Ozone is a molecule which plays an important role in the Earth's atmosphere. It protects the biosphere from harmful UV radiation and controls the thermal structure of the stratosphere. In the middle atmosphere the ozone abundance is governed by transport processes and (photo-)chemical reactions. During a 1-year measurement campaign with ground-based microwave radiometers ozone variations can be measured on diurnal to semi-annual timescales, depending on the instrument capability. The diurnal variations of ozone are mainly controlled by photochemical and catalytic processes. The basic pattern of the diurnal variation from the stratosphere to the mesosphere is well described by a set of pure oxygen reactions derived by . O2+hν→O+OO+O2+M→O3+MO3+hν→O2+OO3+O→O2+O2

During daytime, when the atmosphere is sunlit, odd oxygen (Ox=O+O3) is produced by reaction () and is partitioned between atomic oxygen and ozone by the reactions () and (). The ozone production () depends on the abundance of a third body M and therefore on pressure. In the stratosphere this reaction is very efficient relative to () and all odd oxygen is in the form of ozone which reaches a maximum in the late afternoon. With increasing altitude reaction () gets less efficient relative to () and more odd oxygen is stored as atomic oxygen during daytime. After sunset all the atomic oxygen recombines, which leads to a night-time maximum of ozone. The night-to-day ratio of ozone in the mesosphere increases with altitude. For a realistic description of the ozone cycle additional destruction processes and transport effects have to be taken into account. This includes catalytic ozone depletion by odd hydrogen (HOx), odd nitrogen (NOx) and inorganic halogens (Cl, Br). The magnitude of the diurnal cycle in the different altitude regimes over a year is determined by the season and the latitude with their corresponding daily mean value and variation of the solar zenith angle (SZA) and length of the day. At high latitudes the diurnal cycle is highly dependent on the season because the insolation conditions vary strongly throughout the year. The period of polar night, the period of polar day and the intermediate periods with light and darkness within a day characterize the pattern of the diurnal cycle in the Arctic.

The diurnal cycle of ozone in the mesosphere has been thoroughly studied by means of ground-based microwave instruments and other techniques . A sharp decrease in ozone volume mixing ratio (VMR) after sunrise of up to 65 % at 0.1 hPa was reported. After sunset the night-time values are restored quickly.

Observations from ground-based ozone radiometers from the Network for the Detection of Atmospheric Composition Change (NDACC) were used to study the diurnal cycle in the stratosphere. found an afternoon enhancement in the observations of the ground-based microwave radiometer SOMORA at the Alpine station in Payerne, Switzerland. This enhancement is also seen in the summertime observations of OZORAM at the Arctic station at Ny-Ålesund, Svalbard . A monthly climatology of the diurnal ozone cycle was derived from 17 years of observations of GROMOS which is operated at Bern, Switzerland . used 19 years of observations by a ground-based microwave radiometer at the Mauna Loa Observatory, Hawaii, to analyse the diurnal cycle. Both studies confirm the afternoon enhancement of ozone in the stratosphere.

Satellite-borne observations were also used to study the diurnal cycle of ozone. analysed measurements of UARS MLS, which cycles through 24 h over a given location within 36 days, and derived the diurnal cycle for latitudes between 40∘ N and 40∘ S. Measurements from the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) carried by the International Space Station, which samples 24 h in 60 days, were used by to analyse the diurnal variations in the same latitude band. Both studies show that the relative amplitude of the afternoon maximum with respect to the minimum value at 3 hPa during summer are around 3 % in the tropics and around 7 % in the midlatitudes. The global behaviour of the stratospheric ozone cycle at 5 hPa and a detailed analysis of the contributing chemical reactions are discussed in a model study by .

Measurements of the diurnal cycle in the Arctic are rare. published a time series of stratospheric ozone measured by OZORAM which covers three intervals of several days during summer 2010 and reported the existence of a diurnal variation during polar day. The monthly mean diurnal cycle measured by OZORAM in June 2011 was intercompared to model reanalyses and revealed a relative amplitude of 8 % at 5 hPa .

We present an analysis of the diurnal cycle in the Arctic stratosphere and mesosphere for different insolation conditions throughout the year. The ozone time series used for this study were measured by the two ground-based microwave radiometers GROMOS-C and OZORAM which are located at Ny-Ålesund, Svalbard (79∘ N, 12∘ E), in the Arctic. It is a comprehensive data set of Arctic, middle-atmospheric ozone with a high time resolution. The chemical and photochemical reactions that lead to a diurnal cycle during polar day are studied with the atmospheric model SD-WACCM. The data sets are further intercompared to measurements with balloon-borne ozone sondes and MLS and to the simulations with SD-WACCM.

The remainder of this paper is organized as follows: Sect.  introduces the measurement campaign at Ny-Ålesund. The two ground-based microwave radiometers as well as the other instruments and model used for intercomparison are described in Sect. . Section compares the microwave radiometer measurements with measurements by MLS and balloon-borne ozone sondes and with SD-WACCM simulations. The diurnal cycle of ozone is discussed in Sect.  and measurements of the tertiary ozone maximum are presented in Sect. . Section summarizes the content and draws the conclusions.

A measurement campaign with ground-based microwave radiometers for ozone and water vapour has been taking place at the AWIPEV research base at Ny-Ålesund, Svalbard (79∘ N, 12∘ E), since September 2015. The campaign is a collaboration of the University of Bremen and the University of Bern. The University of Bremen contributes with OZORAM (Ozone Radiometer for Atmospheric Measurements) which is part of the Network for the Detection of Atmospheric Composition Change (NDACC). The University of Bern contributes with the two instruments MIAWARA-C (Middle Atmospheric Water Vapour Radiometer for Campaigns) and GROMOS-C (Ground-based Ozone Monitoring System for Campaigns). Due to its high latitude Ny-Ålesund is the ideal location for observations inside the polar vortex and for studying events related to its dynamics. The scientific focus of the campaign lies on the investigation of dynamical events such as the dynamics of the polar vortex, sudden stratospheric warming and planetary waves, as well as on the link between middle-atmospheric ozone and water vapour chemistry and the analysis of the temporal variability of those atmospheric constituents. The present study covers the temporal variability of ozone.

Svalbard is an archipelago in the Arctic Ocean and the research settlement Ny-Ålesund is located in the north-west of the main island Spitsbergen at the shore of Kongsfjorden. It is one of the northernmost permanent settlements and has been a base for Arctic expeditions and a host to research stations since the early 1900s. At the moment it hosts 15 permanent research stations from 10 different countries mainly focusing on earth and environmental sciences (www.kingsbay.no). Among them is the AWIPEV base, a joint French–German research station operated year-round (www.awipev.eu). At this station the University of Bremen operates ground-based microwave radiometers which measure atmospheric key parameters like water vapour (1998–2003), chlorine monoxide (1993–2003) and carbon monoxide (since 2017). Ozone has been measured with OZORAM since 1993, and ozone sondes have been launched on a weekly basis since 1991.

At this latitude polar day and polar night last for 4 months each. This allows one to study the ozone (photo-)chemistry during day and night conditions. Despite the long winter, the climate at Ny-Ålesund is mild because of the influence of the North Atlantic Current. The result is high humidity of the troposphere compared to other Arctic locations. This affects remote sensing observations of the middle atmosphere where the optical depth of the troposphere has to be taken into account. A low optical depth results in larger amplitudes of the line and reduces the integration time. For microwave radiometry as described in this paper the average optical depth at the GROMOS-C measurement frequency of 110.8 GHz was 0.7 in winter and increased to 1.2 in summer. An opacity value lower than 0.5 is considered to be ideal for such observations . OZORAM measures at 142.2 GHz which is at the wing of a water vapour line. The variability of the opacity is given by changes in water vapour and thus the line at 142.2 GHz is affected more by the variations in tropospheric water vapour than the line at 110.8 GHz for details seeFig. 1.

During the first winter of the campaign a strong and stable polar vortex system formed and stratospheric temperatures were very low. Accordingly the area of polar stratospheric clouds was large and a considerable amount of active chlorine (above 3 ppb) was present in January (WMO Arctic Ozone Bulletin). The subsequent ozone loss in spring was, however, terminated in early March by sudden stratospheric final warming .

In this section the measurements from the ground-based ozone microwave radiometers GROMOS-C and OZORAM are intercompared to the measurements of MLS and radio sondes and to simulations with SD-WACCM.

Figure  shows the ozone time series measured by GROMOS-C during the first year of the Ny-Ålesund campaign. Data gaps are indicated by white vertical lines. During winter they correspond to periods when GROMOS-C switched to carbon monoxide measurements. In summer the data gaps are due to very high opacity values (τ>1.6) and strong precipitation which caused the retrieval process to fail. During the winter half-year the ozone layer is dominated by the dynamics of the polar vortex. Ozone volume mixing ratios decrease sharply when the vortex passes over Ny-Ålesund at a given altitude in the stratosphere. This can be seen in Fig. . The vortex shift of early February and the final stratospheric warming of March led to increases in stratospheric ozone with maximal values of 7 ppm. In summer stratospheric ozone is mostly dominated by photochemistry. Figure  shows the simulation of ozone by SD-WACCM at Ny-Ålesund for the same period. The white lines indicate the polar day and night terminators respectively. At an altitude of 70 km the polar night lasts 2.5 months, whereas at 10 km it lasts 3.5 months. The period with daily sunrise and sunset lasting for 2 months is almost the same at all altitudes. In winter, Fig.  clearly shows all three ozone layers. The main ozone layer at an altitude of 35 km, the secondary ozone layer at 95 km and the tertiary ozone layer at 70 km. SD-WACCM captures the dynamical features of ozone in the main layer very well. The data also show that the secondary ozone layer persists during polar day even though very faint. The two sudden increases in ozone in the second ozone maximum during the sudden stratospheric final warming in the beginning of March are noteworthy. These two increases seem to be connected to an ozone decrease in the tertiary ozone maximum and an increase in stratospheric ozone. However the reason of this phenomena is not known to us and would merit its own investigation. The photochemically induced diurnal variations start shortly after the ending of the polar night at all altitudes. In the stratosphere diurnal variations are seen throughout the polar day, whereas in the mesosphere there is no diurnal variation during polar day.

In order to compare the retrieved profiles with the high-resolution profiles of MLS, SD-WACCM and the ozone sondes, they are convolved with the averaging kernels of the microwave radiometers. The convolution is performed according to xconv=xa-A(xa-x), where xconv is the convolved profile, xa is the a priori profile, A is the averaging kernel matrix and x is the high-resolution profile. For the a priori profile ozone data from an MLS climatology for the years 2004–2013 with monthly mean values are taken. Daily mean values of the convolved time series are taken for the intercomparison. The data set is then averaged over three pressure intervals. The intervals cover the region from 50 to 0.1 hPa and correspond to the middle stratosphere, the upper stratosphere and the lower mesosphere. Ozone sonde measurements are available for the lowest pressure interval only and all individual measurements are displayed. Figures  and show the ozone time series and the relative differences of the microwave radiometers and the convolved data sets. The periods of polar day and night and the intermediate period are indicated by coloured backgrounds.

In the mesosphere (0.1–1 hPa) GROMOS-C is mostly within 20 % of the MLS and SD-WACCM data, and has an offset of 10 %. In the middle and upper stratosphere (10–50 hPa and 1–10 hPa) the relative difference of GROMOS-C and MLS is mostly within 10 % during winter. An exception is October 2015, when GROMOS-C measured too low ozone VMR in the middle stratosphere. In summer GROMOS-C starts to overestimate ozone with 10 %. Compared to the ozone sonde GROMOS-C overestimates ozone during the whole year by 10 %.

In the mesosphere OZORAM agrees with MLS and SD-WACCM within 10 % except during winter, when it underestimates ozone by 20 %. In the upper stratosphere all data sets are within 10 %. However in the middle stratosphere OZORAM underestimates ozone in winter by up to 20 %. During summer it is within 10 % of the MLS measurements again.

Figure  shows averaged ozone profiles and the relative differences of GROMOS-C and OZORAM to the convolved MLS and SD-WACCM profiles. The profiles were averaged during a period in winter and in summer. This comparison shows that the retrieval of OZORAM oscillates during winter but shows good agreement with MLS (within 5 % from 100 to 0.2 hPa) during summer. For GROMOS-C it is the other way around. During winter the average agreement with MLS is within 5 % from 70 to 0.5 hPa. However during summer an offset to MLS is detected but it is still within 10 %.

The difference in the performance of the two radiometers from winter to summer measurements might be caused by the different treatment of the troposphere. A tropospheric correction has been applied to the GROMOS-C spectra such that the retrieval starts at the tropopause level according to . This treatment of the troposphere proved to be robust . For OZORAM, a different retrieval approach is used where the opacity of the troposphere is retrieved together with the ozone profile. In order to do this, the absorption is calculated from standard H2O and O2 profiles using the MPM93 model and scaled along with the O3 profile. For details, see .

In this section we analyse the diurnal cycle of middle-atmospheric ozone above Ny-Ålesund. We use 1 year of measurements from the ground-based microwave radiometers GROMOS-C and OZORAM, as well as ozone simulations from SD-WACCM. The characteristic patterns of the diurnal cycle are shown for the different insolation conditions throughout the year. Furthermore the SD-WACCM model is used to separate the contributions from different reaction pathways to the daily ozone production and loss.

An overview of the diurnal cycle above Ny-Ålesund is given in Fig. a. It shows SD-WACCM ozone VMR over the course of 1 year and at three pressure levels. During this winter, ozone encounters strong fluctuations due to the dynamics of the polar vortex. In the stratosphere these fluctuations are strongest in fall and spring during the formation and the breakdown of the polar vortex. Ozone changes of up to several parts per million per day are seen in the spring and fall period. This has been previously observed in a model study by (their Fig. 6). In spring 2016 exceptionally large changes in ozone VMR were detected which were caused by the sudden stratospheric final warming. During summer the ozone fluctuations decline and the diurnal cycle is clearly visible. In the mesosphere (at 0.1 hPa) the diurnal cycle is strongest during spring and fall. It goes along with the transition from wintertime ozone levels (∼1.7 ppm) to the very low summertime ozone levels, and vice versa. The amplitude of the diurnal cycle is around 1 ppm and the depletion and reformation of ozone are very rapid processes. In the stratopause region and in the stratosphere, the diurnal cycle continues during the entire period of polar day. In the stratopause region (at 1 hPa) the amplitude of the diurnal cycle is largest in the beginning of May with 0.55 ppm. The stratosphere (at 10 hPa) shows the largest amplitudes around summer solstice with 0.25 ppm (Fig. b). During polar night there is no diurnal cycle present at all three pressure levels.

For detailed analysis we look at the averaged diurnal cycle for five time intervals. Because the insolation conditions are symmetric around summer solstice these intervals are taken between February and July 2016 (Figs. –). The time intervals have different lengths depending on the time of the year. From the end of the polar night the length of daytime increases rapidly (23 min per day on average) until the polar day starts. To get a reasonable signal-to-noise ratio of the diurnal variation in the measurements, we choose intervals of 10 days. The shift of the sunrise and sunset time over these 10 days is about 2 h. This implies that the flanks of the depletion at noon are smudged, though this is not critical to our basic objective which is the characterization of the seasonal pattern of the diurnal cycle. During the polar day the changes in the daily minimum solar zenith angle are small and we choose intervals of up to 42 days. For all these intervals we calculate the averaged relative difference to the mean night-time ozone VMR according to ΔO3=O3-O3,nightO3,night, where O3,night is the mean over all measurements performed between 22:00 and 02:00 local solar time in one time interval. Figures – show the relative difference to the mean night-time ozone VMR for GROMOS-C (a), OZORAM (b) and the unconvolved SD-WACCM profiles (c). The diurnal cycle for four pressure levels is additionally shown for the convolved SD-WACCM data (right). The horizontal black lines indicate the altitude of the corresponding pressure levels and the black line in panel (c) indicates the average sunrise and sunset time. The error bars are the standard error of the mean. GROMOS-C is capable of measuring in the four cardinal directions with an observation angle of 22∘ elevation. If not stated otherwise, the eastward GROMOS-C measurements are used for the analysis. OZORAM observes at an elevation angle of 20∘ facing south-east (113∘ azimuth). The SD-WACCM ozone data are taken from the grid point 78.6∘ N, 12.5∘ E which is closest to Ny-Ålesund. To compare the SD-WACCM simulation with the ground-based ozone measurements, the data are convolved with the averaging kernels of GROMOS-C. Since the vertical resolution of the SD-WACCM ozone profiles is degraded by the convolution the unconvolved SD-WACCM profiles are also shown.

In the mesosphere the instruments and the model observe the sharp transitions from night-time ozone VMR to daytime VMR during the period between polar night and day. This transition is caused by the repartitioning of odd oxygen after sunrise. The dip in ozone VMR is deep and narrow in mid-February (70 %), when the Sun is above horizon for about 8 h. One month later the Sun stays above horizon for 16 h and the period with depleted ozone is wider accordingly. In mid-April the polar day has already started in the mesosphere. It is notable that the recovery of ozone at midnight is still present. During the summer months there are no diurnal ozone variations detected in the mesosphere. Around equinox the relative difference in abundance of the daytime ozone to night-time ozone is about 65 %. Studies from the midlatitudes using measurements by UARS MLS and a ground-based microwave radiometer e.g. find the same relative decrease of ozone.

The stratopause region lies in between the mesosphere and the stratosphere at about 1 hPa, and the diurnal cycle is influenced by both regimes. In February a decrease in ozone VMR around noon is detected. In March the diurnal cycle shows the characteristic behaviour of the stratosphere: an ozone minimum in the morning and a maximum in the afternoon. In April the pressure level of 1 hPa lies in between the mesospheric regime with a depletion of ozone during daytime and the stratospheric regime with a morning minimum and an afternoon maximum. For SD-WACCM this results in two ozone maxima, one at 08:00 and the other at 21:00. The instruments can not fully resolve the first maximum since it is smaller in vertical extent than the width of the averaging kernel. In May and June/July the depletion of ozone during day dominates. An ozone maximum around midnight is followed by a minimum around noon. The relative amplitude of the diurnal cycle is largest in May (8 %) and decreases towards summer solstice (5 %). At that pressure level SD-WACCM shows generally larger amplitudes of the diurnal cycle than the microwave instruments. The convolution of the model data with the averaging kernels of GROMOS-C smooths the profiles and brings the simulation closer to the measurements.

In the stratosphere a robust diurnal cycle is identified during the entire period of polar day. At 3 hPa a diurnal variation is already seen in February and at 10 hPa it starts in March. During these 2 months ozone has a maximum around sunrise and decreases towards sunset. In March the error bars at 10 hPa are large (±10 %). This might be caused by the sudden stratospheric warming which lead to an enhanced ozone variability. From April on, the distinct morning minimum and afternoon maximum is seen. At 10 hPa the relative amplitude is largest at summer solstice (7 %), whereas at 3 hPa it is largest in April (13 %). The existence of a diurnal ozone cycle during polar day indicates that the variation of the solar zenith angle over a day is enough to create a change in the ratio of production and loss rates. used OZORAM and WACCM data to analyse the diurnal cycle above Ny-Ålesund in June 2011. The study found a relative amplitude of the diurnal ozone cycle of 8 % at 5 hPa. During the summer solstice at 5 hPa, GROMOS-C measures a relative amplitude of the diurnal cycle of 7 %. Previous studies with ground-based microwave radiometers in the midlatitudes also show a diurnal cycle in the stratosphere. However, the studies from Bern and Payerne, Switzerland (47∘ N; ), show the typical morning minimum and afternoon maximum only below 10 hPa. At 5 hPa, obtained a relative amplitude of 7 %. At Mauna Loa, Hawaii (19.5∘ N), the characteristic diurnal cycle is found between 10 and 4 hPa. At 5 hPa the relative amplitude is 4 % .

The tertiary ozone maximum was first reported by . It is a night-time maximum in ozone volume mixing ratio observed in the winter middle mesosphere at 72 km and close to the polar night terminator. Because of its location, it is also known as the middle mesospheric maximum. At Ny-Ålesund the enhanced ozone VMR in the middle mesosphere lasts for the whole winter, whereas ozone is depleted during summer. This can be considered as a kind of a “super diurnal cycle” by which a day lasts 1 year. The maximum VMR is seen in winter and the minimum in summer (see Fig.  at 0.1 hPa).

The tertiary ozone layer is explained by a decrease in the production of the OH radical relative to the production of odd oxygen. The OH radical is a catalytic destructor of odd oxygen. The main source of OH in the mesosphere is the photodissociation of water vapour by solar radiation with wavelengths λ<184 nm. Close to the polar night terminator, radiation with wavelengths λ<184 nm is strongly attenuated because of the high optical depth of the atmosphere. Hence the production rate of OH diminishes. This decrease in the production rate of OH is not matched by a decrease in the production rate of odd oxygen. The production of atomic oxygen according to the Reactions () and () still continues because the atmosphere is still transparent for radiation in the required frequency range (184 nm <λ<242 nm). After sunset atomic oxygen recombines to form ozone and a night-time ozone maximum builds up. Model simulations of the tertiary ozone maximum reproduced well the altitude and the latitudinal extent of the peak. The models, however, tended to overestimate the peak VMR. The first simulations by predicted a VMR of 7 ppm which was more than twice what satellite measurements suggested with 3 ppm. came to the conclusion that a strong underestimation of the true profile by measurements because of insufficient vertical resolution is unlikely. The results of from GOMOS measurements confirmed the peak VMR of 2–4 ppm, and in simulations by WACCM the peak VMR was only overestimated by 50 %.

In this section we present the GROMOS-C measurements of the tertiary ozone maximum and simulations with SD-WACCM. The retrieval of GROMOS-C is modified for the analysis of the tertiary ozone maximum. The a priori profile is an MLS climatology for September which has no tertiary ozone peak. This precaution is taken to ensure that the information about the tertiary ozone maximum comes from the measurement and not from the a priori. Additionally the GROMOS-C measurements are weighted heavier in the retrieval via a larger covariance matrix of the a priori. The covariance matrix of the a priori has diagonal elements of 0.8 ppm and a Gaussian correlation decay at neighbouring levels. To compare the SD-WACCM simulations to the measurement they are convolved with the averaging kernels of GROMOS-C.

In the measurements of GROMOS-C the tertiary ozone maximum is seen at 0.08 hPa (about 64 km; Fig. a). This is at the upper limit of the retrieval but lies still within the boundaries of a measurement response of 0.8 (indicated by the dashed white line). The tertiary ozone maximum persists over the whole winter from beginning of October to mid-March. The highest ozone VMRs of up to 3 ppm are measured before and after the period of polar night. This is when the polar night terminator is close to Ny-Ålesund. The mean night-time (20–04 h) VMR at 0.8 hPa from 15 October to 1 March is 1.9 ppm.

In the unconvolved SD-WACCM data the tertiary ozone maximum is clearly visible and noticeably detached from the main ozone layer (Fig. c) compared to the GROMOS-C measurements. The highest ozone VMR occurs before and after the polar night with a mean night-time VMR of 2.15 ppm at 0.04 hPa (70 km). This is about 6 km higher than measured with GROMOS-C. During polar night the mean night-time VMR drops to 1.95 ppm at 0.06 hPa (68 km).

The convolved ozone time series of SD-WACCM has a tertiary ozone maximum at the same pressure level as GROMOS-C (0.8 hPa; Fig. b). However the mean night-time peak VMR from 15 October to 1 March is 1.55 ppm, which is 20 % smaller than the peak VMR measured by GROMOS-C. This is in contrast to previous studies that showed an overestimation of the ozone VMR by the models.

A possible enhancement of the peak VMR in the GROMOS-C measurement due to the secondary ozone layer must be considered as the wing of the averaging kernels might extend to these altitudes, though small in extent. In Fig.  we investigate the influence of the secondary ozone layer on the third ozone maximum. Therefore we took an ideal ozone profile from a SD-WACCM simulation which clearly shows the three ozone layers as seen in Fig.  during winter. From this profile a spectrum is generated with ARTS. The ozone profile was then retrieved using the same settings as for the GROMOS-C retrieval. When the secondary ozone layer was not present in the true profile, the magnitude of the tertiary ozone peak was 15 % smaller than for the case with a full secondary ozone layer. In the case of a true profile with a doubled secondary ozone layer the peak VMR of the tertiary maximum was enhanced by 15 %. If SD-WACCM substantially underestimates the VMR of the secondary ozone layer as is suggested by , it would explain the difference between the measurement and the convolved simulation.

If an ideal SD-WACCM profile with no tertiary ozone maximum is taken for generating the spectrum, a tertiary ozone peak is still present in the retrieved profile, which is due to the influence of the secondary ozone layer. The amplitude is, however, significantly lower (about 70 %) than for the case where the ideal profile shows all three ozone maxima. We conclude that the tertiary ozone maximum is a real feature in the GROMOS-C measurements.

The ground-based microwave radiometers GROMOS-C and OZORAM provide measurements of middle-atmospheric ozone from the Arctic station of Ny-Ålesund. The gathered data sets last over more than 1 year and have high time resolution of up to 1 h. With these data sets at hand we are able to present the first comprehensive analysis of the ozone diurnal variations in the Arctic. In this study we analysed the diurnal cycle for different insolation conditions from polar night to polar day and at different altitudes in the stratosphere and mesosphere. Further, an intercomparison of the measured ozone profiles with MLS measurements and SD-WACCM simulations was performed.

The intercomparison of the ozone data from the microwave radiometers with MLS and SD-WACCM shows that instruments and model are generally consistent. The dynamically induced ozone variations are well captured by the nudged model. During summer GROMOS-C overestimates ozone by 10 % compared to the other data sets, whereas the retrieval of OZORAM oscillates during winter.

At Ny-Ålesund the insolation conditions change drastically over the course of 1 year affecting the photochemistry of ozone. In the mesosphere a sharp decrease in ozone of 70 % at sunrise with a subsequent recovery at sunset is seen in February. The depths of the daytime ozone depletion decrease towards polar day and the depletion vanishes in May. In the stratosphere the diurnal variations start in February (at 3 hPa) with a morning maximum and an afternoon minimum in ozone VMR. The typical afternoon maximum is seen from April on and lasts the entire period of polar day. At 3 hPa the largest amplitudes are seen in April (13 %), whereas at 10 hPa the largest amplitudes are seen at summer solstice (7 %). The diurnal cycle at the stratopause (1 hPa) shows the behaviour of both the stratospheric and mesospheric regimes. It shows a morning minimum and an afternoon maximum in March and a depletion at noon in May and June. The pattern of the photochemically induced diurnal cycle is symmetric around summer solstice. During polar night no diurnal variations are detected at all altitudes. Because the diurnal variations of ozone are well modelled by SD-WACCM we use the model to analyse the reaction rates of the ozone production and losses. At equinox the diurnal cycle is dominated by the production and loss rates from the Chapman equations. At summer solstice the reaction rate of the Chapman equations is always positive for the stratosphere and the mesosphere. In the stratosphere the necessary losses for a diurnal cycle result from catalytic reactions. Reactions of ozone with NO and Cl produce the largest losses. For a solar zenith angle larger than approximately 68∘, the combined losses exceed the ozone production by the Chapman reactions, and diurnal variation during polar day is possible. In the mesosphere the Chapman ozone production is balanced by losses through the reaction of ozone with hydrogen.

GROMOS-C measured the tertiary ozone maximum in the Arctic middle mesosphere. The peak is found at an altitude of 64 km with a mean night-time VMR of 1.9 ppm, and it persists throughout the entire winter. SD-WACCM models the peak altitude about 6 km higher in altitude and with a mean night-time VMR of about 2 ppm. The convolved SD-WACCM data, however, match well with the GROMOS-C measurements.