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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, D17311, doi:10.1029/2004JD005713, 2006

Chemical reaction pathways affecting stratospheric and mesospheric ozone J. Lee Grenfell,1,2 Ralph Lehmann,3 Peter Mieth,1,4 Ulrike Langematz,1 and Benedikt Steil5 Received 17 December 2004; revised 7 March 2006; accepted 29 May 2006; published 14 September 2006.

[1] Catalytic cycles and other chemical pathways affecting ozone are normally

estimated empirically in atmospheric models. In this work we have automatically quantified such processes by applying a newly developed analysis package called the ‘‘Pathway Analysis Program’’ (PAP). It used modeled chemical rates and concentrations as input. These were supplied by the ‘‘Module Efficiently Calculating the Chemistry of the Atmosphere’’ MECCA box model, itself initialized by the Free University of Berlin Climate Middle Atmosphere Model with Chemistry. We analyzed equatorial, midlatitude and high-latitude locations over 24-hour periods during spring in both hemispheres. We present results for locations in the lower stratosphere, upper stratosphere and midmesosphere. Oxygen photolysis dominated (>99%) in situ ozone production in the equatorial lower stratosphere, in the upper stratosphere and in the mesosphere. In the lower stratosphere midlatitudes the ‘‘ozone smog cycle’’ (already established in the troposphere) rivaled oxygen photolysis as an in situ ozone source in both hemispheres. However, absolute ozone production rates in midlatitudes were rather slow compared with at the equator, typically 16–50 ppt ozone/day. In the equatorial lower stratosphere, five catalytic sinks were important (each contributing at least 5% to chemical ozone loss): a HOx cycle, a HOBr cycle and its HOCl analog, a water-HOx cycle and a mixed chlorine-bromine cycle. Important in midlatitudes were the HOx cycle, a NOx cycle, the HOBr cycle and the mixed chlorine-bromine cycle. In lowerstratosphere high latitudes, the chlorine dimer cycle and the mixed chlorine-bromine cycle dominated in both hemispheres. A variant on the latter, involving BrCl formation, also featured. In the upper stratosphere high latitudes (where strong negative ozone trends are observed), a nitrogen cycle, a chlorine cycle, and a mixed chlorinenitrogen cycle were found. In the mesosphere, three closely related HOx cycles dominated ozone loss. Citation: Grenfell, J. L., R. Lehmann, P. Mieth, U. Langematz, and B. Steil (2006), Chemical reaction pathways affecting stratospheric and mesospheric ozone, J. Geophys. Res., 111, D17311, doi:10.1029/2004JD005713.

1. Introduction [2] The groundwork for stratospheric ozone photochemistry was discussed by Chapman [1930] and expanded by Bates and Nicolet [1950] amongst others. Oxygen photolysis (l < 242 nm) in the tropics was first established as the dominant photochemical source of stratospheric ozone. In the 1970s, ozone sinks involving NOx- and ClOx-catalyzed

1

Institut fu¨r Meteorologie, Freie Universita¨t Berlin, Berlin, Germany. Now at Institut fu¨r Planetenforschung, Deutsches Zentrum fu¨r Luftund Raumfahrt, Berlin, Germany. 3 Alfred-Wegener-Institut fu¨r Polar- und Meeresforschung, Potsdam, Germany. 4 Now at Institut fu¨r Verkehrsforschung, Deutsches Zentrum fu¨r Luftund Raumfahrt, Berlin, Germany. 5 Max-Planck-Institut fu¨r Chemie (Otto-Hahn-Institut), Mainz, Germany. 2

Copyright 2006 by the American Geophysical Union. 0148-0227/06/2004JD005713

chemistry were established [Crutzen, 1970; Molina and Rowland, 1974; Stolarski and Cicerone, 1974]. A comprehensive review is provided by the World Meteorological Organization (WMO) reports [World Meteorological Organization (WMO), 1995, 1999, 2003]. The contribution of the various cycles varies markedly with altitude, latitude, season and aerosol loading. [3] In the polar lower stratosphere, numerous studies have investigated the cycles associated with rapid springtime ozone decline, now mainly attributed to a chlorine monoxide dimer cycle [Molina and Molina, 1987] and to mixed chlorine and bromine cycles [McElroy et al., 1986; Anderson et al., 1989]. In the midlatitude lower stratosphere, in situ measurements have implied that hydrogen family (HOx) cycles typically account for one half of the overall photochemical ozone sink, and chlorine family (ClOx) cycles may account for a further third [Wennberg et al., 1994; Garcia and Solomon, 1994]. Lee et al. [2002] quantified modeled ozone loss from (specified) catalytic cycles in the southern hemisphere (SH) lower

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GRENFELL ET AL.: STRAT/MESOS OZONE CHEMICAL PATHWAYS

stratosphere. They calculated in situ loss occurring at high latitudes and estimated its impact upon the midlatitudes. Moreover, they found that HOx and ClOx sinks dominated ozone loss in the midlatitudes. Millard et al. [2002] drew broadly similar conclusions for the northern hemisphere (NH), but noted that results were sensitive to the winter-spring meteorology. Fahey et al. [2000] quantified high-latitude summertime ozone loss on the basis of established cycles using species concentrations derived from field measurements and concluded that NOx and HOx cycles dominated. [4] In the middle stratosphere, NOx cycles are the main chemical sinks of ozone at 35 km; ClOx and NOx cycles are of similar magnitudes from 35 to 45 km, whereas HOx and ClOx cycles are the main sinks between 45 and 55 km [Osterman et al., 1997; WMO, 1999]. In the mid to upper stratosphere, the higher abundance of ground-state oxygen atoms (O) favors cycles involving this species. Models mostly underestimate ozone (the ‘‘ozone deficit problem’’) in this region. Details of the ozone deficit problem are provided by Minschwaner et al. [1993], Eluszkiewicz and Allen [1993] and Crutzen et al. [1995]. Improved chlorine chemistry has recently helped reduce this discrepancy [e.g., Lipson et al., 1997, and references therein]. [5] Lary [1996, 1997] discussed the relative importance of established stratospheric catalytic cycles over a range of latitudes and altitudes. He noted the dual importance of HOx and halogen chemistry in the midlatitude lower stratosphere and the dominance of HOx cycles in the upper stratosphere. [6] The mesosphere may act as an ‘‘early warning’’ system for climate change especially in high latitudes [Thomas, 1995, and references therein]. Model studies with coupled chemistry in the mesosphere [Zhu et al., 1999; Sonnemann et al., 1998] implied that HOx chemistry is important for ozone change. HOx is produced mainly via Lyman-alpha photolysis of water vapor from 60 to 80 km, the concentration of which varies seasonally and latitudinally [Jackson et al., 1998]. Mesospheric ozone displays a strongly altitude-dependent diurnal cycle [e.g., Zommerfelds et al., 1989] due to daytime photodissociation and nighttime reformation from molecular and atomic oxygen. Mesospheric chemistry is further complicated by sporadic phenomena such as electron precipitation events [Aikin and Smith, 1999] and meteoric trails [Summers and Siskind, 1999]. [7] In this work, reaction pathways were automatically determined and quantified in terms of their ability to affect ozone in the stratosphere and mesosphere. For this purpose, we took chemical model output (reaction rates and concentrations) and applied a new diagnostic tool (written by R. Lehmann, [email protected]) termed the ‘‘Pathway Analysis Program’’ (PAP). We wanted to investigate the dependence of the results on latitude and height and also to demonstrate the application of PAP to sensitivity analysis (effect of temperature, humidity etc.). We therefore investigated equatorial, middle and high latitudes for both hemispheres in the lower stratosphere in spring. We also analyzed the upper stratosphere at high latitudes (where negative ozone trends are strongest) and the equatorial mesosphere. Section 2 describes the procedure and

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models used; section 3 shows results; section 4 provides a discussion and conclusions.

2. Procedure and Models 2.1. Procedure [8] We took chemical output from a general circulation model (GCM) run and used it to initialize runs with a photochemical box model. We did this because the box model was able to follow a diurnal cycle in the chemistry unperturbed by transport, and it employed a shorter chemical time step (10 min compared with 45 min for the GCM). Also, the box model included bromine chemistry. We took GCM data beginning midnight 2 March in the NH and 1 September in the SH and chose locations at the equator, midlatitudes (grid box closest to 45, i.e., 42N), and high latitudes (70N, 68S) for the lower stratosphere. We chose model levels as close as possible to a constant isentrope in the lower stratosphere, corresponding to 102.7 hPa at high latitudes, 73.4 hPa at midlatitudes and 50.7 hPa at the equator. For the NH, we investigated two cases, with differing transport regimes corresponding to a cold and warm polar vortex. We also analyzed the upper stratosphere (2.68 hPa) at 70N, where strong negative ozone trends occur, and the equatorial mesosphere (0.081 hPa). The GCM chemical data were used to initialize 10-day box model runs at the aforementioned locations and times. We then determined the catalytic ozone production and destruction cycles using the PAP for day 10 of the box model run. The PAP analysis took as input reaction rates and concentrations and gave as output automatically quantified catalytic cycles affecting ozone. We now describe the GCM, the photochemical box model employed and the pathway analysis program. 2.2. Berlin Climate Middle Atmosphere With Chemistry (FUB CMAM CHEM) GCM [9] The GCM runs were carried out with the Freie Universita¨t Berlin Climate Middle Atmosphere Model with online chemistry (FUB CMAM CHEM). The model has been described by Langematz [2000], Pawson et al. [1998] and Langematz et al. [2005]. It employed horizontal resolution T21 and 34 levels in the vertical up to 84 km. We introduced orographic [McFarlane, 1987] and nonorographic gravity wave drag [Hines, 1991] in a coding originally described by Manzini and McFarlane [1998]. Mieth et al. [2004] discuss this further. [10] The chemistry scheme was described by Steil et al. [1998, 2003]. It adopted an improved family approach which included coupling between families via substances included in two or more families. The scheme comprised the following tracers: CH4, N2O, H2O2, HCl, HNO3 + Nitric Acid Trihydrate (NAT), Type 2 Polar Stratospheric Clouds (PSCs), CO, CH3OOH, ClONO2, F11, F12, CH3Cl, CCl4, CH3CCl3, H2, H2O, NOx, ClOx and Ox. The improved family approach featured the use of a non steady state integration scheme to partition the families. A detailed account of the numerics is given by Steil et al. [1998, and references therein]. [11] Heterogeneous reactions were included for sulfate aerosol and for type 1 (NAT) and type 2 (ice) PSCs, as was PSC sedimentation. The chemistry scheme comprised 107

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Table 1. Modeled NOx (Here Defined as NO + NO2), ClOx (=Cl + ClO + 2Cl2O2 + HOCl) and H2Oa 70N Lower stratosphere NOx, ppbv NOy, ppbv ClOx, ppbv H2O, ppmv Upper stratosphere NOx, ppbv NOy, ppbv ClOx, ppbv H2O, ppmv Mesosphere NOx, ppbv NOy, ppbv ClOx, ppbv H2O, ppmv H2O (damp run)

42N Cold 42N Warm Equator

68S

0.0003 1.5 0.49 3.5

0.22 4.3 0.0068 10.6

0.31 3.6 0.0037 9.9

0.32 4.6 0.0093 6.9

0.0001 0.11 0.60 2.0

27.5 34.4 0.88 9.5

5.5 5.5 0.05 10.7 12.8

a The upper stratosphere was analyzed for high latitudes, where O3 trends are high. The mesosphere damp run featured a 20% increase in water compared with the mesosphere run.

gas-phase and photolytic reactions. A zero-flux boundary condition was set at the top layer for all species except NOx, which at high latitudes above 73 km varied according to season and latitude. This was intended to simulate downward fluxes from the thermosphere as observed by the Halogen Occultation Experiment (HALOE) [Steil et al., 2003]. To alleviate a cold pole, we introduced a temperature fix of 12 K in the heterogeneous chemistry module which affected equilibrium NAT and ice concentrations. The temperature-dependent deposition velocities used for calculating denitrification are also based on T + 12 K. In the model it is assumed that ice forms on large NAT particles. Therefore the sedimentation velocity of NAT and ice is assumed to be identical if ice is present, otherwise sedimentation is neglected. The particle number density of ice (nice) is height- (pressure-) dependent and chosen to be nice = 0.03/cm3 below 48 hPa, 0.05/cm3 between 48 and 37 hPa, and 0.1/cm3 above 37 hPa. [12] The GCM run employed a spin-up which entailed seven years without chemistry followed by three years with chemistry. A 20-year equilibrium run was then performed; that is, greenhouse gases (GHGs) were fixed perpetually to year 2000 conditions. Note that such a long run was not needed for the purposes of this paper, but was originally performed to investigate chemistry-climate interaction. We analyzed equatorial, midlatitude and polar scenarios. The latitudes indicated represent the center points of model grid boxes (T21) which most closely corresponded to the equator, 45, and 70 (refer to Table 1). Lower boundary source-gas concentrations were: CO2 = 375 ppmv, CH4 = 1810 ppbv, N2O = 319 ppbv, F11 = 289 ppt and F12 = 545 pptv. Concentrations of selected species for the various runs are detailed in Table 1. [13] Table 1 indicates only snapshot values; model variability will clearly result in departures from climatological values, sometimes to significant extents. The control run reproduced well the distribution of O3 in the middle atmosphere (MA). Climatological peak values of 9 – 10 ppmv O3 occurred at 10 hPa. The peak migrated seasonally across the equator in a reasonable manner.

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Despite introducing gravity waves (GWs), the model still featured a cold pole by up to 10 K and a rather strong polar night jet (PNJ). Mieth et al. [2004] have described strengths and weaknesses in the GW implementation. Long-term tracers such as CH4 were around 20% too high in a mean sense in the PNJ, reflecting the lack of subsidence in this region. The cold pole led to strong denitrification and dehydration; that is, both HNO3 and H2O values were too low by about a factor of two at high latitudes. Despite this, the GCM calculated rather weak active ClOx, especially in high latitudes because the model smeared out steep gradients across the vortex edge. 2.3. Mainz Photochemical Box Model [14] The Mainz photochemical box model included the ‘‘Module Efficiently Calculating the Chemistry of the Atmosphere’’ (MECCA) and has been described by Sander et al. [2005]. The comprehensive chemistry scheme employed the same mechanism as the European Centre for Medium-Range Weather Forecasting Hamburg Climate Model version 4 (ECHAM4). We used a stratospheric version containing 50 species and 132 reactions, including bromine chemistry. The Mainz model is initialized from GCM output of FUB-CMAM-CHEM. The Mainz box model used a similar (but not identical) version of the original FUB GCM photolysis code, i.e., the delta twostream approximation method based on Landgraf and Crutzen [1998]. Mainz photolysis rates were scaled to ensure the same, original photolysis coefficients as in the FUB-CMAM-CHEM at local noon. [15] Total bromine was set to 20 pptv [Sinnhuber et al., 2002; Schofield et al., 2004; WMO, 1999, section 7.38] for all altitudes except in the mesosphere where it was set to zero. In the real atmosphere, mesospheric Bry levels are not zero. However, bromine compounds important for ozone loss (e.g., BrO) peak in the lower stratosphere, decreasing by about an order of magnitude from about 20 km to 32 km [Dorf et al., 2006; WMO, 1999]. Therefore mesospheric bromine cycles do not feature in the runs discussed here. For the stratospheric runs, starting values for the individual inorganic bromine species were: HBr = 5 pptv, HOBr = 7 pptv, BrNO2 = BrNO3 = BrC l = Br2 = 2 pptv. The HOBr value (7 pptv) was derived from (HOBr/HBr) = 1.4 on the basis of Johnson et al. [1995]. The HBr value (5 pptv) is uncertain and may be somewhat high in the model since measurements by Nolt et al. [1997] and Johnson et al. [1995] suggest 1 –2 pptv and 2 – 3 pptv HBr in the lower stratosphere, respectively. The model calculated a midday value of (BrO/Bry) = 0.50 on day 10, i.e., [BrO] = 10 pptv, consistent with the studies of Pundt et al. [1998] and Harder et al. [1998] which suggested a range of 5 – 15 pptv. 2.4. Pathway Analysis Program (PAP) [16] An algorithm for the automatic determination of all significant pathways (= reaction sequences) in a chemical reaction system was presented by Lehmann [2004]. It requires the following input: (1) list of the chemical reactions of the system (as ASCII file); (2) concentration of all species, averaged over a time interval of interest; and (3) rates of all reactions, averaged over the same time interval. The output consists of a list of the most important pathways and their rates. From that, all pathways that

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produce, destroy or recycle a species of interest (ozone in the present study) can be selected. [17] The algorithm operates as follows: starting from the individual reactions, pathways are constructed step by step by connecting shorter (partial) pathways. For this, the chemical species in the system are consecutively considered as ‘‘branching points,’’ beginning with the most short lived species. For every branching point species Sb, each pathway Pk producing Sb is connected with each pathway Pl consuming Sb. The rate of the corresponding newly formed pathway is calculated as the rate of Pk multiplied by the probability that Sb is consumed by Pl. This latter probability (‘‘branching probability’’) can be calculated from the rate of Pl and the rate of the total consumption of Sb. [18] If a newly formed pathway contains subpathways, e.g., zero cycles, it is split into these subpathways. In order to avoid an intractable number of pathways (‘‘combinatorial explosion’’), pathways with a rate smaller than a prescribed threshold are deleted. The summary effect of all deleted pathways is calculated, so that the accuracy of the final results can be estimated. [19] In the present analysis, the input information listed in points 2 and 3 above is supplied by the chemical box model (section 2.3). The time interval of interest is 1 day. The threshold for the deletion of minor pathways was set to 108 ppb/h for the lower-stratospheric cases and to 107 ppb/h for the upper stratosphere and mesosphere, reflecting the generally larger reaction rates compared to the lower stratosphere. [20] For analyzing a time interval as long as 24 hours, there are two possibilities: (1) Average the reaction rates over short time intervals, e.g., 1 hour, apply the pathway algorithm to each of these intervals separately and add the rates (of the pathways) obtained, or (2) average the reaction rates over the whole time interval, e.g., 24 hours, and apply the pathway algorithm once to these rates. Method 2 is better suited to the analysis of the daily mean ozone destruction than method 1: the latter yielded noncatalytic ozone production pathways at sunrise, e.g., initiated by N2O5 photolysis: N2 O5 þ hv ! NO2 þ NO3 NO3 þ hv ! NO2 þ O O þ O2 þ M ! O3 þ M

At sunset the effect was reversed by noncatalytic ozone destruction pathways that reformed N2O5. Therefore, over the 24-hour period analyzed, the sunrise and sunset effects tend to cancel with respect to ozone destruction. Method 2 automatically combines them into a zero cycle. [21] Over long time intervals the branching probabilities may change. In order to estimate the impact of this effect, we analyzed the ‘‘cold’’ stratospheric midlatitude example by both methods. This example appeared to be a suitable test case, because species from five different families (Ox, HOx, NOx, ClOx, BrOx) competed in the ozone destruction. It turned out that the rates of all pathways calculated by the two methods deviated by less than 0.5% (relative to the total

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ozone destruction). On the basis of this, for the remainder of the study we applied method 2.

3. Results 3.1. Validation of the Freie Universita¨t Berlin Climate Middle Atmosphere Model With Interactive Chemistry (FUB-CMAM-CHEM-GCM) [22] Modeled ozone in the FUB-CMAM-CHEM-GCM was well captured in tropical and midlatitudes. The model underestimated adiabatic subsidence in the vortex, which implied that further adjustment of the gravity wave parameterization was required [Mieth et al., 2004]. The strong latitudinal gradients observed for active chlorine, ClONO2 and HNO3 were ‘‘smeared out’’ in the model by the excessive diffusion associated with the rather low resolution (T21). These species therefore exhibited lower concentrations by approximately a factor of two compared with observations [e.g., Santee et al., 1996, 1999]. HNO3 was further suppressed in polar regions because of excessive denitrification associated with the cold pole. Modeled CO values were consistent with those observed [de Reus et al., 2003] (who reported 30– 60 ppbv in the lower stratosphere during various aircraft campaigns from February to September over a wide range of latitudes). Modeled OH concentrations were comparable with available observations in the upper stratosphere, e.g., Jucks et al. [1998], who reported (15 – 20) 106 molecules cm3 at 0915 LT, 69N 149W on 30 April 1997. An exact comparison was not possible given the diurnal variation and the dependence on local factors such as meteorology, water vapor etc. [23] In the mesosphere, our GCM captured diurnal ozone changes satisfactorily, similar to other modeling studies [e.g., Sonnemann et al., 1998; Zhu et al., 1999]. Ozone in the model varied from around 0.2 (midday) to 1.2 ppmv (midnight) at 65 km, similar to observations [Sandor et al., 1997]. 3.2. Pathway Analysis Results [24] Figure 1 (lower stratosphere analysis) and Figure 2 (upper stratosphere and mesosphere analysis) show the most significant cycles constructed by the pathway analysis program for the various latitudes. Figures 1 and 2 include all cycles contributing individually more than 5% to the ozone production or destruction. The remaining, ‘‘missing’’ fraction of the ozone production or destruction is due to the contribution of several pathways which individually fall below this threshold. Unless specifically stated, the cycles mentioned are already established in the literature [see, e.g., Wayne, 2000; Lary, 1997; WMO, 1995, 1999]. However, even if a complete list of all possible cycles is available, it is not always straightforward to know which cycles are important under which conditions. 3.2.1. Equator [25] Oxygen photolysis at the equator (Figure 1) dominated chemical production (>99%) as expected. A straightforward HOx cycle constituted 40% of chemical loss, an HOBr cycle and its HOCl analog together made up 15%, a cycle involving water made up a further 6% and the mixed Br-Cl cycle (the same cycle generally accepted to be important for ozone loss at high latitudes [e.g., Anderson et al., 1989]) contributed 5%. Regarding the water cycle, the

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Figure 1. Twenty-four-hour ozone loss via catalytic cycles determined by the pathway analysis program for the lower stratosphere scenarios. Values (in ppbv or pptv as indicated) represent rates integrated over the 24-hour period analyzed. Percentages represent the production (loss) as a % of total ozone production (loss) over the same period. The cutoff level is 5%. ‘‘Cold’’ and ‘‘warm’’ refer to two different springs with cold and warm conditions inside the polar vortex, respectively. reaction of O(1D) with water is a well-known ozone sink in the troposphere. The PAP has traced the fate of the two OH molecules produced by this reaction. It has identified reaction with HO2 and reaction with ozone. Since the

former reaction produces a water molecule, the PAP has gone on to construct a cycle which involved ozone loss, catalyzed by destruction and subsequent reproduction of water. It is possible that the OH radicals produced partici-

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Figure 2. As for Figure 1 but for the upper stratosphere (70N, 2.68 hPa) and the mesosphere (0N, 0.08 hPa). ‘‘Damp’’ denotes a run with water increased by 20%. For the upper stratosphere, closely related cycles which differed only in the origin of O (see text above) were combined, whereas for the mesosphere we show such cycles individually.

pate in the catalytic destruction of several O3 molecules before the termination reaction OH + HO2 ! H2O + O2. However, this effect is included in the rate of the dominant HOx cycle (and the HOBr and HOCl cycles). 3.2.2. NH Midlatitudes [26] Figure 1 presents midlatitude data for two different NH springs. ‘‘Cold’’ denotes a cold, undisturbed PNJ, whereas ‘‘warm’’ denotes a warm, disturbed Polar Night Jet (PNJ). Compared to the equator where oxygen photolysis contributed over 99% of the ozone production, in the midlatitudes, oxygen photolysis contributed 58% and 39% for the cold and warm cases respectively. The PAP results in Figure 1 suggested that the so-called ‘‘smog mechanism,’’ more frequently associated with the troposphere, also played a role in the midlatitude lower stratosphere. The smog mechanism [Haagen-Smit, 1952] involves the attack of a hydroxyl radical on a volatile organic compound (VOC) such as methane or carbon monoxide. This forms a peroxy radical which can convert NO into NO2 which in turn photolyzes to release O and ultimately results in ozone formation as shown in Figure 1. The contribution from the

photochemical smog mechanism increased from 23% to 42% for the cold and warm midlatitude cases respectively, consistent with an increase in CO, from 28 to 41 ppb. There was an additional effect: ozone production in the smog cycle proceeds via the reaction of HO2 with NO. However, there is a competing reaction for HO2, namely HO2 + O3 ! OH + 2O2. This reaction proceeded more slowly for the warm case, implying less competition for HO2 in the smog cycle, hence more ozone production. The slowing in the above reaction was related to a reduction of O3 for the warm case. The ozone mixing ratio in midlatitudes is strongly dependent on transport. In an averaged sense we would expect warmer temperatures to imply more transport hence higher ozone values in midlatitudes but this need not be true for a single analysis, as the case in hand shows. [27] The possible role of the smog cycle in lower stratosphere midlatitudes is an interesting result; a caveat however is the absolute rate of the smog cycle of the order 16 – 42 pptv/day. This was rather slow compared with dynamical changes which arose from the meridional circulation transporting ozone from the tropics (where oxygen

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photolysis was the dominant in situ source). The HOx cycle contribution to ozone destruction decreased from 51% to 39% from the cold to the warm midlatitude cases in Figure 1 respectively, associated at least partly with a lowering in water vapor by 7% as well as the 30% ozone decrease already mentioned. Lee et al. [2002] and Millard et al. [2002] also found that this HOx cycle was the most effective (20 – 40%) at destroying ozone during midlatitude spring. 3.2.3. NH High Latitudes [28] Weak insolation at high latitudes implied negligible ozone production (0.01% of the rate of ozone destruction). Important ozone loss cycles were the mixed chlorinebromine cycle (48%) [McElroy et al., 1986], the chlorine dimer cycle (35%) [Molina and Molina, 1987] and a variant of the mixed chlorine-bromine cycle (10%) which involved BrCl formation (see Figure 1). 3.2.4. SH High Latitudes [29] Columns five and six in Figure 1 show results for SH high latitudes for two ClOx scenarios. The ‘‘GCM ClOx’’ scenario featured ClOx taken directly from the GCM, which was weak (around 0.5 ppbv). This was due to excessive diffusion across the vortex edge at T21 (see section 2.3). As a test we performed a sensitivity run with ClOx values of 2 ppbv. This led to the dimer cycle increasing to 55% of overall loss, compared with 43% for the GCM ClOx scenario. This run suggested a total O3 destruction (by all pathways) of around 15 ppbv/day on the level analyzed (102 hPa, i.e., about 419 K). Bevilacqua et al. [1997] however, suggested observed ozone loss of around 55 ppb/day in SH high latitudes on the 450 K isentrope. Related to this point, we checked whether ClOx deactivated during the run; it did not, remaining at 1.8 – 2.0 ppbv throughout. Rex et al. [2003] noted that models underestimate observed ozone loss, suggesting a possible reevaluation of kinetic data may be required, although this is unlikely to explain entirely the factor of three difference suggested by this work. 3.2.5. NH Upper Stratosphere [30] Results are shown in Figure 2. Note that the initial temperature calculated by the GCM was unusually cold for the period chosen to be analyzed. On timescales of days to weeks, wave activity in the model can lead to large fluctuations in temperature. In the upper stratosphere the ozone photochemistry is sensitive to temperature, e.g., via strong dependencies in the rate of the reaction between O and O3. As a test, we performed an upper stratosphere run with chemical output from the GCM but with a climatological temperature. This is the run which we show in Figure 2. Although this correction led to reasonable ozone levels, there is clearly now an inconsistency between the chemical fields and the temperature. [31] In Figure 2, oxygen photolysis dominated ozone production. Nitrogen and chlorine cycles accounted for 35% and 22% of the ozone loss budget, respectively. A mixed chlorine-nitrogen cycle accounted for a further 11%. The mixed cycle was similar to the NOx cycle except that the reaction NO + O3 ! NO2 + O2 was replaced by two reactions, namely NO + ClO ! NO2 + Cl and Cl + O3 ! ClO + O2, which together have the same net effect. [32] When presenting PAP results, one can either combine the effect of closely related cycles, or show them

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separately. For example, O may be produced either directly via ozone photolysis, or via ozone photolysis forming O(1D) followed by quenching. In Figure 2, for the upper stratosphere, closely related cycles which differed only in the origin of O in this way were combined, whereas for the mesosphere we show such cycles individually. 3.2.6. Mesosphere [33] Figure 2 columns 2 and 3 show results for the mesosphere at the equator. The run marked ‘‘damp’’ (column 3) investigated a 20% increase in water vapor compared with the control. For both these runs, oxygen photolysis dominated ozone production and closely related mesospheric HOx cycles dominated ozone loss. For the ‘‘damp’’ sensitivity run, the percentage contributions in HOx (and the quenching cycles) were similar, but ozone decreased by 8.4% compared with the control (not shown in Figure 2). The decrease occurred because water increased, therefore HOx, which destroyed more O3. [34] In Figure 2, why did the damp run feature less O3 loss via HOx (12.3 ppmv) compared with the control (13.2 ppmv)? O3 production in the damp run is smaller (23.3 ppmv) than in the control (25.1 ppmv). There are three main reactions which control the fate of O in the model: (1) O + O2 + M ! O3 + M, (2) OH + O ! H + O2, and (3) HO2 + O ! OH + O2. Whereas reaction 1 produces O3, reactions 2 and 3 (together with H + O2 + M ! HO 2 + M, and O2 + hv ! 2O) lead to a zero cycle, with O being recycled instead of being converted to O3. For the damp run, the fraction of O participating in this zero cycle increased compared with the control, since H2O hence HOx were enhanced. In other words, in the damp run, the importance of an O3 zero cycle increased relative to the cycle which produced O3, so the damp run featured less overall O3 production. Further, since O3 is in equilibrium, the damp run therefore also featured less O3 destruction. This was also reflected in the rates of the individual destruction cycles. Increased HOx (in the damp run) would normally be associated with higher rates of O3 destruction via HOx cycles. This work suggests an opposing effect if it is taken into account that all oxygen atoms O contribute to the production of Ox = O+O3 (by definition), but not all of them lead to the formation of O3 molecules. 3.3. Families of Pathways [35] Up until now we have discussed only the effect of individual (dominant) pathways. We now consider the effect of all pathways together. To do this, species were first assigned to families as described below. Then, pathways were assigned to families according to which species reacted in the reactions of those pathways. If a pathway belonged to more than one family, its contribution to ozone destruction was distributed evenly between families. Species were assigned to families as follows: Ox = (O, O(1D), O(3P)), HOx = (H, OH, HO2, H2O2, HOCl, HOBr), NOx = (N, NO, NO2, NO3, N2O5, ClNO3, BrNO2, BrNO3), ClOx = (Cl, ClO, Cl2, Cl2O2, OClO, HOCl, BrCl, ClNO3), and BrOx = (Br, BrO, Br2, HOBr, BrCl, BrNO2, BrNO3). A reaction was considered to be an Ox family reaction only if at least two reactants were Ox species, because otherwise all ozone loss cycles would belong to the Ox family. Table 2 summarizes PAP output assigned according to families.

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Table 2. Contribution to in Situ Ozone Loss From Ox, HOx, NOx, ClOx and BrOx Cycles for the Various Latitudes Analyzeda Family

70N

42N Cold

42N Warm

Equator

68S

5 62 11 9 13

4 51 19 9 17

3 65 9 11 12

0 1 0 77 22

0 100 0 0 0

Lower stratosphere Ox 0 HOx 2 NOx 0 ClOx 67 BrOx 31 Upper stratosphere Ox 10 HOx 13 NOx 43 ClOx 34 BrOx 0 Mesosphere Ox HOx NOx ClOx BrOx a

Contribution is in %. The mesosphere result is valid for both the control and the damp runs. The 68S run featured 2 ppbv ClOx.

3.3.1. Comparison of Quantified Ozone Loss for Published Works [36] Table 3 compares results of published works which have quantified catalytic ozone loss. In Table 3, clearly a direct comparison is difficult because of differences in space and time. Johnston and Podolske [1978] analyzed stratospheric ozone photochemistry by considering an extended oxygen family definition (not including bromine) hence determining a net chemical production and destruction rate for this family. A similar technique was applied by Crutzen et al. [1995] (Table 3) who used HALOE data plus a chemical box model, thirteen reactions from which were identified to be most relevant for the ozone budget (their Table 1). Note that the lower boundary of the Crutzen et al. data is near 25 km, whereas our results are for 20.5 km. From 25 to 20 km, HOx-cycle contributions increase with decreasing height whereas for NOx-cycles the reverse is true. This explains some of the discrepancy in %HOx and %NOx in Table 3. Also, NOx concentrations in our model are rather low, as already discussed. The Wennberg and Jucks studies quote errors in the lower stratosphere of up to a factor two to three arising, e.g., because of reaction rate and transport uncertainties. Also, the Wennberg study was affected by enhanced sulphate aerosol from the Pinatubo eruption. 3.3.2. Comparison of Analysis Methods [37] Here we investigate the effect of applying different analysis methods which quantify ozone loss using the same input data, i.e., the equator data from the FUB GCM. Table 4 shows results for three differing analysis

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approaches. Analysis (a) is based on a modified odd oxygen family, Ox*, definition: Ox* ¼ O3 þ O3 P þ O1 D þ NO2 þ 2NO3 þ 3N2 O5 þ HNO3 þ HNO4 þ ClO þ HOCl þ 2Cl2 O2 þ 2ClONO2 þ HO2 þ BrO þ 2BrONO2

This Ox* is an extension of the odd oxygen family in the work by Crutzen et al. [1995]: we have added BrO, 2BrONO2 (analogous to 2ClONO2), and HO2 (to prevent the recycling reaction NO + HO2 ! NO2 + OH from being counted as a source). As in the Crutzen work, we take into account how many Ox* molecules are destroyed or produced by a particular reaction. Then production and destruction reactions are assigned to a particular family as described at the start of 3.3. If a reaction belongs to two families then half of its rate is assigned to both. In Table 4, analysis (a) uses the Ox* definition. Since we wanted to investigate how the results of the method depend upon the definition of the odd oxygen family, analysis (b) uses the following extended family: Ox** ¼ Ox* þ 2OClO:

Analysis (c) shows results from the PAP method for the equator, as already shown in Table 2: [38] Table 4 shows that the % contribution of a particular family shows a small dependence upon how one defines the oxygen family. The PAP results, which, unlike analyses (a) and (b) are derived by constructing pathways and cycles, lie roughly in the midrange between results from analyses (a) and (b).

4. Discussion and Conclusions [39] The PAP has proved a new and innovative tool for analyzing the output of chemistry models. Lehmann [2004] has presented a full description. The analysis required only chemical output at given grid points and did not involve any alteration of the model source code. The PAP provides a more reliable means of quantifying catalytic cycles compared to the usual approach of estimating them empirically. [40] An interesting result is the potentially important role of the ‘‘ozone smog mechanism’’ (established in the troposphere) operating in the lower stratosphere. Nevertheless, results reflect only in situ chemical rates of change. Therefore ozone produced in the tropics via oxygen photolysis, then transported poleward results in a dynamical rate of change at higher latitudes, hence will not be analyzed by the PAP there. In this sense our results will overestimate the role of the smog mechanism. Also, our GCM tracer fields were rather problematic at high latitudes because of a cold

Table 3. Comparison of Techniques Quantifying Ozone Loss % Loss Reference

Latitude

Height

Date

Ox

HOx

NOx

ClOx

BrOx

Crutzen et al. [1995] based on HALOE Jucks et al. [1996] (radical method) Wennberg et al. [1994]

23S 34N 38 – 50N mean

25 km 20 km 20 km

Jan 1994 Sep 1989 Apr/May 1993

8 not quoted not quoted

25 38 50

55 40