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Synopsis

REACTION OF NOx AND HALOGEN-CONTAINING TRACE GASES ON CIRRUS CLOUDS – IMPLICATIONS FOR UTLS CHEMISTRY. R.A.Cox , M.A.Fernandez, R.G.Hynes, and J.C.Mossinger Centre for Atmospheric Science, University of Cambridge, Department of Chemistry, Lensfield Rd, Cambridge, UK.. (rac26@cam.ac.uk).

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  1. REACTION OF NOx AND HALOGEN-CONTAINING TRACE GASES ON CIRRUS CLOUDS – IMPLICATIONS FOR UTLS CHEMISTRY. R.A.Cox , M.A.Fernandez, R.G.Hynes, and J.C.MossingerCentre for Atmospheric Science, University of Cambridge, Department of Chemistry, Lensfield Rd, Cambridge, UK.. (rac26@cam.ac.uk)

  2. Synopsis • Ozone production and loss in the tropopause region - role of heterogeneous reactions • Cirrus clouds in the tropopause region • Laboratory studies of HNO3 uptake on ice surfaces • Scavenging of HNO3 by cirrus clouds • Laboratory studies of Cl and Br Activation on ice • Recyclingof tropospheric ClOx and BrOx by cirrus clouds • Influence on ozone

  3. Upper Tropospheric ozone - production and loss

  4. Ozone production due to NOx • RO2 + NO Æ NO2 + RO (R = H or Organic) (1) • NO2 + hnÆ NO + O(3P) (2) • O(3P) + O2 + M Æ O3 + M (3) • Ozone production depends on local [RO2] and [NO]; typical values (Wennberg et al, 1998): [HO2] = 6 x 107 molecle cm-3, [NO] = 3 x 108 molecule cm-3 ; k1 = 1.1 x 10-11 cm3s-1 at 220 K • P(O3) = 2 x 105 molecle cm-3s-1 (t = 90 days)

  5. Ozone Loss due to HOx photochemistry • Photochemical loss • O3 + hnÆ O2 + O(1D) J1 • O(1D) + H2O Æ 2OH f • f = fraction of O(1D) reacted falls off with altitude due to drop in [H2O]. t > 1yr at 8 km ( n.b.~ 1 mo. at surface) • Chemical loss due to HO2 O3 + HO2Æ 2O2 + OH (4) • If [HO2] = 6 x 107 molecle cm-3, k4 = 1.15 x 10-15 cm3s-1 at 220 K; t = 170 days at 8 km • Transport loss: mean exchange time for air in UT t ~10 days

  6. Ozone Loss due to Halogens • HO2 + XO Æ O2 + HOX (X = Br, I or Cl) (5) • HOX + hnÆ OH + X (6) • X + O3Æ XO + O2 (7) • Ozone loss depends on local [HO2] and [XO]; typical values: [HO2] = 6 x 107 molecule cm-3 (Wennberg et al, 1998); [BrO] = 2 x 107 molecule cm-3 (Fitzenburger et al, 2000); k5 = 4.4 x 10-11 cm3s-1 at 220 K • L(O3) = 5.3 x 104 molecule cm-3s-1 (t = 219 days)

  7. Role of Heterogeneous reactions • Scavenging reactions: remove reservoirs and hence reduce reactive NOx and halogen species, e.g.HNO3, HCl, HBr; n.b. mean exchange time for air in UT ~10 days - leading to loss in wet precipitation • Activation reactions: convert stable reservoirs into photochemically active NOx and halogen species e.g. NO2, Cl2, BrCl, Br2

  8. Properties of Cirrus Clouds • Cirrus form 1 - 3 km below tropopause; dominant S/V • consist of ice crystals - columns, plates; 2-5:1 AR • Particles <100 mm (BG cirrus) evaporate in 1.5 km cloud layer - ads. HCl/HNO3 reversible; • Precipitating cirrus (~250 mm) in ‘warm clouds’- weaker adsorption <1000 mm2cm-3

  9. Uptake experiments on water-ice • Ice films prepared at 258 K; • Experiment temperatures: 205-235 K • PHCl, PHNO3: (0.3-2.0)  10-6 Torr • (~300 ppbv at 1.7 Torr) • Flow rate = 500 sccm; flow velocity = 1500cm/s • Observables: • Sticking coefficient (g) and the Surface coverage () as a function of temperature and reactant partial pressure. • Ct = C0 exp (-kobsz/v) • g = 2r kW/w

  10. HNO3 uptake on ice desorption S.I. down Uptake saturates T = 225 K P HNO3 = 8  10-7 Torr P H2O = 10-2 Torr S.I. up  = 0.008; time dependent Near supercooled liquid region T = 210 K P HNO3 = 8  10-7 Torr P H2O = 6  10-3 Torr continuous uptake  = 0.04; continuous

  11. Surface chemistry model for gas adsorption on ice Langmuir isotherm for adsorption with dissociation. HX  H+ (ads) + X- (ads) Keq = kads/kdes = C T-1/2 exp (-DHads/RT) C depends on the number of surface sites = 1015 cm-2 Equilibrium surface coverage

  12. HNO3 surface coverage on ice Determined by integrating the drop in signal and the recovery to saturation 218 K 228 K q Abbatt, 228 K Cox group, 218 K Field measurements, 200 K Polstar, 1998, Kramer et al. P(HNO3) X 106 / Torr Temperature / K Hads = – (54.0  10.0) kJ/mol.

  13. Atmospheric Implications: Lifetime of HNO3 in the presence of Cirrus • [HNO3] UT-LS (210K) 760 – 1200 pptv free troposphere(230 K) 20 – 760 pptv • Lab. Uptake measurements (Q, g) indicate lee wave cirrus and dense BG cirrus clouds should completely scavenge gas phase HNO3 ; also no activation of NO2 seen. • Removal on precipitating cirrus limited by kinetic and surface area restrictions (Tabazedeh et al, 1999) • Field measurements(Meilinger et al,1999; Weinheimer et al, 1998) suggest uptake less than expected; new analysis by Kremer et al, 2002, may resolve this.

  14. Heterogeneous halogen activation on cirrus clouds in the UT region Formation of dihalogens from reservoirs: ClONO2 + HCl ÆCl2 + HNO3 HOCl + HCl ÆCl2 + H2O HOBr + HCl ÆBrCl + H2O HOBr + HBr ÆBr2 + H2O Photolysis of dihalogens (visible light): Cl2+ hnÆ Cl + Cl BrCl + hnÆ Br + Cl Br2 + hnÆ Br + Br [HCl] in UT 100 - 500 pptv

  15. HCl surface coverage on ice film -  Determined by integrating the drop in signal and the recovery to saturation q Langmuir adsorption model PHCl X 106 / Torr Hads = – (49.010.0) kJ/mol.

  16. Ely-Rideal mechanism for HOBr + HCl reaction Loss HOBr = Production BrCl

  17. Rate of reactive uptake in terms of surface coverage of HCl Data ofChu and Chu, 1999 Langmuir isotherm for HCl on ice (Hynes et al, 2001) T = 221 K T = 190 K go ~ 0.4 (independent of temperature) (Mossinger et al., 2002)

  18. Recycling tropospheric ClOx and BrOx by HOBr + HCl reaction on cirrus clouds • t HCl ~ 5 days due to physical removal; significant Cl activation only in Cirrus with high s/v (> 2000mm2 cm-3 ) • t HOBrdue to heterogeneous activation is comparable to photolysis (~7 min) and much shorter than physical removal; explains the elevated concentrations of BrO recently observed in the free troposphere .

  19. Impact of heterogeneous reactions on ozone in the tropopause region • Meilinger et al . (2001) show that net effect of heterogeneous chemistry on P(O3) is controlled by NOx . • In the UT loss of O3 due to BrOx unimportant relative to HOx; • In LS ClOx activation occurs enough to reduce average P(O3) by <10% (Bregman et al, 2002) • Heterogeneous loss of HO2 is important in the UT (Meilinger et al . 2001)

  20. Co-adsorption of HCl and HNO3 The ice surface at 218 K is exposed to ~110-6 Torr HCl added via the sliding injector HCl displaced by HNO3 HCl signal (mV) HNO3 injector up injector down time (s)

  21. Competetive adsorption on ice surfaces • HCl less strongly adsorbed than HNO3 (c.f.Hads = – 49 kJ/mol. vs -56 kJ/mol.) • HNO3 displaces adsorbed HCl - reduction of Cl activation(?) • Organic oxygenates less strongly adsorbed than HCl (Sokolov & Abbatt, 2002) • Organic acids/alcohols not consistent with 2 species Langmuir model (Sokolov & Abbatt, 2002)

  22. Conclusions • Adsorption properties of HNO3 and HCl on ice surfaces have been determined in 210-230 K range; • Observed HNO3 partitioning in lee wave cirrus clouds consistent with lab studies; • HCl + HOBr reaction on ice parameterised in terms of HCl surface coverage; • Heterogeneous recycling of BrOx can account for enhanced tropospheric [BrO] • Effects on ozone are small but significant in LS

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