Upper Stratospheric and Lower Mesospheric HO x :Theory and Observations Tim Canty 1, Herb Pickett 2, Brian Drouin 2, Ken Jucks 3, Ross Salawith 2 HO x Chemistry 1 University of Maryland, College Park, MD NASA Jet Propulsion Laboratory, Pasadena, CA Harvard Smithsonian Center for Astrophysics, Cambridge, MA Conclusions Motivation HO x Dilemma HO x Dilemma Revisited Ozone Deficit Problem Ozone Deficit Problem - MLS Affiliations and Acknowledgements Two long standing problems in middle-atmospheric photochemistry HO x Dilemma: Before the MLS observations, models were unable to agree with available observations of both stratospheric and mesospheric HO x (HO x =OH+HO 2 ). No combination of kinetic parameters could solve this issue. O 3 Deficit Problem: Models have a tendency to underestimate O 3 at altitudes where O 3 is controlled by HO x photochemistry. Again, no combination of kinetic parameters have successfully resolved this issue. We re-examine both of these problems through the analysis of observational data and comparisons with photochemical model results. Model results agree with MLS OH observations without the need for changes to kinetics parameters as suggested in the literature (results do not exhibit a HO x dilemma) Taken at face value, our simulations suggest a continued need to resolve the ozone deficit problem without recourse to major perturbations in the kinetics parameters that regulate HO x Observations and Model Description 1) Production StratosphereH 2 O + O( 1 D)  2OH MesosphereH 2 O + h  OH + H 2) Loss OH + HO 2  H 2 O + O 2 3) Partitioning (numerous) HO 2 + O  OH + O 2 OH + O  O 2 + H HO x + O 3  products Observations : Microwave Limb Sounder (MLS): v2.2 retrievals of OH, HO 2 and HO x pre-cursors Balloon OH (BOH): functionally identical to THz module on MLS Far-infrared Spectrometer (FIRS-2): Balloon-borne thermal emission far-infrared Fourier transform spectrometer (for more info, see Pickett et al., GRL, 2006, Canty et al., GRL, 2006) 1-D chemical model : Assumes 24hr balance between production and loss Constrained by MLS observations of O 3,H 2 O, N 2 O, and temp. JPL 2006 kinetics reproduces solar variability, improved Lyman-  Ground based microwave observations of HO 2 [Clancy et al. JGR, 1994] were used to suggest a 60-80% decrease in the rate of HO 2 + O  OH + O 2 (1) Mesospheric MAHRSI OH observations (Summers et al., Science, 1997) led to the suggestions of either decreasing (1) by 50% or reducing (1) by 20% and a 30% increase in HO 2 +OH  H 2 O +O 2 (2) M iddle A tmospheric H igh R esolution S pectrograph I nvestigation Standard model overpredicts OH at secondary peak (70km) and underpredicts OH at primary peak (40km) (Conway et al., GRL, 27, 2000). No combination of kinetic parameters yields good agreement between models and measurements at all altitudes. NOTE: Without simultaneous observations of OH and HO 2, it is difficult to attribute the above discrepancies to HO x loss, production, or partitioning. MLS zonal mean selected to match SZA range of MAHRSI (SZA=32 ° - 49 ° ) Above 45 km, MLS and MAHRSI 2 σ uncertainties overlap. However, at the 70 km peak, MAHRSI is 20% lower than MLS. At the 42 km peak, MAHRSI is nearly 50% higher than MLS. Bottom Line: Differences between observations could be related to the 8 year separation between observations and consequent changes in HO x pre-cursor species (O 3, H 2 O, CH 4, etc). Though it is unlikely that changes in the concentrations of the species would be large enough to explain the discrepancy between MLS and MAHRSI. Fig 6-17 WMO 1998, “Purple Book” Photochemical Loss O 3 > Prod O 3, for Z > ~45 km Hence, models tend to underestimate measured [O 3 ] (Jucks et al., JGR, 1996, Osterman et al., GRL, 1997) Proposed solutions have not worked (see below). GRL, 1997 Solutions? Vibrationally excited O 2 (Miller et al., 1994) Collisional removal dominates photodissociation (Slanger and Copeland, 2003) Rxn's involving vibrationally excited O 2 and OH could solve HO x dilemma as well (Varandas, 2004) Mechanism rate constants are insufficient to produce significant changes (Smith and Copeland, 2005) 50% decrease in O+HO 2 (Summers et al., 1997) Solves problem but leads to disagreement between models and observations of HO x (Canty et al., 2006) OH tied up as HO 3 (Murray et al., JPC, 2007) Analysis only goes up to 30 km. May lead to increase disagreement between models and observations of HO x O x (O x =O 3 +O) is controlled by HO x above 45 km. Model constrained to either MLS or FIRS-2 observations (H 2 O, O 3, CH 4, etc.) shows good agreement with observed OH and HO 2 (top panels) The model uses standard JPL 2006 kinetics with modifications (within recommended uncertainty) to the rate of O+OH ( Smith and Stewart, J. Chem. Soc. Faraday Trans.,1994) and OH+HO 2 (20% increase) to achieve overall good agreement between measured and modeled OH,HO 2,HO x, and HO 2 /OH. However, the model over-predicts O 3 loss above 45 km (bottom panels),where O 3 photochemistry should be controlled by OH and HO 2. Slightly better agreement between model and MLS but for a very limited altitude range. MLSFIRS-2 The fastest reactions involving HO X simply repartition the concentration of OH and HO 2 without changing the overall amount of HO x.