11 Halogens in the Troposphere AOSC 637 Atmospheric Chemistry Russell R. Dickerson Finlayson-Pitts Chapt. 4,6 Seinfeld Chapt. 6 OUTLINE History & Importance Detection Techniques Sources and Sinks Global Chemistry Remaining Challenges References

2 History Duce (1963) measured sea salt aerosols and found depletions of Cl and Br but enrichments of I. Molina and Rowland (1974) showed that chlorine is important in the strat but it was generally thought that there was no halogen chemistry in the trop. Chameides and Davis (1980) hypothesized that iodine chemistry could be an ozone sink in the marine boundary layer. Barrie et al. (1988) observed rapid ozone destruction in polar sunrise and attributed this to Br chemistry mediated by ice. Finlayson-Pitts et al. (1989) observed the formation of ClNO 2 in laboratory reactions on salt-containing aerosols.

3 Importance Ozone destruction at polar sunrise Ozone destruction in the marine boundary layer Ozone production in polluted atmospheres Conversion of elemental mercury to reactive mercury. Removal of Methane.

4 Detection Techniques Tandem mass spectroscopy (e.g., Spicer et al. 1998). –H 3 O + generated and it reacts with Cl 2 Chemical ionization Mass Spec (CIMS) Mist chamber DOAS Resonance fluorescence

VonGlasow and Crutzen (2007) 5

Sea salt aerosol production. 6

Halogen atoms can destroy ozone in the unpolluted trop. Where X represents Cl, Br, or I, but not F. F + H 2 O  HF + OH And HF is stable. X + O 3  XO + O 2 XO + hv + (O 2 )  X + O Do nothing X + O 3  XO + O 2 XO + HO 2  HOX + O 2 HOX + hv  X + OH OH + CO (+O 2 )  CO 2 + OH O 3 + CO  CO 2 + O 2 net

Destruction can also proceed through a halogen dimer. This cycle proceeds in the Arctic where X represents Br, and BrO concentrations are high. It can also proceed with one Br and one Cl. 2(X + O 3  XO + O 2 ) XO + XO  2X + O 2  X 2 + O 2 X 2 + hv  2X O 3  3O 2 net

Chlorine atoms can initiate ozone production in the polluted trop. as OH does. HCl is pretty much dead in the troposphere. It is lost by dry & wet deposition. Bromine atoms, in contrast, do not react with hydrocarbons. They do react with aldehydes though. R-H + Cl  HCl + R ● R ● + O 2 + M  RO 2 + M RO 2 + NO  NO 2 Etc.

Bromine atoms destroy ozone in the absence of NOx Barrie et al. (1988) Used these reactions to explain rap[id ozone loss in the Arctic spring (polar sunrise). Keene et al. (1990) and Vogt et al. (1996) proposed this mechanism as a path to ozone destruction in the marine boundary layer.

11 Proof? The concentrations of Br (and Cl) are so small as to be nearly impossible to detect. Is it possible to infer the existence of halogens from their effect on the chemistry of ozone?

Diel cycle of ozone over the Indian Ocean 12

13

The vertical structure of ozone shows that the upper trop and strat are sources and the MBL is a sink for ozone. 14

15

The Model of Chemistry Considering Aerosols (MOCCA) evaluates trace gas concentrations in an environment with clouds or aerosols. For the marine boundary layer the rate of change in concentration is derived from gas-phase reactions, input from the free troposphere and ocean surface, and exchange with aerosols. Where C g is the gas-phase concentration, P g and L g are the gas-phase photochemical production and loss terms, E is the emission or entrainment rate, Z is the MBL height, V d is the deposition velocity L is the liquid water content (LWC), k t is the gas-aerosol exchange coefficient, and C g,eq is the gas-phase concentration in equilibrium with the aqueous phase (Henry’s Law). 16

17

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19 The vapor-phase concentrations of the molecules Br 2 and BrCl reach ppt levels at night.

The presence of halogens can explain the systematic destruction of ozone during the daylight hours. Because much of the Earth’s surface is oceanic, bromine multiphase reactions may be a substantive sing on a global scale. 20

21 Example from the Arctic

New Paper by Thornton et al. (2010) Trop halogen chemistry had been thought to be a marine or coastal problem. Nitryl chloride (ClNO 2 ) observed far from ocean. ClNO 2 acts as a reservoir for NOx. ClNO 2 + hv → Cl + NO 2 See Finlayson page 120 for the absorption spectrum of ClNO 2.

23

JA Thornton et al. Nature 464, (2010) doi: /nature08905 Schematic of chlorine activation by night-time NO x chemistry.

JA Thornton et al. Nature 464, (2010) doi: /nature08905 Time series of key quantities observed in Boulder, Colorado, from 11 to 25 February Three days showing high (left), moderate, and low (right) RH.

JA Thornton et al. Nature 464, (2010) doi: /nature08905 Observed and modelled relationships of ClNO 2 and particulate chloride. Left: Observed ClNO 2 and particulate chloride. Right: Observed and modeled ClNO 2 and particulate chloride. Solid lines are model results

JA Thornton et al. Nature 464, (2010) doi: /nature08905 Annual average components of P ClNO over the US. 2 a) Annual average NOx emissions over the US in Kg/yr. b) Annual average fraction of total nitrate (0-2km) formed via N 2 O 5. c) Yield of ClNO2. d) Production in g(Cl)/yr log scale startng at g(Cl)/yr.

28 Remaining Challenges Where does the Cl come from in the middle of a continent? What is the efficiency of ClNO 2 production? Direct measurements of HOBr and HOCl in the trop. If Thornton et al are right it will explain why N 2 O 5 does not seem such a major NOx sink, suggest that NOx is longer lived, and suggest Cl controls.

29 References Barrie, L. A., J. W. Bottenheim, P. J. Crutzen, and R. A. Rasmussen (1988), Ozone destruction at polar sunrise in the lower Arctic atmosphere, Nature, 334, Chameides, W. L. and D. D. Davis (1980), Iodine: It's possible role in tropospheric photochemistry, J. Geophys. Res., 85, Dickerson, R. R., K. P. Rhoads, T. P. Carsey, S. J. Oltmans, J. P. Burrows, and P. J. Crutzen (1999), Ozone in the remote marine boundary layer: A possible role for halogens, Journal of Geophysical Research-Atmospheres, 104, Duce, R. A., J. T. Wasson, J. W. Winchester, and E. Burns (1963), Atmospheric Iodine, Bromine, and Chlorine, Journal of Geophysical Research, 68, Finlaysonpitts, B. J., M. J. Ezell, and J. N. Pitts (1989), Formation of Chemically Active Chlorine Compounds by Reactions of Atmospheric NaCl Particles with Gaseous N 2 O 5 and ClONO 2, Nature, 337, Keene, W. C., A. A. Pszenny, D. J. Jacob, R. A. Duce, J. N. Galloway, J. J. Schultz-Tokos, H. Sievering, and J. F. Boatman (1990), The geochemical cycling of reactive chlorine through the marine troposphere, Glob. Biogeochem. Cycles, 4, Spicer, C. W., E. G. Chapman, B. J. Finlayson-Pitts, R. A. Plastridge, J. M. Hubbe, J. D. Fast, and C. M. Berkowitz (1998), Unexpectedly high concentrations of molecular chlorine in coastal air, Nature, 394, Thornton, J. A., J. P. Kercher, T. P. Riedel, N. L. Wagner, J. Cozic, J. S. Holloway, W. P. Dube, G. M. Wolfe, P. K. Quinn, A. M. Middlebrook, B. Alexander, and S. S. Brown (2010), A large atomic chlorine source inferred from mid-continental reactive nitrogen chemistry, Nature, 464, Vogt, R., P. J. Crutzen, and R. Sander (1996), A mechanism for halogen release from sea salt aerosol in the remote marine boundary layer, Nature, 383, von Glasow, R. (2010), ATMOSPHERIC CHEMISTRY Wider role for airborne chlorine, Nature, 464,