O3 Destruction Research Articles

STRATAQ: A three-dimensional Chemical Transport Model of the stratosphere

Abstract. A three-dimensional (3-D) Chemical Transport Model (CTM) of the stratosphere has been developed and used for a test study of the evolution of chemical species in the arctic lower stratosphere during winter 1996/97. This particular winter has been chosen for testing the model’s capabilities for its remarkable dynamical situation (very cold and strong polar vortex) along with the availability of sparse chlorine, HNO3 and O3 data, showing also very low O3 values in late March/April. Due to those unusual features, the winter 1996/97 can be considered an excellent example of the impact of both dynamics and heterogeneous reactions on the chemistry of the stratosphere. Model integration has been performed from January to March 1997 and the resulting long-lived and short-lived tracer fields compared with available measurements. The model includes a detailed gas phase chemical scheme and a parameterization of the heterogeneous reactions occurring on liquid aerosol and polar stratospheric cloud (PSC) surfaces. The transport is calculated using a semi-lagrangian flux scheme, forced by meteorological analyses. In such form, the STRATAQ CTM model is suitable for short-term integrations to study transport and chemical evolution related to "real" meteorological situations. Model simulation during the chosen winter shows intense PSC formation, with noticeable local HNO3 capture by PSCs, and the activation of vortex air leading to chlorine production and subsequent O3 destruction. The resulting model fields show generally good agreement with satellite data (MLS and TOMS), although the available observations, due to their limited number and time/space sparse nature, are not enough to effectively constraint the model. In particular, the model seems to perform well in reproducing the rapid processing of air inside the polar vortex on PSC converting reservoir species in active chlorine. In addition, it satisfactorily reproduces the morphology of the continuous O3 decline as shown by the satellite during the investigated period, with a tendency, however, to underestimate the total column values inside the polar vortex during late winter. As possible causes of this model/observation difference we suggest an incorrect estimation of the vertical transport and of the tropospheric contribution.Key words. Atmospheric composition and structure (Middle atmosphere-composition and chemistry) Meterology and atmospheric dynamics (middle atmosphere dynamics)

Observational evidence for the role of denitrification in Arctic stratospheric ozone loss

Severe and extensive denitrification, chlorine activation, and photochemical ozone loss were observed throughout the lower stratosphere in the 1999–2000 Arctic vortex. A large number of air parcels sampled between late February and mid‐March, 2000, were photochemically intercomparable for chemical O3 loss rates. In these air parcels, the temporal evolution of the correlations of O3 with the NOy remaining after denitrification provides strong evidence for the role of NOy in moderating O3 destruction. In 71%‐denitrified air parcels, a chemical O3 destruction rate of 63 ppbv/day was calculated, while in 43%‐denitrified air parcels the destruction rate was only 43 ppbv/day. These observational results show that representative denitrification models will be required for accurate prediction of future Arctic O3 changes.

On the impact of heterogeneous chemistry on ozone in the tropopause region

We examine the impact of heterogeneous chemistry involving liquid aerosol and ice particles on net ozone (O3) production rates under conditions representative of the midlatitude upper troposphere (UT) and lowermost stratosphere (LS). We demonstrate that heterogeneous effects are controlled by nitrogen oxides (NOx) and by the location of the air masses relative to the tropopause (TP). The net effect of heterogeneous chemistry is to decrease net O3 production below the TP (via heterogeneous HO2 loss) and to cause O3 destruction above the TP (via heterogeneous chlorine (Cl) activation). In the UT, gas phase chemistry due to non‐methane hydrocarbons (NMHCs) can become as important for O3 chemistry as heterogeneous reactions, and removal of HO2 by particles can become more important than changes of hydrogen oxides (HOx) through heterogeneous bromine (Br) chemistry. In the humid LS, Cl activation can become sufficiently large, so that O3 depletion occurs at all conceivable values of NOx. Such cold and humid conditions occur frequently enough to reduce the average ozone production rates in the midlatitude LS by more than 10%.

Numerical Investigation of the Effects of Boundary-Layer Evolution on the Predictions of Ozone and the Efficacy of Emission Control Options in the Northeastern United States

This paper examines the effects of two different planetary boundary-layer (PBL) parameterization schemes – Blackadar and Gayno–Seaman – on the predicted ozone (O3) concentration fields using the MM5 (Version 3.3) meteorological model and the MODELS-3 photochemical model. The meteorological fields obtained from the two boundary-layer schemes have been used to drive the photochemical model to simulate O3 concentrations in the northeastern United States for a three-day O3 episodic period. In addition to large differences in the predicted O3 levels at individual grid cells, the simulated daily maximum 1-h O3 concentrations appear at different regions of the modeling domain in these simulations, due to the differences in the vertical exchange formulations in these two PBL schemes. Using process analysis, we compared the differences between the different simulations in terms of the relative importance of chemical and physical processes to O3 formation and destruction over the diurnal cycle. Finally, examination of the photochemical model's response to reductions in emissions reveals that the choice of equally valid boundary-layer parameterizations can significantly influence the efficacy of emission control strategies.

Rate Constant Measurements for the Reaction of HO2 with O3 from 200 to 300 K Using a Turbulent Flow Reactor

The rate constant for the reaction of HO2 with O3 was measured using a turbulent flow reactor with tunable diode laser absorption detection of HO2. Two separate methods were used to determine k1 from the pseudo-first-order decay of HO2 in excess O3. The isotopic labeling method used H18O2 in excess 16O3 to avoid reformation of the reactant. The OH scavenger method used trifluorochloroethylene to remove the OH product and prevent re-formation of HO2. The turbulent flow reactor allowed measurements at temperatures between 297 and 197 K at total pressures from 80 to 175 Torr, spanning a wide range of stratospheric conditions. The temperature-dependent rate constant is given by the three-term expression k1(T) = {(103 ± 57) exp[−(1323 ± 160)/T] + 0.88} × 10-15 cm3 molecule-1 s-1, reflecting its non-Arrhenius behavior at low temperatures. These results demonstrate that the catalytic destruction of lower stratospheric O3 by HOx radicals (OH and HO2) may proceed faster than current models predict. The rate consta...

Ozone destruction and production rates between spring and autumn in the Arctic stratosphere

In situ measurements of radical and long‐lived species were made in the lower Arctic stratosphere (18 to 20 km) between spring and early autumn in 1997. The measurements include O3, ClO, OH, HO2, NO, NO2, N2O, CO, and overhead O3. A photochemical box model constrained by these and other observations is used to compute the diurnally averaged destruction and production rates of O3 in this region. The rates show a strong dependence on solar exposure and ambient O3. Total destruction rates, which reach 19%/month in summer, reveal the predominant role of NOx and HOx catalytic cycles throughout the period. Production of O3 is significant only in midsummer air parcels. A comparison of observed O3 changes with destruction rates and transport effects indicates the predominant role of destruction in spring and an increased role of transport by early autumn.

Delivery of pure ozone in ultrahigh vacuum

An apparatus designed to produce and deliver pure ozone to a sample inside a metal ultrahigh vacuum apparatus is described. Ozone of 96%–97% purity is prepared in a glass vacuum system and converted to a molecular beam of known flux using an all glass doser. The storage lifetime of the O3 at Torr pressure in the glass storage vacuum system (containing a small surface area of metal) has been measured, and it is shown that the rate of O3 destruction during storage is inconsequential for hours of storage.

Diurnal ozone cycle in the tropical and subtropical marine boundary layer

A conceptual analysis of diurnal ozone (O3) changes in the marine boundary layer (MBL) is presented. Such changes are most pronounced downwind of O3 sources in tropical and subtropical latitudes and during summer at higher latitudes. Previously, it has been assumed that daytime photochemical O3 loss and nighttime replenishment through entrainment from the relatively O3‐rich free troposphere explain the diurnal O3 cycle. We show, however, that in a net O3 destruction environment (low NOx) this diurnal cycle can be explained by photochemistry and advection, which establish a horizontal O3 gradient that is typical for the MBL. We support this hypothesis first by calculations with a conceptual one‐dimensional (1‐D) advection‐diffusion model and second by simulations with an interactive 3‐D chemistry transport model. The results are in good agreement with observations, for example, in the Indian Ocean Experiment.

The role of BrNO3 in marine tropospheric chemistry: A model study

Laboratory studies have shown that bromine nitrate (BrNO3) reacts on sulfuric acid and on ice particles. Here we investigate the potential role of BrNO3 in the marine boundary layer (mbl) assuming that it reacts on sea‐salt particles as well. Using the chemical box model MOCCA we find that heterogeneous reactions of BrNO3 on aerosol particles could affect the chemistry in four major ways: 1) They increase loss of NOx (=NO+NO2) from the gas phase; 2) They accelerate loss of bromide and chloride from sea‐salt aerosols. This dehalogenation occurs without the consumption of aerosol acidity; 3) The resulting loss of NOx and the increase of gas‐phase bromine species both lead to O3 destruction; 4) The resulting increase of reactive chlorine species affects gas‐phase hydrocarbons as well as S(IV) oxidation by HOCl in sea‐salt aerosols.

Role of nitrogen oxides in the stratosphere: A reevaluation based on laboratory studies

The changes in chemical partitioning and stratospheric O3 abundance due to the recently measured rate coefficients for the O + NO2, OH + HNO3, and OH + NO2 reactions are examined using a two‐dimensional model. The rate constant changes increase NOx abundance (up to 40%) and NOx‐catalyzed O3 destruction, and extend down by several kilometers the altitude region where NOx dominates catalytic O3 destruction. Reductions in the abundance of HOx (10–30%) and ClOx (20‐40–) in the lower stratosphere partially buffer the effect on column O3 amount. Column O3 at middle and high latitudes is reduced by 2–10% depending on season for current halogen levels. The model derived long‐term O3 trend at midlatitudes due to increases in anthropogenic halogens is reduced by approximately 30%.

Tropospheric O3 distribution over the Indian Ocean during spring 1995 evaluated with a chemistry‐climate model

An analysis of tropospheric O3 over the Indian Ocean during spring 1995 is presented based on O3 soundings and results from the European Centre Hamburg (ECHAM) chemistry‐general circulation model. The ECHAM model is nudged toward actual meteorology using European Centre for Medium‐Range Weather Forecasts analyses, to enable a direct comparison between model results and in situ observations. The model reproduces observed CO levels in different air mass categories. The model also reproduces the general tendencies and the diurnal variation in the observed surface pressure, although the amplitude of the diurnal variation in the amplitude is underestimated. The model simulates the general O3 tendencies as seen in the sonde observations. Tropospheric O3 profiles were characterized by low surface concentrations (<10 ppbv), midtropospheric maxima (60–100 ppbv, at 700–250 hPa) and upper tropospheric minima (<20 ppbv, at 250–100 hPa). Large‐scale upper tropospheric O3 minima were caused by connective transport of O3‐depleted boundary layer air in the intertropical convergence zone (ITCZ). Similarly, an upper tropospheric O3 minimum was caused by Cyclone Marlene south of the ITCZ. The midtropospheric O3 maxima were caused by transport of polluted African air. The ECHAM model appears to overestimate surface O3 levels and does not reproduce the diurnal variations very well. This could be related to unaccounted multiphase O3 destruction mechanisms involving low level clouds and aerosols, and missing halogen chemistry.

Heterogeneous Kinetics of the Uptake of HOBr on Solid Alkali Metal Halides at Ambient Temperature

Bromine contg. compds. are thought to play an important role in the O3 destruction in various regions of the atm. Recent studies of atm. Br chem. suggest that HOBr is one of the important Br compds. The rapid photolysis of HOBr in daytime releases Br atoms and OH radicals, which causes ozone destruction. In this study, the reactivity of HOBr with solid alkali metal halides is reported which affords insights into the importance of heterogeneous reactions of HOBr with salt, which in the atm. act as substrate particles for the reactions. The heterogeneous reactions of HOBr with solid cryst. NaCl [HOBr(g) + NaCl(s) -> BrCl(g) + NaOH(s)] and KBr [HOBr(g) + KBr(s) -> Br2(g) + KOH(s)] substrates at ambient temp. have been investigated using a Teflon coated Knudsen cell reactor. Powder, grain, and spray-deposited salt substrates were used for the measurement of the HOBr reactivity. The obsd. uptake probability depends on the total external surface area of the salt substrates. For NaCl substrates, Br2 and BrCl are obsd. as products; for KBr substrates, Br2 is obsd. as the sole product. In both cases, a dependence of the initial uptake probability g0 on HOBr flow rate has been obsd. The initial uptake is large at low flow rate and 10 times smaller at high flow rate. Values of g0 ? (6.5 +- 2.5) * 10-3 for NaCl and g0 ? 0.18 +- 0.04 for KBr are obtained under our exptl. conditions of limiting low flow rates akin to atm. conditions. The prodn. of Br2 is obsd. even for HOBr interacting on solid NaNO3, a non-halogen contg. substrate. The yield measurements imply that a HOBr self-reaction occurs on salt surfaces according to 2HOBr -> Br2 + H2O + 1/2O2. The decrease in Br2 yield with increasing HOBr flow rate from 100 to 50% indicates that a competition between the heterogeneous reaction of HOBr with NaCl or KBr and the self-reaction of HOBr takes place on the solid salt surface under lab. exptl. conditions. The decrease of g0 with time indicates that approx. 5-10% of the Br atoms on a KBr surface interact with HOBr.

Implications of large scale shifts in tropospheric NOx levels in the remote tropical Pacific

A major observation recorded during NASA's western Pacific Exploratory Mission (PEM‐West B) was the large shift in tropical NO levels as a function of geographical location. High‐altitude NO levels exceeding 100 pptv were observed during portions of tropical flights 5–8, while values almost never exceeded 20 pptv during tropical flights 9 and 10. The geographical regions encompassing these two flight groupings are here labeled “high” and “low” NOx regimes. A comparison of these two regimes, based on back trajectories and chemical tracers, suggests that air parcels in both were strongly influenced by deep convection. The low NOx regime appears to have been predominantly impacted by marine convection, whereas the high NOx regime shows evidence of having been more influenced by deep convection over a continental land mass. DMSP satellite observations point strongly toward lightning as the major source of NOx in the latter regime. Photochemical ozone formation in the high NOx regime exceeded that for low NOx by factors of 2 to 6, whereas O3 destruction in the low NOx regime exceeded that for high NOx by factors of up to 3. Taking the tropopause height to be 17 km, estimates of the net photochemical effect on the O3 column revealed that the high NOx regime led to a small net production. By contrast, the low NOx regime was shown to destroy O3 at the rate of 3.4% per day. One proposed mechanism for off‐setting this projected large deficit would involve the transport of O3 rich midlatitude air into the tropics. Alternatively, it is suggested that O3 within the tropics may be overall near self‐sustaining with respect to photochemical activity. This scenario would require that some tropical regions, unsampled at the time of PEM‐B, display significant net column O3 production, leading to an overall balanced budget for the “greater” tropical Pacific basin. Details concerning the chemical nature of such regimes are discussed.

An assessment of ozone photochemistry in the extratropical western North Pacific: Impact of continental outflow during the late winter/early spring

This study examines the influence of photochemical processes on tropospheric ozone distributions over the extratropical western North Pacific. The analysis presented here is based on data collected during the Pacific Exploratory Mission‐West Phase B (PEM‐West B) field study conducted in February‐March 1994. Sampling in the study region involved altitudes of 0–12 km and latitudes of 10°S to 50°N. The extratropical component of the data set (i.e., 20–50°N) was defined by markedly different photochemical environments north and south of 30°N. This separation was clearly defined by an abrupt decrease in the tropopause height near 30°N and a concomitant increase in total O3 column density. This shift in overhead O3 led to highly reduced rates of O3 formation and destruction for the 30–50°N latitude regime. Both latitude ranges, however, still exhibited net O3 production at all altitudes. Of special significance was the finding that net O3 production prevailed even at boundary layer and lower free tropospheric altitudes (e.g., ≤ 4 km), a condition uncommon to Pacific marine environments. These results reflect the strong impact of continental outflow of O3 precursors (e.g., NO and NMHCs) into the northwestern Pacific Basin. Comparisons with PEM‐West A, which sampled the same region in a different season (September‐October), revealed major differences at altitudes below 4 km, the altitude range most influenced by continental outflow. The resulting net rate of increase in the tropospheric O3 column for PEM‐West B was 1–3% per day, while for PEM‐West A it was approximately zero. Unique to the PEM‐West B study is the finding that even under wintertime conditions substantial column production of tropospheric O3 can occur at subtropical and mid‐latitudes. While such impacts may not be totally unexpected at near coast locations, the present study suggests that the impact from continental outflow on the marine BL could extend out to distances of more than 2000 km from the Asian Pacific Rim.

HALOE observations of the Arctic Vortex during the 1997 spring: Horizontal structure in the lower stratosphere

In late March, 1997, HALOE observed very low ozone (less than 1 ppmv at 48OK), low HCl, generally high NOx, and normal levels of H2O within the Arctic vortex. Trajectory mapping techniques show that the lower stratospheric Arctic vortex was locally chemically perturbed during this period. Regions of low HCl and low NOx mixing ratios found within the Arctic vortex indicate that partitioning within the odd chlorine family had been shifted from reservoir species to active species. Low O3 mixing ratios within these regions strongly suggest that chlorine‐catalyzed photochemical destruction of O3 had recently taken place. Regions of low HCl and higher NOx mixing ratios are also observed within the Arctic vortex. Correlations of O3, H2O, NOx and HCl with HF are used to quantify the changes in these constituents during March and early April, 1997. Temporal changes in the correlation of HCl and NOx with HF indicate increases in these species. No trend in the correlations of H2O or O3 with HF was found in the lower stratosphere.

A parameterization of mixdown time for atmospheric chemicals

The finite spatial resolution of grid‐based numerical chemical transport models can lead to overestimation of chemical reaction rates that are in reality limited by the rate of mixing. A parameterization has been developed to compensate for this effect. It estimates the time taken to mix chemical features down from the model grid scale to the physical scale at which they really react, then builds an appropriate time lag into the chemistry by reducing the rate constant. The parameterization is shown to work in idealized tests, where it allows low‐resolution parameterized experiments to mimic the results of higher‐resolution unparameterized experiments in terms of the total reaction product. The parameterization is applied in a single‐layer chemical transport model of the winter stratosphere to the deactivation of ClOx. by reaction with NOx. It is found to have a large effect on the deactivation of ClOx. The resulting impact on the rate of catalytic destruction of O3 by ClOx is estimated to be significant.

Results from the Intergovernmental Panel on Climatic Change Photochemical Model Intercomparison (PhotoComp)

Results from the Intergovernmental Panel on Climatic Change (IPCC) tropospheric photochemical model intercomparison (PhotoComp) are presented with a brief discussion of the factors that may contribute to differences in the modeled behaviors of HOx cycling and the accompanying O3 tendencies. PhotoComp was a tightly controlled model experiment in which the IPCC 1994 assessment sought to determine the consistency among models that are used to predict changes in tropospheric ozone, an important greenhouse gas. Calculated tropospheric photodissociation rates displayed significant differences, with a root‐mean‐square (rms) error of the reported model results ranging from about ±6–9% of the mean (for O3 and NO2) to up to ±15% (H2O2 and CH2O). Models using multistream methods in radiative transfer calculations showed distinctly higher rates for photodissociation of NO2 and CH2O compared to models using two‐stream methods, and this difference accounted for up to one third of the rms error for these two rates. In general, some small but systematic differences between models were noted for the predicted chemical tendencies in cases that did not include reactions of nomnethane hydrocarbons (NMHC). These differences in modeled O3 tendencies in some cases could be identified, for example, as being due to differences in photodissociation rates, but in others they could not and must be ascribed to unidentified errors. O3 tendencies showed rms errors of about ±10% in the moist, surface level cases with NOx concentrations equal to a few tens of parts per trillion by volume. Most of these model to model differences can be traced to differences in the destruction of O3 due to reaction with HO2. Differences in HO2, in turn, are likely due to (1) inconsistent reaction rates used by the models for the conversion of HO2 to H2O2 and (2) differences in the model‐calculated photolysis of H2O2 and CH2O. In the middle tropospheric “polluted” scenario with NOx concentrations larger than a few parts per billion by volume, O3 tendencies showed rms errors of ±10–30%. These model to model differences most likely stem from differences in the calculated rates of O3 photolysis to O(1D), which provides about 80% of the HOx source under these conditions. The introduction of hydrocarbons dramatically increased both the rate of NOx loss and its model to model differences, which, in turn, are reflected in an increased spread of predicted O3. Including NMHC in the simulation approximately doubled the rms error for O3 concentration.

Ozone destruction on solid particles

Ozone (O3) is an important constituent of the Earth atmosphere, either stratosphere, where it has a beneficial role to protect Earth's surface from harmful UV-B radiation, or troposphere where it is considered an air pollutant. We investigated the ozone destruction on solid particles of natural or anthropogenic origin as: silica-gel, pollen, coal fly ash, titanium dioxide with different specific surface (s) and sodium halides (NaCl, NaBr and NaI). The experiments were conducted in a fluidized bed reactor with elevated ambient concentrations of O3 (100 ppb) employed. The results indicate that the destruction of O3 depends upon: sample quantity (silica-gel with equal s), sample surface (TiO2 with different s) and chemical composition (coal fly ash comparative to wood ash). Interesting results were obtained with sodium halides: no effect on O3 concentrations was detected with NaCl, NaBr shows a certain destruction, while NaI removes completely O3 from the air stream. In the experiments with NaI doped NaCl, the destruction of O3 was dependent on NaI quantity.

Ozone, hydroperoxides, oxides of nitrogen, and hydrocarbon budgets in the marine boundary layer over the South Atlantic

The NASA GTE TRACE A mission sampled air over the South Atlantic and western Indian Oceans. Thirteen flight legs were flown within the marine boundary layer (MBL). The MBL was typically the cleanest air sampled (e.g., CH4 < 1680 ppb, CO < 70 ppb, C2H6 < 400 ppt, C3H8 < 40 ppt, NOx < 15 ppt, and midday NO < 5 ppt) but was overlain by polluted air. The photochemistry of the MBL was influenced by oceanic emissions, surface deposition, and entrainment of pollutants from aloft. Chemical budgets were constructed for several species in the MBL in order to investigate these effects and are presented for ethane, ethylene, propane, propylene, n‐butane, formic acid (HFo), methylhydroperoxide (CH3OOH), oxides of nitrogen (i.e., NO, NO2, PAN, HNO3), hydrogen peroxide (H2O2), and ozone (O3). A photochemical point model was used to evaluate local chemical production and loss. An entrainment model was used to assess material exchange between the lower free troposphere (FT) and the MBL and a resistance deposition model was used to evaluate material exchange across the air‐sea interface. The results suggested the ocean to be the source of measured alkenes in the MBL and to be the most likely source of the shorter‐lived alkanes: propane and n‐butane. Ethane was the only hydrocarbon for which input from aloft may have exceeded its photochemical destruction. The estimated hydrocarbon sources from the ocean were in agreement with prior analyses. Transport from the lower FT together with surface loss could not account for measured concentrations of CH2O, HFo, and HNO3. The transport of peroxyacetylnitrate (PAN) from the FT to the MBL exceeded the rate of HNO3 production and was more than sufficient to maintain observed NOx levels without having to invoke an oceanic source for NO. The flux of NOx, PAN, and HNO3 was in balance with the surface deposition flux of HNO3. However, the predicted rates of HNO3 formation from the oxidation of NO2 and HNO3 entrainment from aloft were inadequate to maintain observed levels of HNO3 unless HNO3 was partitioned between the gas phase and a more slowly depositing aerosol phase. The estimated dry deposition flux of HNO3 to the South Atlantic during TRACE A, 2–4 × 109 molecules cm−2 s−1, was about 10 times the annual average estimate for this region. The destruction of O3 within the MBL was found to be exceeded by transport into the MBL from aloft, 6 ± 2 × 1010 compared to 11 ± 10 × 1010 molecules cm−2 s−1. The principal O3 destruction process was mediated by the formation and surface deposition of H2O2 and CH3OOH, 4 ± 4 × 1010 and 1.1 ± 0.5 × 1010 molecules cm−2 s−1. The direct loss of O3 to the sea surface was estimated to be 1.7 ± 0.2 × 1010 molecules cm−2 s−1. CH3OOH was lost to the sea and transported into the FT from the MBL. Its first‐order loss rate was estimated to be 7 × 10−6 s−1 for a mean MBL height of 700 m. H2O2 and CH2O losses from the MBL were estimated at rates of 1.3 × 10−5 s−1 for both species. The inclusion of surface deposition improved the agreement between predicted and measured concentrations of HNO3, CH3OOH, H2O2, and CH2O. However, model CH2O remained significantly greater than that measured in the MBL.

Analysis of UARS data in the southern polar vortex in September 1992 using a chemical transport model

We have used a new, isentropic‐coordinate three‐dimensional chemical transport model to investigate the decay of ClO and evolution of other species in the Antarctic polar vortex during September 1992. The model simulations cover the same southern hemisphere period studied in a companion data paper by Santee et al. [this issue]. The model is initialized using the available data from the Microwave Limb Sounder (MLS) and Cryogenic Limb Array Etalon Spectrometer (CLAES) on the Upper Atmosphere Research Satellite (UARS). During the model initialization chemical inconsistencies in the UARS data became evident. Fields of odd nitrogen (NOy) derived from CLAES N2O underestimated the sum of the direct observations of the major NOy species. Results from the model integrations at 465 K and 585 K are sampled in the same way as the various UARS instruments and compared to the observations both directly and by considering average quantities in the inner and edge vortex regions. Sampling the observed species in the same way as the UARS instruments is important in removing any spurious trends due, for example, to changing solar zenith angle. While the model can reproduce the magnitude of the MLS ClO observations at 585 K, this is not possible at 465 K. The model partitions too much ClO into Cl2O2 to reproduce the observed ClO which is around 2.0 parts per billion by volume (ppbv) averaged within the polar vortex. The model also underestimates CLAES ClONO2 in the inner vortex at 465 K due to heterogeneous processing. The observations require that effectively all of the inorganic chlorine is in the form of ClO and ClONO2 in the inner vortex at this altitude. In the basic model run, the decay of ClO produces ClONO2 which is not observed by CLAES. Our results indicate the potential importance of the speculative reaction between OH and ClO producing HCl for the recovery of HCl in the Antarctic spring. By including this reaction, the decay of model ClO into HCl is enhanced, yielding better agreement with HCl data from the Halogen Occultation Experiment (HALOE) data. Similar results can also be obtained by including the reaction between HO2 and ClO to produce HCl with a 3% channel. The model generally reproduces the observed O3 destruction during September. The most significant discrepancy for O3 is in the inner vortex at 465 K where the model underestimates the observed O3 loss rate, especially when the effects of vertical motion are included.