1 / 22

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).

jontae
Download Presentation

Synopsis

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related