Ozone distribution in the middle latitude ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D04305, doi:10.1029/2010JD014393, 2011

Ozone distribution in the middle latitude mesosphere as derived from microwave measurements at Lindau (51.66°N, 10.13°E) P. Hartogh,1 Ch. Jarchow,1 G. R. Sonnemann,1,2 and M. Grygalashvyly2 Received 21 April 2010; revised 22 September 2010; accepted 27 September 2010; published 22 February 2011.

[1] Mesospheric ozone measurements between 50 and 80 km altitude were carried out in middle latitude at Lindau (51.66°N, 10.13°E), Germany, by means of the microwave technique between 1993 and 2006, with an interruption from 1997 to 1998. We utilize data obtained between 1998 and 2006. The measurements show a tendency of typical ozone features that occurs in each year. The most marked patterns are a late summer maximum of ozone in the middle and lower mesosphere which is shifted into autumn and winter in the stratopause region, and a distinct nighttime maximum around 72 km during the winter season, whereas no annual maximum occurs there during daytime. A clear annual asymmetry of the nighttime ozone distribution exists in this domain, marked by a decline of the mean ozone values in January/February and an increase to a subsidiary annual maximum a few kilometers higher in March/April. This asymmetry at the height of the well‐known middle mesospheric maximum of ozone (MMM, often called the tertiary ozone maximum) results from the asymmetric occurrence rate of sudden stratospheric warmings (SSWs) occurring more frequently after winter solstice than before. Additionally, the asymmetric annual variation of water vapor with lowest values just around spring equinox influences the annual variation of ozone. A strong influence on the nighttime ozone concentration is the zonal wind. The night‐to‐day ratios (NDRs) in the middle to upper mesosphere display a distinct winter anomaly marked by values more than twice as high as in summer. The NDR is modulated by pronounced oscillations with a planetary time scale. The maximum effect occurs at 65–70 km, clearly below the height of the MMM. Citation: Hartogh, P., Ch. Jarchow, G. R. Sonnemann, and M. Grygalashvyly (2011), Ozone distribution in the middle latitude mesosphere as derived from microwave measurements at Lindau (51.66°N, 10.13°E), J. Geophys. Res., 116, D04305, doi:10.1029/2010JD014393.

1. Introduction [2] Besides water vapor, ozone is the most important key constituent for understanding all aeronomic processes of the middle atmosphere. It governs both the chemistry and the dynamics by absorption of solar shortwave radiation and emission of infrared radiation. While the stratospheric ozone was a subject of intense investigation in the context of the anthropogenic decline of the ozone layer, the mesospheric ozone did not attract this attention. The global models were often confined only to the stratosphere. However, it becomes more and more clear that the atmosphere can only be understood if it is considered as a unity. There are important exchange processes between the layers, and consequently the

1 Max‐Planck‐Institute for Solar System Research, Katlenburg‐Lindau, Germany. 2 Leibniz‐Institute of Atmospheric Physics at the University of Rostock in Kühlungsborn, Ostseebad Kühlungsborn, Germany.

Copyright 2011 by the American Geophysical Union. 0148‐0227/11/2010JD014393

mesosphere has recently moved more to the center of scientific interest. [3] With the beginning of the satellite era in the early 1970s, the mesospheric ozone became a subject of intense exploration using the occultation technique [e.g., Hays and Roble, 1973; Riegler et al., 1976, 1977; Lippert et al., 1976; Sonnemann et al., 1984; Ohle et al., 1984; Fichtelmann and Sonnemann, 1989; Fussen et al., 2000; Kyrölä et al., 2006]. The Halogen Occulatation Experiment (HALOE) on board the Upper Atmosphere Research Satellite (UARS) uses this method. A large body of publications exists about mesospheric ozone measurements by means of various further techniques including microwave measurements [e.g., Trinks, 1975; Krueger et al., 1980; McPeters, 1980; Rusch et al., 1983; Thomas et al., 1983, 1984; Vaughan, 1984; Keating et al., 1987; Bevilacqua et al., 1990, 1996; Takahashi et al., 1996; Ricaud et al., 1996; Froidevaux et al., 1996; Pumphrey and Harwood, 1997; Sandor et al., 1997; Summers et al., 1997a, 1997b; Sandor and Clancy, 1998; Marsh et al., 2002; Kaufmann et al., 2003; Hartogh et al., 2004; Degenstein et al., 2005; Seppälä et al., 2006; Sofieva et al., 2009; Smith et al., 2009]. Since then different empirical models

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HARTOGH ET AL.: OZONE DISTRIBUTION IN THE MIDLATITUDE

of mesospheric ozone have been established [Krueger and Minzner, 1976; Sonnemann et al., 1984; Keating et al., 1990, 1996]. The measurements were used to derive an ozone reference model by Keating et al. [1990] and the so‐ called improved model by Keating et al. [1996]. In the international Network for the Detection of Atmospheric Composition Change (NDACC, www.ndacc.org) there are more than 70 high‐quality, remote‐sensing research stations for observing and understanding the physical and chemical state of the stratosphere and upper troposphere and for assessing the impact of stratosphere changes on the underlying troposphere and on global climate. The network began to operate in January 1991. The monitoring of ozone is one of the most prominent goals of this network. The microwave measurements at Lindau discussed in this paper are not part of the NDACC network. [4] A so‐called tertiary ozone maximum was detected in high latitudes at around 72 km altitude during the winter season [e.g., Froidevaux et al., 1996; Jarchow et al., 2000; Marsh et al., 2001; Hartogh et al., 2004; Degenstein et al., 2005; Seppälä et al., 2006; Kyrölä et al., 2006; Sofieva et al., 2009; Smith et al., 2009]. The term “tertiary ozone maximum,” indeed a somewhat confusing term as it is located between the primary permanent main maximum and the almost permanent secondary ozone maximum, only reflects the historical sequence of its detection. In addition to this, it is not a permanent feature of the ozone mixing ratio (even less of the number density). These facts were the reason for calling this phenomenon “middle mesospheric maximum of ozone” (MMM) by Hartogh et al. [2004], a term which we will also use. Although 3D‐model calculations reproduced this phenomenon [Sonnemann et al., 1994; Körner, 2002], it was not appreciated as a separate effect. This situation changed as the effect was also measured. We will discuss the MMM in more detail in section 4.1. [5] Although the general behavior of mesospheric ozone is widely known, there are many details which are still not understood. In particular, the long‐term ground‐based measurement above a fixed geographic point is a useful mean to investigate the variations both in time and in altitude. [6] A powerful tool for continuous monitoring of the mesospheric ozone is the microwave technique. In Lindau (51.66°N, 10.13°E), ground‐based millimeter wave measurements were carried out from April 1993 to October 1995 and from December 1998 until now, detecting the rotational transition of ozone at 142 GHz. Between October 1995 and June 1996, the instrument operated at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) facility in Norway. We used the results at Lindau to investigate the mean behavior of ozone in middle latitude in the mesosphere (50–80 km) and discuss some interesting phenomena of the mean annual ozone variation. In section 2 we describe the instrument very briefly. In section 3 we present the results and consider first the mean annual variation of ozone and then their night‐to‐day ratio (NDR). In section 4 the results will be discussed in terms of dynamics and chemistry considering different parameters such as the wind components, the temperature or sudden stratospheric warmings (SSWs), and planetary waves. Finally, in section 5 we summarize the most important results and refer to the 3D

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calculations on the basis of a new model of the dynamics and chemistry of the middle atmosphere.

2. Microwave Technique [7] The investigation of atmospheric trace gases by ground‐based millimeter wave heterodyne spectroscopy started in the mid‐1970s with the observation of carbon monoxide at 115 GHz [Kakar et al., 1976] in the atmosphere of Venus. Wilson and Schwartz [1981] and Lobsiger and Künzi [1986] demonstrated the capability of this technique by detecting the diurnal variation of ozone in the Earth mesosphere at 110 and 142 GHz, respectively. Zommerfelds et al. [1989] provided the first quantitative analysis of the mesospheric diurnal variation of ozone between 54 and 76 km altitude based on observations of the 142 GHz ozone line. Ricault et al. [1991] reported on the diurnal and seasonal variation of ozone between 33 and 55 km based on 110 GHz observations. Pardo et al. [1998] observed eight different ozone lines between 142 and 359 GHz and compared the ozone profiles retrieved from the spectra. In the new millennium a number of comparison campaigns between ground‐based and satellite‐borne observations took place, e.g., recently those by Boyd et al. [2007] and Hocke et al. [2007]; however, a long‐term data set based on mesospheric ozone observations has not been presented and analyzed thus far. [8] Our instrument was developed in 1990/1991. The original device and refurbishments have been described by Hartogh et al. [1991] and Hartogh and Jarchow [1995]. The instrument consists of a so‐called radiometer front end and a spectrometer back end. The front end is a heterodyne receiver which detects the 142.175 GHz ozone line. The atmospheric signal is first filtered by a Martin‐Puplett single sideband filter, then combined with a local oscillator signal using a folded Fabry‐Perot, and afterward fed back into a cooled single‐ended Schottky mixer. The mixer then provides a down‐converted signal which is amplified and finally analyzed in the spectrometer back end. The spectrometer back end, of specific importance for accurate mesospheric measurements, is described by Hartogh and Hartmann [1990], and a refurbished version is described by Hartogh and Jarchow [1994] and Hartogh [1998]. The back end consists of a broadband filter bank covering 1.2 GHz bandwidth and a high‐resolution (44 kHz) chirp transform spectrometer covering the inner 40 MHz with 1024 equidistant channels; i.e., the channel spacing is smaller than the frequency resolution. Data are taken with a fixed elevation angle of 30° and calibrated every 4 s using two external reference loads at 77 K and ambient temperature. [9] The instrument has a single sideband noise temperature of 500 K (the older, nonrefurbished 660 K) which allows retrieval of mesospheric ozone profiles every few minutes. However, a longer integration time is usually applied in order to obtain a better altitude resolution. Here we show single day and night averages. The resulting high‐resolution spectra allow the retrieval of middle atmospheric ozone profiles from 15 km to approximately 80 km. The essential component for analyzing the data is a radiation transfer model calculating the atmospheric brightness at ground level as a function of the ozone profile. In addition to this profile, the following additional parameters need to be specified: tem-

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perature profile, pressure profile (both according to the National Centers for Environmental Prediction (NCEP)), line intensity and partition function (according to Poynter and Pickett [1985]), and pressure broadening coefficient Dn C = (2.5 MHz mbar−1)p(300 K/T)0.7 [Rosenkranz, 1993, p. 86]. The inversion of the spectra has been carried out by means of the optimal estimation method given by Rodger [1976]. This algorithm needs the radiation transfer model yn = F(xn) which relates to an ozone profile xn, to the corresponding spectrum yn, to the measured spectrum y, to its covariance matrix Sy, and to a so‐called a priori profile x0 providing just that part of profile information that cannot be derived from measurement. The use of additional a priori information makes this technique especially suited for ill‐posed inversion problems. [10] A typical ozone profile for Lindau and ALOMAR has been employed as an a priori profile for all retrievals. The a priori covariance matrix S0 has been set to reflect a volume mixing ratio uncertainty of ±1 ppmv at each altitude. We separated the measured data into daytime and nighttime spectra, which then have been integrated into a single daytime and nighttime spectrum for each day. For more details, see the report by Jarchow and Hartogh [1994]. The typical signal‐ to‐noise (SNR) ratio of any retrieved spectrum presented in this study is >1000, providing a vertical resolution between approximately 7 km (middle stratosphere) and 10 km (middle and upper mesosphere). This SNR is sufficient to resolve the transition region of collisional and Doppler broadening at around 75 km of the 142 GHz ozone line. Therefore, the maximum altitude we retrieve is 75 or 80 km, depending on whether we apply the maximum of the uppermost averaging kernel or its upper full width half maximum (compare, e.g., Ricaud [1991]) as a criterion. Note that the 142 GHz ozone line is the best compromise detecting upper mesospheric ozone, because (1) the line amplitude is rather large (compared, for instance, with the 110 GHz line), (2) the frequency is still rather low (i.e., the transition region of the collisional/ Doppler broadening appears at a considerably higher altitude than those for the even stronger transitions beyond 230 GHz), and (3) the tropospheric transmission is high compared to the stronger lines above 230 GHz. [11] Uncertainties in the temperature profile of 10 K RMS (root mean square) caused an error of less than 5% in the retrieved profile. We have chosen a temperature uncertainty of 10 K RMS in our error estimation since Lübken and von Zahn [1991] showed that the fluctuations of the temperature at Andenes, Norway (69°N), deviate from the CIRA 86 climatology between about 5 K RMS in summer and less than 15.1 K RMS in winter in the altitude range from 50 to 80 km. The same order of temperature deviation to CIRA 86 was found for middle latitudes [Rauthe, 2008]. The estimated error of the microwave measurements at Lindau amounts to about 5% [Jarchow, 1999]. [12] During daytime the small ozone values range only slightly above the noise level, so that the calculated night‐to‐ day ratio can be influenced by the noise of the daytime data. The noise may also be responsible for an overestimation of ozone in the two upper panels at 75 and 80 km. In case the number density of ozone is relatively large above 80 km (for instance, at the secondary ozone maximum roughly 10 km higher), these values can also amplify the inferred ozone retrieval. One must consider the values at 80 km with caution,

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as frequently an ozone minimum occurs just at this height, above which ozone strongly increases again [e.g., Fussen et al., 2000; Kaufmann et al., 2003; Hartogh et al., 2004; Degenstein et al., 2005].

3. Results 3.1. Annual Variation of Mesospheric Ozone in Middle Latitude [13] First, we will present our observations, followed by an interpretation of the findings. Ozone chemistry is very complex and depends on both chemical and dynamic influences. In order to understand the particularities of the annual variation in ozone, we must assess and discuss all impacts separately. This may seem somewhat tedious, but this is seemingly the only way to understand the different partially contradictorily acting influences. [14] Figure 1a shows the single night averages of the ozone mixing ratio (called nighttime ozone values) at Lindau between 50 and 80 km for different heights for the period December 1998 to December 2004 (black points) and the running average (red lines) using a Gaussian function with an 8 day full width at half maximum (FWHM). Generally, the gaps of the measurements have been linearly interpolated. Figure 1a clearly exhibits the recurrence of typical annual patterns, but it also displays pronounced short‐term variations, particularly in the middle to upper mesosphere and at the stratopause. Figure 1b depicts the same state of affairs as shown in Figure 1a but for the daytime values. There are no strong annual and short‐term variations in the upper domain. Typically, the strongest variations occur around the stratopause. [15] Figure 2 compares a mean annual variation of the nighttime ozone values based on the diurnally averaged data of all available years using for presentation the first six harmonics of a Fourier analysis (red line) with the running mean for the years 2003/2004 according to results shown in Figure 1a (black lines). The period 2003/2004 belongs to a period of mean solar activity. In December 2003/January 2004 an SSW event occurred which may be responsible for the ozone increase at 75 km and above. Several proton events occurred in October and November 2003, forming nitric oxide [Seppälä et al., 2004], but these events had obviously no distinct influence on ozone at Lindau. The chemistry in the middle latitude mesosphere is almost pure: an odd oxygen‐ odd hydrogen chemistry [Crutzen et al., 1995]. An ozone decrease at 50 km in December 2003 could be interpreted by positive feedback between ozone and the ozone dissociation rate introduced by Sonnemann and Hartogh [2009]. The first six harmonics consider intraannual ozone variations possessing time scales between 1 year and 2 months. It suppresses all variations smaller than 2 months. A maximum occurs between 65 and 80 km around winter solstice. It is most pronounced at 70 and 75 km. For both upper panels of Figure 2 the winter maximum is split. Clearly a subsidiary peak occurs in March–April. Above 65 km after summer solstice the annual minimum appears. At 65 km a slight secondary summer maximum can be recognized (a weak semiannual variation) which becomes dominant for the lower panels in Figure 2. Below 65 km the annual period is subjected to a phase jump of approximately 180°. For lower altitudes the annual ozone maximum is shifted from summer

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Figure 1a. Nighttime ozone mixing ratio at Lindau (51.66°N, 10.13°E) between 50 and 80 km for different heights for the period December 1998 to December 2006 (black points) and the running mean using a Gaussian function with an 8 day FWHM.

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Figure 1b. Same as Figure 1a but for daytime values.

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Figure 2. Mean annual variation of ozone showing a time interval of 2 years according to the measurements exhibited in Figure 1a derived on the basis of a Fourier analysis displaying the sum of the first six harmonics (red line). Also shown is the running mean for the years 2003/2004 using the Gaussian function with an 8 day FWHM.

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Table 1. Mean Ozone Concentration, Amplitude of the Annual Period, and Phase of the Occurrence of the Annual Amplitude for Different Heights According to a Fourier Analysis Altitude (km)

Average (ppmv)

Annual Amplitude (ppmv)

Phase (days)

80.0 77.5 75.0 72.5 70.0 67.5 65.0 62.5 60.0 57.5 55.0 52.5 50.0 47.5 45.0

1.108 1.285 1.431 1.477 1.385 1.233 1.151 1.182 1.340 1.600 1.853 2.052 2.335 3.088 4.222

0.2724 0.3401 0.4001 0.4317 0.3957 0.2739 0.0956 0.0852 0.1790 0.1757 0.1051 0.0603 0.1375 0.1814 0.1468

26.50 18.78 11.25 5.70 0.36 354.45 339.64 190.94 177.02 175.90 186.01 267.49 306.01 307.24 289.05

into fall with decreasing height. The deviations of the current ozone values from the mean annual variation are most marked at the stratopause at 50 km. Table 1 lists, according to a Fourier analysis, the yearly averages, the amplitude of the annual period, and their respective phases in days starting on 1 January with 1. The annual period is smallest around 65 km. [16] Figure 3a displays the mean ozone mixing ratio in a contour plot based on a Fourier analysis using the first 12 Fourier coefficients. Compared with Figure 2, it exhibits more details of the intraannual variations. Figure 3a shows one after another two identical years so that the annual variation of the mean ozone‐mixing ratio during summer and particularly during winter is good to recognize without a step on the first January. As Figure 3a makes clear, a subsidiary maximum appears around spring equinox. Its altitude lies somewhat higher than that of the main maximum around winter solstice. There is a remarkable asymmetry of the ozone distribution in this region, although the solar insolation is symmetric with respect to the time from the solstices. The reason for this asymmetry we will discuss in section 4. The ozone values are clearly larger in the domain of the middle mesospheric maximum (MMM) during the first half of the winter season than in the second half. A small premaximum occurs as early as November. The phase jump below 65 km can be clearly recognized. During the summer months the strong ozone depletion at 80 km can be seen. As a simple visual inspection seems to reveal, the subsidiary maximum could result from a decrease of ozone in January/February. However, this does not explain the seasonal asymmetry, marked by a slower decrease of ozone toward the summer minimum than the respective increase toward the winter maximum. Figure 3b shows the same state of affairs as presented in Figure 3a for the daytime values. The mixing ratios are very small above 65 km, whereas below this height the annual variations with the largest values in summer are distinctly recognizable. A faint secondary maximum also occurs in winter. With decreasing height all maxima are shifted from the solstices to later days. 3.2. Night‐to‐Day Ratios of Mesospheric Ozone [17] The ozone concentration increases during the night when the photodissociation of ozone is absent and atomic

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oxygen is converted into ozone. Above about 60 km atomic oxygen is increasingly the dominant odd oxygen constituent (O and O3) during the daytime, so it is able to enhance the ozone concentration considerably during the night. Figure 4 displays the night‐to‐day ratios (NDRs) for the same period as depicted in Figure 1. The black points exhibit the ratio of the running averaged nighttime and daytime values using the same Gaussian function. The red lines represent the mean behavior according to a Fourier analysis summing up the first six harmonics. In the bottom panel the ratio is close to unity, although at the stratopause ozone varies more strongly than in both levels above. The reason is that ozone became the dominant odd oxygen species, so the conversion of atomic oxygen into ozone during the night cannot strongly change the ozone concentration. [18] In the upper stratosphere the nighttime values are even smaller by a few percent than the daytime values, as the ozone destruction is a little stronger than the contribution of the ozone formation from the atomic oxygen reservoir. The NDR increasingly begins to rise above the first two levels. A pronounced winter enhancement compared with the summer values can be recognized, particularly at the 65 and 70 km levels. Especially at 65 km the NDR is asymmetric, again with the largest values before winter solstice. The winter‐to‐ summer ratio becomes smaller in the upper two panels. For these panels an enhancement around spring equinox occurs which seems to be related to the subsidiary nighttime ozone maximum. The curves presented in Figure 4 are modulated by oscillations with a planetary time scale, meaning with periods on the order of a few weeks. The real absolute values of the NDR are probably somewhat larger in the upper two panels because, as mentioned in section 2, the daytime values are influenced by the noise of the instrument. [19] Figure 5 displays the NDR together with the individual nighttime (blue) and daytime (orange) measurements and their sliding averages for the two selected years 2004 and 2005. The black curve displays the sliding average as described above, and the red curve represents the mean variation according to the Fourier analysis shown in Figure 4. The smoothing procedure suppresses variations with periods below about 1 week.

4. Discussion 4.1. Middle Mesospheric Maximum of Ozone (MMM) [20] In the middle and upper mesosphere the enhancement of the nighttime ozone mixing ratio marked by oscillatory patterns, as shown in Figure 1a, is a regular feature at Lindau in the winter season. We define nighttime value as the average between sunrise and sunset. An explanation for the nighttime enhancement of ozone in high latitudes in winter (MMM, often called the tertiary ozone maximum) was first given by Marsh et al. [2001] and later given in more detail by Hartogh et al. [2004]. According to the suggestion of these groups, the general cause of the formation of the MMM is given by the fact that the solar radiation dissociating water vapor, producing hydrogen radicals in this process which destroy ozone, decreases faster with increasing solar zenith angle than that part of radiation dissociating molecular oxygen and thus forming ozone. There is an increasing imbalance with the rising solar zenith angle between ozone production and hydrogen radical formation. The increase of ozone for this

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Figure 3a. Contour plot of the mean nighttime ozone mixing ratio using the first 12 Fourier coefficients. The figure clearly shows two annual maxima.

increasing solar zenith angle is confined by the fact that the radiation which dissociates ozone is nearly not absorbed, even for grazing incidence of radiation penetrating the domain of the MMM [Hartogh et al., 2004]. [21] The dissociation of ozone results in the formation of O(1D), which is able to oxidize water vapor, and molecular hydrogen also producing ozone‐destroying hydrogen radicals. Hartogh et al. [2004] presented an inequality of the dissociation rates for ozone, molecular oxygen, and water vapor normalized to unity for overhead sun. A normalized dissociation rate of the constituent x is defined by the expression NJx(c, z) = Jx(c, z)/Jx(c = 0, z) with c solar zenith angle and z altitude. There is also an increasing imbalance between atomic oxygen formation from the ozone dissociation and the photolysis of molecular oxygen, almost the only term of net odd oxygen production, whereas the ozone dissociation does not form odd oxygen but only transforms one odd oxygen constituent into another. The effect of the MMM formation is strongest in the vicinity of the polar night terminator, but it extends into middle latitudes with decreasing amplitude. Yet even this explanation is still incomplete, as we will discuss later. Looking at the daytime ozone measurements, the increase is not much larger in winter compared with the summer values: at 70 km the ozone mixing ratios are rather smaller in winter than those in summer. The imbalance between odd oxygen and odd hydrogen production should already act during the daytime. Apparently, the explanation of the MMM is more complex than can be described by only a pure imbalance of the dissociation rates. [22] Another phenomenon which must be understood and interpreted consists of the seasonal asymmetry of the annual

variation of the ozone mixing ratios which are modulated by wave‐like oscillations. The strong year‐to‐year variability indicates that the effect also depends on variable conditions in the mesosphere changing considerably from year to year. The solar insolation is, of course, symmetric with regard to the solstices. The water vapor distribution shows an asymmetric annual variation, and the molecular hydrogen varies inversely to water vapor [Sonnemann et al., 2005a]. However, water vapor as source gas for the hydrogen radicals is at a maximum in late summer and a minimum at the time of the subsidiary nighttime ozone maximum [e.g., Bevilacqua et al., 1985, 1989; Summers et al., 1997a, 1997b; Seele and Hartogh, 1999, 2000; Körner and Sonnemann, 2001; Sonnemann and Grygalashvyly, 2005a; Sonnemann et al., 2005a; Hartogh et al., 2010]. The annual variation amounts to a factor of 2 between summer maximum and winter minimum. This behavior contradicts the fact that the largest ozone values are measured during the first half of the winter season as Figure 2 shows, but the late winter subsidiary ozone maximum is in accordance with the late winter water vapor minimum. The distribution of SSWs has a clear annual asymmetry. More SSWs occur in statistical mean after winter solstice (see also the ERA 40 or NCEP statistics of major SSWs: www.appmath.columbia.edu/ssws). A stratospheric warming is connected with a cooling of the mesosphere above about 65 km. This cooling starts in the lower thermosphere around a week earlier than the beginning of the stratospheric warming [e.g., Holton, 1976; Schoeberl, 1978; Walterscheid et al., 2000; Liu and Roble, 2002]. During the SSW event characteristic changes of the circulation take place [e.g., Labitzke, 1981]. For a so‐called major

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Figure 3b. Same as Figure 3a but for daytime values.

warming, the zonal wind switches from westward to eastward. The direction of the meridional wind depends on the wave 1 structure and can be directed equatorward or poleward. This is, of course, an idealized picture, and the whole event has an exceedingly more complicated and complex dynamic behavior in the middle and upper mesosphere. In a case study Sonnemann and Grygalashvyly [2003] investigated the change of composition in the mesosphere in coherence with SSWs. We will now discuss the influence of the different dynamic parameters upon the ozone chemistry particularly in the domain above 60 km. 4.2. Zonal Wind [23] First of all, we want to discuss the effect of changing dynamics on the ozone chemistry, and we start with the influence of the zonal wind on the chemistry of the mesosphere. A large body of publications has shown that the photochemical system of the mesosphere represents an enforced nonlinear chemical oscillator driven by the diurnally periodic solar insolation [e.g., Sonnemann and Fichtelmann, 1987, 1997; Fichtelmann and Sonnemann, 1992; Feigin et al., 1998; Sonnemann and Grygalashvyly, 2005b]. Resonance occurs for a driven photochemical oscillator when the period of the external periodic radiative excitation power (1 day in the Earth’s atmosphere if the zonal wind is zero) agrees with the mean system internal chemical response time of the photochemical system, also called the characteristic system time (see also Sonnemann and Fichtelmann [1997] and Körner and Sonnemann [2001] for a definition). The resonance occurs only at a distinct altitude in the upper mesosphere/mesopause region. The precise altitude, where

the period of excitation agrees with the chemical system time, depends on specific conditions, such as the water vapor mixing ratio, the ratio of daytime to nighttime hours, etc. In the mesosphere/lower thermosphere the characteristic system time decreases with decreasing height, thereby making the area in the middle mesosphere and below the system far from the resonance case. [24] The photochemical Doppler effect or zonal wind effect on the photochemistry introduced by Sonnemann [2001] consists of the fact that the period of insolation changes when an air parcel moves in a zonal direction with or against the Earth’s rotation. The period of insolation is then shortened or prolonged. Depending on the zonal wind velocity and latitude, the period can vary by many hours (+4 and −7 h within the mesospheric wind jet under climatological mean conditions calculated by a modified Doppler formula [Sonnemann, 2001]). We abbreviate this effect as DSE (Doppler‐Sonnemann effect) [Sonnemann et al., 2006b, 2007]. The characteristic chemical system time rises with increasing height from a few hours in the mesosphere to 1 day in the mesopause region and many days in the lower thermosphere. The characteristics of resonance consist of a phase shift of p/2 between external exciting force (radiation) and system response (formation of odd oxygen including ozone). Coming from the mesosphere, the daytime ozone maximum is shifted with increasing height into the sunset hours above 80 km. In this case no daytime maximum occurs. This exact effect was presented by Degenstein et al. [2005], who found an extension of the “tertiary ozone maximum” with increasing altitude into low and equatorial latitudes.

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Figure 4. Night‐to‐day ratio (NDR) for the period exhibited in Figures 1a and 1b at discrete height levels. Black points represent the ratio of the running means using a Gaussian function with an 8 day FWHM. The red line results from a Fourier analysis summing up the first six harmonics.

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Figure 5. Smoothed annual variations of the NDR for two selected years (2004/2005) and the individual nighttime (blue) and daytime (orange) ozone measurements at different height levels. The black line represents the running means using a Gaussian function with an 8 day FWHM, and the red line stands for the Fourier analysis according to Figure 4. 11 of 17

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which is dominant in the upper mesosphere becomes less important in the lower mesosphere. This cycle has a limited influence in the region of the MMM. The chief odd oxygen‐ destroying cycle runs over the reaction scheme:

net:

Figure 6. Ratio of the dissociation rates of ozone to that of molecular oxygen (upper curve) and of water vapor to that of molecular oxygen again (lower curve) normalized to unity depending on the solar zenith angle at 70 km. [25] The crucial problem in the middle mesosphere, as mentioned above, seems to be that the chemical system is far from the resonance case. However, the characteristic time ranges in the order of the duration of sunset of a few hours. As stated above, there is an increasing imbalance between odd oxygen formation by photolysis of O2 and the dissociation of ozone with rising solar zenith angle. The latter photolysis only converts one odd oxygen species (O3) into another one (O). [26] Figure 6 shows the ratios of the normalized dissociation rates of ozone to that of molecular oxygen (upper curve) and of water vapor to that of molecular oxygen again (lower curve) depending on the solar zenith angle at 70 km. The opposite behavior for growing solar zenith angles is clearly recognizable. Depending on the height, the largest quantity of O returns to O3 in the three‐body reaction O + O2 + M → O3 + M depending quadratically on the air density. This returning ozone will be subjected to further dissociation acts. (We note that the chemical lifetime of ozone during the day amounts to only about 100 s.) However, a certain quantity of atomic oxygen is included in different loss processes. The first process is given by the reaction O + O3 → 2O2. This reaction has a relatively small reaction rate and requires large ozone concentrations. It only becomes effective in the stratopause region and below, but not in the domain of the MMM. In the mesosphere different catalytic cycles act, destroying odd oxygen. Due to the fact that atomic hydrogen is effectively transformed into HO2 by the three‐body reaction H + O2 + M → HO2 + M also depending quadratically on the air density, the concentration of atomic hydrogen becomes increasingly smaller with decreasing altitudes so that the cycle

net:

O3 þ H ! O2 þ OH OH þ O ! H þ O2 O þ O3 ! 2O2

H þ O2 þ M ! HO2 þ M HO2 þ O ! OH þ O2 OH þ O ! H þ O2 O þ O ! O2

Both cycles include the reaction of O with OH. [27] The reaction rates of the reactions involving atomic oxygen are very large. Consequently, during sunset when the odd oxygen formation slows down but ozone is still dissociated producing atomic oxygen, this constituent becomes permanently lost so that finally the quantity of daytime atomic oxygen will only be transformed into nighttime ozone to a certain amount. The nighttime ozone level increases, but essentially less strongly than it would increase if all atomic oxygen were transformed into ozone. As long as atomic oxygen and hydrogen radicals are available, these cycles run. It is evident that a shortening of the duration of the time of sunset relatively increases the nighttime ozone level. Exactly this is what happens with a wintry west wind system. The zonal wind velocity during sunset is decisive for the nighttime level of ozone. This wind normally differs from the diurnally averaged wind as it is superimposed by tidal winds and other natural variations. [28] If the wind direction changes from a wintry west wind system blowing with the Earth’s rotation, shortening the time of sunset into an east wind system as a result of an SSW, the period of sunset is prolonged. This picture is also valid from the point of view of a fixed observer because the observer monitors passing air parcels also being subjected to a changed period of solar insolation. The wind reversal during a major SSW or the final warming in April has the effect of reducing the MMM. The climatologic mean of the middle atmospheric wind jet in winter has its strongest velocities at an altitude in the vicinity of 60 km at about 50° latitude. Due to the decrease of the length of the parallels toward the pole, the maximum effect is shifted into this direction. At the height of the maximum of the MMM, the daytime atomic oxygen concentration is roughly 2 orders larger in magnitude than the ozone concentration. However, the increase of the nighttime ozone amounts to only a factor of 6–8 (less than 1 order) as the NDR indicates. In other words, only a small part of O will be converted into nighttime ozone. At 60 km both daytime odd oxygen concentrations (ozone and atomic oxygen) have comparable concentrations, but the NDR ranges clearly below 2. These numbers demonstrate that the odd oxygen loss during sunset is a dominant process, and they indicate that small variations, which result from the DSE, can entail pronounced variations of the nighttime ozone level. [29] The variation of the black curve in Figure 5 looks like the time behavior of the winter anomaly of the ionospheric absorption of radio waves in the D layer being actually an anomaly of the electron density [e.g., Schwentek, 1971]. The cause of the variation of the winter anomaly in the plasma parameters consists of an NO transport from the thermosphere modulated by planetary waves. Precisely this question

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arises also for the NDR. The modulation of the zonal wind by planetary wave activity results in a modulation of the NDR. 4.3. Meridional Wind [30] Typical meridional wind velocities range on the order of a few tens of meters per second. An air parcel moving with a speed of 10 m s−1 travels 864 km d−1 or crosses about 7.8° in latitude. The chemical characteristic time of the odd oxygen family is quite variable in the domain of microwave measurements at Lindau. In the daytime it amounts to a maximum of a few hours. Under this condition a far‐range transport of odd oxygen is impossible: the concentration more or less obeys a local chemical equilibrium depending on the season, height, and latitude, and on aeronomic parameters such as temperature and humidity. The behavior is clearly different during the night when ozone becomes almost the only odd oxygen constituent. Particularly in the vicinity of the polar night terminator, the ozone distribution can be influenced most strongly in the late night hours. The monitoring station Lindau lies outside the direct domain of the MMM. The effect extends into middle latitudes only with reduced amplitude, but the ozone mixing ratio possesses still values of 2 ppmv at the maximum of the MMM, as Figure 2 demonstrates, in contrast to 1 ppmv in summer. During an SSW the meridional wind also alters its direction and velocity, but the change is not as uniform as in the case of the zonal wind and temperature. As Dowdy et al. [2004] and Hoffmann et al. [2007] showed, the wind direction depends on the planetary wave 1 structure and can be opposite at different remote locations (e.g., Poker Flat 65°N, 147°W, and Andenes 69°N, 16°E) during an SSW. Seele and Hartogh [1999] and Flury et al. [2009] found an increase of the water vapor concentration in connection with SSW events indicating a poleward transport of relatively humid air from lower latitudes. Independent of the wind direction, the meridional wind tends to decrease the amplitude of the MMM close to the polar night terminator. A wind blowing into the dark polar night region transports ozone from the MMM into this domain. Ozone is destroyed relatively slowly there. However, it reduces the ozone maximum in the MMM area as the air is replaced by air from lower latitudes which is poorer in ozone. When the wind blows toward the equator, the air directly in the MMM region is replaced by air from the polar night region marked by very small ozone concentrations, but the air coming from the MMM increases the ozone mixing ratio in middle latitudes. With respect to the meridional wind, the highest ozone concentration in the MMM domain can be found under quiet conditions. The meridional extension of the MMM is smallest then. In middle latitudes the meridional wind influences the variability of ozone according to its direction and the meridional ozone gradient. 4.4. Vertical Wind [31] The vertical wind controls the seasonal variation of long‐lived constituents such as water vapor and molecular hydrogen. Both constituents vary in opposite manner [Körner and Sonnemann, 2001; Sonnemann et al., 2005a]. While in the mesopause region the gradient of the water vapor mixing ratio is large (the mixing ratios decrease strongly with height), it is relatively weak in the domain below. An SSW is also connected to a change of the vertical wind system; however,

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this change is also not so clear as in the case of the zonal wind [Sonnemann et al., 2006a]. The cooling of the upper mesosphere in connection with an SSW mainly results from an adiabatic cooling by upward blowing winds, whereas under normal winter conditions a downward wind is responsible for the high winter temperatures above the middle mesosphere. Measurements carried out at Lindau showed that in the aftermath of an SSW the humidity also increases, but it does not reach summer values [Hartogh et al., 2010]. A certain variation of the water vapor mixing ratio is connected with the SSW, but a drastic change needs more time than the duration of an SSW event. The variation of water vapor is able to modulate the ozone concentration within certain limits during the SSW period [Sonnemann et al., 2006a]. In the lower thermosphere, the downward transport of particularly atomic oxygen has a strong influence on ozone (which increases drastically) and the other species changing their composition (see also model calculations of Sonnemann et al. [2006a, 2006b]), but below 80 km the influence is confined by the decreasing chemical lifetimes of the chemical families. The mean vertical velocity is on the order of a few centimeters per second, meaning an air parcel would be lifted or lowered by only a few kilometers per day [Fauliot et al., 1997]. However, the tidal winds are usually 1 order in magnitude larger than the mean vertical wind; consequently, the tidal wind can influence the diurnal variation of the water vapor mixing ratio in limited areas. Another possibility of vertical transport is linked by a gravity wave transport as investigated, e.g., by Hickey et al. [2000]. Gravity waves convey atomic oxygen downward from the lower thermosphere. The transport by wave‐induced vertical mixing is a robust effect and could be responsible for the oscillatory patterns found in the ozone observations. [32] Atomic hydrogen, and consequently the odd hydrogen family, has a steep gradient in the domain of the MMM, with mixing ratios increasing strongly with height. Ozone itself possesses a marked minimum of the mixing ratio at about 80 km, so a direct transport from the secondary ozone maximum into the MMM domain is excluded. However, the transport of odd oxygen takes place by a transport of all members of the family. An upward wind lowers the concentration of the odd oxygen destroyers, and the opposite effect is valid for a downward wind. However, atomic oxygen, and consequently odd oxygen, possesses a positive gradient above the MMM. There is a minimum of odd oxygen in the middle mesosphere roughly around 60 km. Thus, it is generally not that simple to estimate the net effect of the vertical wind on the MMM. This must be done for concrete conditions in the frame of an advanced model. 4.5. Temperature [33] The influence of the temperature T on the MMM is apparently relatively simple to estimate. The specific temperature dependence of the chemical reaction rates is such that the ozone concentration increases with decreasing temperature. This assertion is valid for the ozone production rate O + O2 + M → O3 + M, with k1 = (6 × 10−34)(300/T )2.3 cm6 s−1 increasing with decreasing temperature as well as for the main loss reaction H + O3 → OH + O2, with l1 = 1.4 × 10−10 exp(−470/T) cm3 s−1 decreasing with decreasing temperature. For example, a drop in T by 30 K being typical during an SSW in the upper mesosphere, the calculated ozone

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increases on the basis of these reactions amounts approximately to a factor of 2. The production of ozone is the loss of atomic oxygen, so of course the temperature dependence of all constituents must be taken into calculation. While O3 rises, O decreases, meaning the odd oxygen balance with respect to the ozone production reaction does not depend on temperature. The odd oxygen production is almost completely determined by the photolysis of O2. The most important O loss process O + OH → O2 + H with l2 = 2.2 × 10−11 exp(120/T ) cm3 s−1 becomes larger if T decreases, entailing a reduction of odd oxygen so that this effect also dampens the increase of ozone. [34] Considering the current odd hydrogen concentration, the production term of H is given by the main net loss term of O increasing with T and the main loss term by the reaction with ozone which decreases with T . Effectively, H increases and OH decreases with decreasing T. This discussion may illustrate that the net effect cannot be derived from the consideration of only one species. However, as a net effect we can state that ozone increases with decreasing temperature, but not as strongly as the simple estimation indicates if we consider only the response of the ozone production and loss reaction rate on temperature. With regard to the annual temperature variation, the assertion is valid that the enhanced wintertime temperatures dampen the ozone increase. The response drastically changes with altitude. The response also depends on local time, and it is particularly different between day and night. [35] In conclusion, the winter season asymmetry of ozone at the height of the MMM essentially seems to be caused by the frequent occurrence of SSW. In this context the subsidiary weak ozone maximum is a result of the relative ozone decrease in January/February by the SSW activity. It is amplified by small water vapor concentrations in late winter during the period before the wind reverses. The DSE plays an important role for the formation of the MMM and its variation. It influences, in particular, the NDR above 60 km close to the mesospheric winter jet. The meridional and vertical wind components also have an impact on the variability of the MMM. The varying gravity wave activity could be responsible for planetary wave‐like oscillations of the ozone mixing ratio, but this subject is still under investigation. 4.6. Ozone in the Middle and Lower Mesosphere [36] The discussion of the ozone variation in the mesosphere below about 65 km is apparently much simpler. Solar insolation, temperature, and water vapor concentration are the decisive parameters of influence. They vary partly in opposite manner and entail a complicated annual variation different at the various height levels. A late summer maximum occurs in the middle to lower mesosphere between 60 and 70 km, resulting generally from the more intense solar insolation. This maximum appears the later and the lower the altitude is. The maximum occurs for all 200 m lower approximately 1 day later. Rather, in the stratopause region a slight spring/ early summer minimum and a late autumn/winter maximum exist. The low winter temperatures in the stratosphere/lower mesosphere support the formation of a maximum, whereas the small solar insolation acts in a contrary manner. This fact may be the reason for the small minimum around winter solstice. Obviously, the strong temperature increase and the large water vapor values around the stratopause in summer

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overcompensate higher solar insolation. The temperature has its maximum in the stratopause in early summer (May/June) when the water vapor concentrations are already high. On the other hand, the final warming occurring in early spring decreases the ozone concentration when the insolation is not yet strong. Due to the short lifetimes of the odd oxygen family, a long‐distance transport of ozone is impossible in the lower mesosphere, meaning variations must be explained in terms of local chemistry. One fact to be considered is that the ozone dissociation rate depends on the ozone distribution itself. At the stratopause the relative decrease of the dissociation rate with increasing solar zenith angle and with decreasing height is strongest in the whole atmosphere. This feeds back in a nonlinear manner to the chemistry [Sonnemann et al., 2005b, 2009; Flury et al., 2009]. The ozone dissociation products O(1D) and O destroy ozone directly and via the cycle introduced in section 4.2, or they act indirectly due to the formation of hydrogen radicals from water vapor by O(1D) oxidation.

5. Summary and Conclusions [37] The mean annual variation of the mesospheric ozone in middle latitude has been derived from continuous monitoring of ozone by means of the microwave technique at Lindau (51.66°N, 10.13°E). The measurements brought evidence that the middle mesospheric maximum of ozone (MMM) regularly extends into middle latitudes. A weak subsidiary maximum of the MMM was detected in late winter/spring equinox. This subsidiary maximum was explained by the decrease of ozone after winter solstice due to the more frequent occurrence of sudden stratospheric warming (SSW) before this period and by the occurrence of the annual minimum of water vapor during this period. The SSW events are connected with a cooling of the mesosphere above about 65 km which should increase ozone. However, they are also connected with a drastic change of the dynamic patterns. The zonal wind considerably influences the night‐to‐day ratio as a result of the photochemical Doppler‐ Sonnemann effect. The magnitude of the MMM is considerably determined by this effect. The change of the zonal wind from a west wind into an east wind system reduces the NDR and reduces the nighttime ozone concentration. The NDR displays a wintertime enhancement modulated by a planetary wave‐like variation resembling the winter anomaly of the plasma parameter of the D layer. A possible reason for these patterns could be linked to the gravity wave activity. The meridional wind is able to disperse the MMM, but for a southward blowing wind it can also transport air rich in ozone from high latitudes into middle latitudes. The annual variation of the water vapor mixing ratio also considerably influences the annual variation of ozone, particularly at the upper levels as water vapor is the source gas for the hydrogen radicals destroying ozone. [38] In the middle to lower mesosphere an ozone maximum occurs in late summer. With decreasing height this maximum is shifted into autumn. A spring minimum and a broad autumn/winter maximum have been observed at the stratopause. The concrete annual ozone variations in different heights depend on the annual variations of the solar insolation, the temperature, and the water vapor concentration. There are pronounced ozone variations, particularly around

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the stratopause, which are connected with SSW events and water vapor variations. On the whole, the measured values agree fairly well with the Ozone Reference Model [Keating et al., 1990] presenting day values. However, in detail larger differences also occurred. In a paper about chemical modeling of the mesophere/lower thermosphere region we will present calculations based on a new global real‐time 3D model and compare the ozone measurements for selected years with model results for the same period. [39] Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) grants HA 3261/1‐2 and So268/4.

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