30 ppt    3 - 4 days     Anthropogenic emissions
CH2I2      up to 1 ppt      5 min

CH2ClI     up to 1 ppt      10 h

CH2BrI     up to 1 ppt     45 min
      Marine Boundary Layer
• MBL: the lowest, 500-1,000m deep part of
  the troposphere that is in direct contact
  with the sea surface
• Separated from the free troposphere by a
  temperature and humidity inversion and is
  generally well mixed
• Halogens are very abundant in the form of
  sea salt aerosols which contain chloride
  and bromide
         1. Sea salt aerosol
• Produced at the sea surface by the
  bursting of air bubbles
• Bubble bursting produces small droplets
  from the film of the air bubbles as well as
  large jet droplets.
• Even larger spray droplets are produced
  by strong winds blowing over wind crests.
• Global flux of sea salt: 1500Tg/year-
  104Tg/year
                 1. Sea salt aerosol




Figure 10: Four stages in the production of sea salt aerosol by the bubble-burst
mechanism. (a) A bubble rises to the ocean surface thereby forming a thin film
at the interface which begins to thin. (b) Flow of water down the sides of the
cavity further thins the film which eventually ruptures into many small sea spray
particles. (c) An unstable jet, produced from water flowing down the sides of
the cavity, releases a few large sea spray drops. (d) Tiny salt particles remain
airborne as drops evaporate; a new bubble is formed. Note the scale change
between Figures (a) to (c) and Figure (d) (after Pruppacher and Klett (1997)).
                1. Sea salt aerosol
                         Ionic composition of sea water

ion                Cl-     Na+   Mg2+ SO42- K+       Ca2+   HCO3- Br-    I-

Conc.(mmol/l)      550     470   53    28      10    10     2     0.85   10-3



      pH of ocean surface water is around 8.2, buffered by HCO3-
      Uptake of acids from the gas phase leads to acidification of
      the particles.
      Keene and Savoie(1998,1999): pH values for moderately
      polluted conditions at Bermuda were in mid-3s to mid-4s
         1. Sea salt aerosol
• Major differences between reactions on
  sea salt aerosol and in free troposphere:
• Acidity
• Semi-liquid layer on the surface
         2. Reactive chlorine
• Reactive chlorine in the MBL is important
  for its roles in the acidity budget (HCl), the
  aqueous phase oxidation of S(IV) by HOCl,
  and the oxidation of organics and DMS by
  the chlorine atom.
        2. Reactive chlorine
• Many sea salt aerosol composition
  measurements found chlorine deficits
• main reason: the release of HCl from sea
  salt aerosol by acid displacement:
        2. Reactive chlorine
• “Hydrocarbon clock” method for
  estimating Cl concentrations: by
  measuring changes in hydrocarbon
  relative abundances, the concentration of
  the Cl radical can be determined.
• Wingenter et al. (1996): 3.3*104atoms/cm3,
  6.5*104atoms/cm3
         3. Reactive bromine
• Many field measurements show not only a
  depletion of Cl- in aged sea salt but often even
  more so of Br-
• On average at least 50% of the bromide is lost in
  the sampled aerosols. The effective solubility for
  bromide is about 600 times greater than for
  chloride (Brimblecombe and Clegg, 1989) so
  that HBr, unlike HCl, is not affected by acid
  displacement. Therefore, other mechanisms that
  involve photochemical processes are the reason
  for a release of bromine from the aerosol.
3. Reactive bromine
        3. Reactive bromine
• When sufficient Br is available:
                    -
          4. Reactive iodine
• In sea water, iodide concentration is very
  low compared to chloride and bromide.
• In sear salt aerosol, Cl and Br are usually
  depleted whereas I is strongly enriched.
• 500-1000 times in rain compared to sear
  water -> a major additional iodine source

        Biogenic?        Anthropogenic?
          4. Reactive iodine
• Main source of iodine in the MBL:
  emission of biogenic alkyl iodides like CH3I,
  C3H7I, CH2ClI or CH2I2 and inorganic
  iodine like I2 by various types of macro-
  algae and phytoplankton that live in the
  upper ocean and in tidal areas along the
  coast.
• Other sources
5. Halogen – sulfur interactions
• DMS and halogen



• S(IV) and halogen
5. Halogen – sulfur interactions
       Ozone Depletion Event in
            Polar Region
              Low surface ozone level
              (below 10ppb,even reach zero value)
Discovery     in Arctic region in late winter/early spring
                 were measured by
              (1)Oltmans(1981) at Barrow, Alaska.
              (2) Bottenheim(1986) at Alert, North Canada.



             1.Polar Meterology:
             Stable, Stratified in vertical
Why?         Prevent downward ozone from stratosphere
(Possible    2.Less VOCs, NOx pollutants
  reason)    3. Active halogen catalyzed ozone
                destruction chain.
        Why ODEs event happen?




BrO and ozone time series measured at Ny   http://www.iup.uni-bremen.de
AAlesund,Spitsbergen during ARCTOC96       /doas/scia_data_browser.htm
by Tuckermann et al. (1997)                        SCIAMACHY
 Meteorological analyses show that
ODEs only occurred, when air masses
 have been in contact with the Arctic
Ocean surface (Worthy et al. (1994))    Heterogenous
                                           reaction




                                        Bottenheim et al.
                                        (2002b)

                                        Transport:
                                        advection of an
                                        airmass in
                                        which O3
                                        depletion had
                                        already
                                        occurred.
Major Chemcial mechanism of polar
             ODEs

                        XONO2
            NO2                      H2O

                  HO2                HOX(aq)
hv      XO                HOX
                  O3 hv                    X-,Y-,H+
     XO,YO,NO
                                                      Aqueous
                           hv
                   X            XY    XY(aq)          phase

                hv NO2
Gas phase
                XNO2                  XNO2

                                            X-
                N2O5                  HNO3
       Sources of active bromine


     Less than
                                                    N2O5 and sea
    One-year-old           Frost Flower
                                                      Salt NaBr
      Sea ice



• When frozen                                        Do not need acidity
                         • Large surface areas
halide concentrated                                  during the reaction.
                         • Potential frost flower
on the surface                                       Due to low NOx
                         Area(PFF) region
• When melt,                                         Concentration,
                         Lead to regions with
lowered freezing                                     It is not an important
                         enhanced BrO
Point, greater density                               source
          The different roles of Bromine and
                Chlorine in Polar ODEs




Time series of O3, Br2, BrCl, and global irradiance at Alert for 10 – 11March 2000. Spicer et al.(2002)



1.In the ARCTOC 1996 campaign, the time integrated concentration of Cl
   was a thousand times smaller than that of Br.(Ramacher et al.1999)
2.Ozone loss by ClO-BrO catalysis is much smaller than by the BrO-BrO.
                         (Jobson et al 1994)
   The different roles of Bromine and
         Chlorine in Polar ODEs

                                        • 3.Fickert et al. (1999)
                                        find:
                                        The yield of Br2 and BrCl was
                                        found to depend on the Cl−
                                         to Br− ratio




Iodine plays a more important role in ODEs in marine Boundary layer.
 Halogen chemistry in Salt lake
   1. Measurement of high BrO concentration at a
   site downwind of Dead sea area.
    Hebestreit et al (1999) ,BrO up to 90pmol/mol
    Matveev et al.(2001) ,BrO up to 200pmol/mol


2. Stutz et al.(2002) in 2000 detected ClO 5~15pmol/mol
at the Great Salt Lake in Utah.(Br-/Cl- is only 0.0007)


3.In summer 2001 Zingler and Platt(2005) identified
IO mixing ratio 0.5~6pmol/mol in the Dead Sea Basin.
(Possible Oxidizing bacteria produce idoine)
                                                  Chemical mechanism
                                                     Matveev et al.(2001)
                                                     Concluded: bromine
                                                  release from salt deposit,
                                                     autocatalytic reaction
                                                      HOBr(aq)+ H++Br-
                                                          Br2(aq) + H2O




                                                   Salt lake gas and aerosol
BrO, O3 and NO2 levels at the Dead Sea southern   Phase cycling are similar to
    site, 5 August 2001.(Tas et al.,2005)
                                                          Polar region
               Conclusion

• 1.Halogen activation from aqueous phase
  to gas phase plays a critical role in Ozone
  depletion in polar region.

• 2. ODEs in polar region will probably
  increase.

				
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posted:7/19/2011
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