The transmitter of the system (Fig. 1, Table 1) is based on a KrF excimer laser (Lambda Physik, LPX 250, maximum repetition rate 100 Hz) consisting of a tunable narrowband oscillator and a three-pass power amplifier. CaF2 is used for transmitted optics. CaF2 is not birefringent, and thus polarization effects (Kempfer et al., 1994) and ageing are avoided. The energy was considerably enhanced by anti-reflection (AR) coating on the outer side windows of the amplifier gas cell and the beam splitter in front of the energy monitor. A pulse energy of up to 540 mJ was measured several metres away from the laser where divergent components also emerging from the amplifier can be separated and blocked by an aperture. For the lidar measurements the laser energy is usually set to 400 mJ. The unstable cavity of the amplifier yields a highly collimated rectangular beam with a divergence of 0.2 mrad. The wavelength was set for maximum output and the prism-grating combination never touched again. In 2010 measurements with a HighFinesse WS6 (Δλ=0.6 pm) wavelength meter carried out over several days yielded 248.5078 nm ± 0.0060 nm, in agreement with the results of Kempfer et al. (1994). The spectral bandwidth is specified as 0.2 cm-1 (6 GHz). Locking the amplifier to the oscillator can be nicely verified by an enhancement of the pulse energy by up to 70 mJ under our standard operating conditions.

The output of the KrF laser is split by a 50 % beam splitter and focused into two Raman cells with f=1.0 m AR-coated plano-convex CaF2 lenses. One cell is filled with hydrogen and the other one with deuterium, and the same pulse energy per cell as previously used for a single cell (Kempfer et al., 1994) is ensured (almost 0.2 J). A total of six Stokes components are generated in hydrogen, just 277.124 nm (S1) and 313.188 nm (S2) are taken (Table 1). For deuterium the second Stokes (S2) component (291.838 nm) is used. The outer surfaces of the CaF2 windows of the Raman cells are AR-coated. The inner ones are not coated because of the possibility of ageing in the presence of photolysed hydrogen. The pump radiation leaving the evacuated Raman cells is of the order of 160 mJ. The output of the Raman cells is combined with a pair of dichroic beam combiners and collimated with an f=5 m, 150 mm diameter concave spherical mirror. The beam combiners reflect 99 % of the 292 m radiation at 45∘ and transmit 88 to 90 % of all the other relevant spectral components. Overlap and pointing of the 292 nm beam are optimized by placing a wire cross in front of the D2 cell or behind the second beam combiner by watching the images of the cross in front of mirror M4.

The Raman conversion efficiency obtained with the LPX 250 laser system is lower than that previously published (Kempfer et al., 1994). We ascribe this to the smoother energy distribution in the beam profile of the new laser. As an example, Fig. 3 shows the conversion efficiencies obtained for hydrogen for a laser pulse energy of almost 200 mJ per Raman cell, attenuated by the optics, in particular by the single-side AR-coated cell entrance windows. The sum of all conversion efficiencies is less than 1.0 starting at already low pressures. This loss of overall energy is tentatively ascribed to optical breakdown. Above 3 bar the loss starts to level off. The non-negligible fourth Stokes emission (Kempfer et al., 1994) was not determined. The maximum second Stokes conversion efficiency for deuterium is approximately 17 % (at 11 bar). The operating pressures have been chosen at around 3.3 and 11 bar for H2 and D2, respectively.

The conversion efficiency was determined for a laser repetition rate of 10 Hz in order to avoid damage to the power meter used. During the lidar measurements it turned out that the second Stokes output may increase when selecting a repetition rate of 100 Hz, sometimes even leading to range signal overflow in the transient digitizer. This effect was unexpected and must be taken into account when setting the detector supply voltages. We did not analyse this behaviour in detail.

Linear polarization is important for single-line output of the Raman shifters (Kempfer et al., 1994). Thus, we placed a Glan prism and a Fresnel rhomb (both from Halle) in the beam between the oscillator and amplifier. All mirrors and beam splitters of the transmitter section were manufactured with minimum polarization sensitivity. The Fresnel rhomb is rotated for optimum backscatter signal (Fig. 4). The strong modulation of the lidar signal in Fig. 4 is mainly caused by the holographic gratings used in the receivers (Sect. 3).

Due to the high average power of the laser system the time for a single ozone measurement, carried out with a repetition rate of 99 Hz, is as short as 41 s.

The pump laser of the mobile DIAL was a frequency-quadrupled Nd:YAG laser with a 30 Hz repetition rate and pulse energies of up to 140 mJ at 266 nm (Continuum, Powerlite 9030). The laser was selected because of a remote control option. The manufacturer promised external control of warm-up and rotation of the frequency doubling and quadrupling crystals. The 1064 and 266 nm powers were measured by two Molectron power meters for a PC-based power optimization. However, the computer control never worked properly: automatic warm-up of the laser was never achieved. The reason was a conflict with “keep-alive” pulses that had to be sent by the external control.

The quadrupling was achieved by using BBO (beta barium borate). This approach yielded high conversion efficiency and moderate thermal loading. However, after more than 1 year of infrequent operation of the lidar the surface of the crystal started to degenerate. This turbid layer did not strongly reduce the UV emission and polishing was therefore postponed.

At maximum pump energy (1.6 J at 1064 nm) the 266 nm radiation exhibited a ring-shaped mode at a pulse-energy level of 140 mJ. We reduced the pulse energy to 1.1 J. Still, 120 mJ could be produced, now with a filled beam profile. However, a hot spot formed that focused in the Raman-shifting compartment and we reduced the UV output to about 70 mJ for safety reasons. This hot-spot problem was solved by the manufacturer in a later (“precision”) version of the laser.

A ceramics shutter was added to the exit holes of the Powerlite laser that was controlled by both the safety system and the lidar PC. Closing the shutter was preferred to switching off the laser oscillator in order to maintain stable thermal conditions in the laser during an interruption.

A side view of the lidar including the entire transmitter is given in Fig. 2, which is the lower level of the frame in Fig. 5. Figure 5 shows the Raman-shifting compartment that also contained a 6:1 beam expander used for reducing the beam divergence. Rotating beam splitters were used for directing the laser pulses into the H2 and D2 cells. These beam splitters were based on circular quartz plates differently coated on the two halves of the surface: high-reflecting for the lidar wavelengths on one half and high-transmitting on the other one. The rotation was synchronized to the laser pulses. The control unit issued pulses for identifying the Raman cell actually passed for the data acquisition system. Two precision motors with measured out-of-axis rotation of just about ±2 and ±40 µrad, respectively, were chosen (KaVo, model EWL 4025; with custom-made electronic control).

Due to high thermal sensitivity the emission wavelengths of Nd:YAG lasers may vary considerably from model to model. We derive a guess of the unknown pump wavelength of our Powerlite laser model from Trickl et al. (1989; 2007) and wavelength measurements for three other injection-seeded Nd:YAG lasers in our laboratory. The average pump wavelength is 266.120 nm ± 0.011 nm. This yields first-Stokes-shifted wavelengths of 289.103 nm (in D2) and 299.209 nm (in H2).

By focusing the 266 nm beam with an f=1.0 m plano-convex lens we reached maximum first Stokes (S1) conversion efficiencies of almost 50 % in both hydrogen and deuterium at pressures as low as 0.9 and 1.6 bar, respectively. This is remarkable in two respects: the theoretical Raman conversion efficiency reaches 50 % at higher pressures and the Raman gain of deuterium is substantially smaller than that of hydrogen (de Schoulepnikov et al., 1997). A total of 5 Stokes orders and 1 anti-Stokes order were visually observed for hydrogen, with fewer orders for deuterium. There was some contribution of the second Stokes order (particularly low at 1 bar due to gain competition with S1), but those for the higher orders were below the 1 mW detection threshold of the power meter used. Starting at pressures below the threshold for Raman conversion absorption was realized and, in H2, the conversion efficiency rapidly dropped to zero above about 1 bar. The same effect was also observed in pure helium and argon. Thus, we ascribe these observations to laser-induced breakdown. The role of the hot spot in igniting this breakdown could not be examined. Quite obviously, the Stokes emission was emitted prior to the breakdown maximum (see also Trickl, 2010a). In any case, the high conversion efficiency achieved was more than enough for the lidar operation.

Motivated by the hot-spot problem the focusing lens was replaced by a pair of crossed f=1.0 m cylindrical lenses during the final phase of operation of this lidar system. As suggested by Perrone and Piccinno (1997) this may result in a softer focus, a larger focal volume, and higher Raman conversion. The maximum possible distance between the two lenses was about 12 cm and was chosen for the lidar operation. In Fig. 6 the conversion efficiencies as a function of cell pressure for this distance and also for the minimum possible distance of about 5 cm is given. A clear change in behaviour was seen. The transmitted pump energy no longer dropped to zero above 1 bar. As one would expect the depletion for pressures up to 2 bar is smaller for the larger distance between the two lenses. Quite interestingly, the pump depletion in D2 was much less pronounced than that in H2. Despite these obvious improvements, the maximum conversion efficiency just rose for H2 (to 61 %, comparable to the results by de Schoulepnikoff et al., 1997).

The rectangular beam-steering mirror was mounted on two mutually orthogonal rotation stages (OWIS). The beam pointing angle was set on the lidar PC.

The optical layout of the IFU lidar systems built or modernized since 1990 is based on several design principles:

the use of Newtonian telescopes for a less critical alignment than in the case of a Cassegrain telescope and for an easier discrimination of the near-field signal; and

The detectors are key components of our lidar development, which calls for an explicit description. As suggested by Kempfer et al. (1994), we exclusively used the 14-stage EMI 9893B photomultiplier tubes (PMTs) between 1994 and April 1996. For linear performance the 9893B detectors were operated with maximum analogue signal levels below 10 mV (50 Ω termination). This means that the very high gain of this 14-dynode PMT (up to 8 decades) is completely unnecessary. The big plus was range gating (Kempfer et al., 1994), lifting the far-field signal level to values mostly well above the electronic imperfections of the signal processing system. The range-gating circuit was further improved for repetition rates of more than 20 Hz.

However, after very positive testing in 1995, we introduced Hamamatsu H5783P-06 PMT modules to both DIAL systems in spring 1996 (Brenner et al., 1997; Eisele and Trickl, 1997). The miniature PMT features a built-in Cockroft–Walton power supply, an 8 mm diameter photocathode, and six mesh dynodes, leading to a maximum current gain of 3×105. This gain is sufficient for obtaining a very big lidar signal. This module is extremely linear over at least 5 decades for analogue signals up to at least 100 mV (50 Ω termination) in the operating voltage range around the most recommended 800 V. Fluorescence-free Corion SB-300-F short-pass filters were placed on the PMTs and efficiently removed radiation for wavelengths beyond 320 nm.

The small size of the modules allowed us to achieve a very compact design of the polychromators of the two lidar systems. In particular, side-by-side operation of all three PMTs in the spectrographs of the stationary DIAL became possible. These modules were used in our stationary system for more than 15 years without discernible signs of ageing.

Finally, driven by the hope for further improvement, we replaced the Hamamatsu H5783P-06 modules in 2012 with an actively stabilized version optimized for us in 1999 for our three-wavelength aerosol lidar (Kreipl, 2006) by Romanski Sensors (RSV). This device had to be based on the follow-up PMT version Hamamatsu R7400U-03 because the 5600 series was longer available. The socket was further modified to deliver optimized single-photon spikes without the ringing of the original PMTs (Fig. 9a and b). The power connection cable is shielded, but the shield is grounded just on one side.

Similar to the Hamamatsu module the RSV socket generates a clean reference voltage (5 V). This voltage is produced from the 15 V supply voltage. The 5 V reference, corresponding to a PMT voltage of 1000 V, is then returned to the power supply where it is divided to the adjustable final control voltage level (0 to 5 V) that is sent back to the detector (Fig. 3.12 of Kreipl, 2006). This loop was necessary to clean the lidar signals to a level below 10-5 of the peak signal. Sending in just an external control voltage resulted in an unacceptable baseline crossing of about 10-4 of the peak lidar signal.

The diameter of these detector modules, 50 mm, was too large for operating the PMTs for 277 and 292 nm side by side in the spectrograph of the stationary system. In order to make this possible, RSV delivered four of the modules with the small PMT tubes mounted off-axis.

Testing of the PMTs in our three-wavelength aerosol lidar showed that above peak signals of 40 mV signal-induced nonlinearities become observable that are attributed to photocathode overload (Fig. 3.10 of Kreipl, 2006; English version: http://www.trickl.de/PMT.PDF, last access: 19 November 2020). However, this result was obtained for a PMT supply voltage of the order of just 450 V and therefore corresponded to an excessive photon flux (see Fig. 10 for a gain curve). For voltages around 800 V (maximum: 1000 V), as recommended for photon counting, the incident radiation levels for creating the same signal are roughly 100 times lower. As a consequence, much higher signal levels can be afforded, and in recent years we have routinely set the peak signals in the far-field receiver to 70 mV, this being a rather conservative choice. This setting was motivated by the decision to stay within the 100 mV input range of the transient digitizer (Sect. 4.5).

We ascribe this unprecedented performance to the mesh layers of the dynode stages that likely act as electrostatic kinetic energy filters for the electrons. A pulse-height spectrum of one of the PMTs for the recommended operating voltage of 800 V is shown in Fig. 11. This spectrum was derived from a time scan with a 1 GHz digital oscilloscope (Tektronix, DPO 7104). No rise in photon counts towards 0 V pulse height is seen that would indicate signal-induced cathode emission, this result being limited by the chosen trigger level of the scope of -1.5 mV. It is important to mention that the pulse-height distribution does not end at -23 mV. As can be concluded from Fig. 9a and b, much higher pulses exist that can reach almost -200 mV. For 1 h measurements with our Raman lidar (Klanner et al., 2020) we did not observe dark counts in 7.5 m bins for discriminator thresholds of 4 mV and PMT supply voltages beyond 900 V.

In the far-field receiver we found that a high number of photons is more important than a high peak analogue voltage because the photon noise dominates the signal at large distances. Thus, we no longer attenuate the signals and irradiate the photocathode with all the light emerging from the spectrograph. For compensation we reduce the PMT voltage to about 700 V. Now, the 70 mV signal level corresponds to about 2.5 times more photons per time interval than before. This change has resulted in considerable lowering of the ozone noise for the 292–313 nm wavelength combination in recent years. Photon counting at 700 V, along with the resulting much lower single-photon amplitudes, has not been tested so far (Sect. 4).

A really bad surprise was that the 7400 PMT is more than 1 order of magnitude more susceptible to daylight than the old modules. The H5783P-06 modules stayed linear up to about 12 mV of constant-background analogue signal. Now, the constant signal background must be kept below 1 mV. This task is demanding at 313 nm during the brightest part of the day, aggravated by the degraded surface of the primary mirror and in the presence of clouds. In spring and summer signal undershoot to below the signal baseline has even been observed during the hours around noon. We added a 5.7 nm (f.w.h.m.) filter from Laseroptik for additional background blocking. Still, mathematical corrections had to be made, which were particularly important for optimum aerosol retrievals. A filter with a 0.5 nm flat top and very steep edges is needed. Additional solutions could be an additional light baffle above the telescope and replacing the aged primary mirror of the telescope.

For the digitization of the analogue signal a 12 bit transient digitizer was found to be sufficient for avoiding the influence of single-bit steps since the shot-to-shot noise is larger than a least significant bit (LSB). This was anticipated by numerical simulations with artificial noise before the 1994–1995 upgrading of the stationary system that demonstrated the absence of steps for a noise amplitude of 4 LSBs. A sawtooth generator built for randomizing the single-bit steps turned out to be unnecessary. By contrast, Langford (1995) reported a significant improvement in his system achieved by modulating the signal.

In the upgraded stationary system, a 12 bit, 20 Hz system from DSP Technology was used until 2003. Since the mobile system was built 1 year later, the first 12 bit, low-noise 20 Hz transient digitizers systems from Licel became available and were used. The performance was excellent with lower noise than in the DSP system. In 2013, the Licel transient digitizers were upgraded at our request by introducing custom-made ground-free input amplifiers. This latest version has led to an unprecedented performance with a relative noise level of about ±1×10-6 of the full 100 mV voltage range after minor smoothing (Sect. 7.1), also yielding highly sensitive aerosol measurements at 313 nm despite the short wavelength. This unprecedented performance has made it possible to operate the system without photon counting with very little loss of quality.

Though being much noisier, the DSP Technology system was more linear than that of Licel as resolved down to a level of 2×10-5 of the full scale (Kreipl, 2006; Fig. 3.10: http://www.trickl.de/PMT.PDF, last access: 19 November 2020). When firing the laser of our mobile aerosol lidar nearly horizontally onto a rock at a distance of 9 km, where the peak equalled the signal maximum, the return from beyond the rock instantaneously and exactly returned to zero. By contrast, the Licel system yields a small undershoot for distances beyond remote clouds that is larger for larger signal areas. Of course, the performance is perfect in the absence of clouds that generate very pronounced spikes. The performance of the most recent version of the Licel system is discussed further in Sect. 7.1.

In order to lift the PMT output, typically around 10 mV for the old PMTs and 70 mV for those from Hamamatsu (into 50 Ω), to the coarsest range of the transient digitizers, adjustable-gain pre-amplifiers were used until 2011 (Analog Modules, model 351, bandwidth 4 MHz, gain-adjustable between 1 and 10). In two of the far-field channels (on wavelengths) these pre-amplifiers produced some very small ringing. Between 1997 and 2003 these problems were overcome by using photon-counting data. For many years of exclusively using analogue data the ringing had to be removed by mathematical corrections. The ringing and the additional noise finally completely disappeared after disconnecting the zero voltage. After introducing the latest (ground-free) version of the Licel input stage the pre-amplifiers were removed.

In the stationary ozone DIAL single-photon counting was applied between spring 1997 and 2003 with an FDC700 1 GHz photon-counting system from Optec. The signals were fully linear starting in the middle troposphere but produced extra counts at lower altitudes, presumably due to pile-up effects of the PMT ringing (Fig. 9a). The signal for photon counting was separated from the analogue output by an impedance-matched junction containing an adjustable discriminator custom-made by RSV. In the first version the discriminator level could not be reduced to below 11 mV. This level had to be chosen to ensure linear performance and maximum signal (Fig. 11). The unit was upgraded several years ago for picosecond time resolution and discriminator levels down to 2 mV.

The new PMT units delivered by RSV are free of the ringing of the original Hamamatsu tubes (Sect. 4.4) and feature pulse widths of about 1.5 ns (Fig. 10). In order to benefit from this considerable time resolution we recently purchased MCS6 and MCS6A five-channel high-speed photon-counting systems from Fast Comtec for several of our lidar systems. The signals are scanned for selectable pulse edges at intervals of 100 ps, which means a maximum count rate of about 5 GHz for equidistant picosecond pulses. For both reasons a highly linear photon-counting performance was achieved that is presented in detail in the parallel publication on our Raman lidar for water vapour and temperature (Klanner et al., 2020).

The simultaneous analogue and photon-counting measurements from a single PMT lead to a deterioration of the analogue signal with an artificial perturbation of the signal of the order of 10-4 of the peak voltage. This could be reduced by 1 order of magnitude by adding an optocoupler to the trigger input of the counting system. However, the shape of the perturbation was somewhat complex and thus difficult to correct mathematically. In addition, we do not have experience with photon counting at the currently preferred PMT voltages of around 700 V or less (see above). At this time the simultaneous application of photon counting is postponed until a better solution becomes available.

All connections between electronic components of the two DIAL systems are ground-free. The trigger pulse is derived from a photodiode and subsequently distributed into numerous output channels via optocouplers (Ingenieurbüro W. Funk). The supply voltages for the PMTS, pre-amplifiers, and discriminators (Ingenieurbüro W. Funk) are generated through high-quality DC–DC converters (TRACO POWER, models TYL 05-05S30 and TYL 05-15W05). They are transferred to the different devices in shielded cables. The shields of the cable leading to the PMTs are open on the side of the detectors. The supply voltage can be set by the lidar PC via an I2C bus, but this option has never been used in the stationary system because of the rather stable clean-air conditions at Garmisch-Partenkirchen. Also, the opening and closing of the flap in the roof was initiated via an I2C bus.

Electromagnetic interference from outside (e.g. the laser) has been kept at a negligible level by using doubly shielded signal cables (Suhner, G03332; the outer shield is left open on one side) and ground-free circuits. The trigger pulses were obtained from photodiodes and then distributed via optocouplers.

The firing of the XeCl laser was initiated via RS232 remote control of the computer of the excimer laser. The power for the high-voltage circuits of the laser is supplied by a separate source. The laser PC was connected to the clean power in the lidar laboratory. The laser itself is controlled by its computer via optical fibres. Finally, both cables connecting the lidar laboratory and the laser PC are shielded, which successfully removed any interference from the high-voltage pulses (Eisele and Trickl, 1997).

Both DIAL systems have been extensively operated under automatic control by the lidar PC. In the mobile system an external start and warm-up of the laser was not possible due to issues in the programmes delivered by Continuum. The laser output was continuously controlled: the measurements were interrupted if the 1064 and 266 nm power levels were below maximum.

Among the various error conditions the most important ones are rain and high wind speed. This results in an immediate closing of the flap in the roof. As to the KrF laser the high-voltage is shut down, and as to the Nd:YAG laser the output shutter is closed, with the laser continuing to fire in order to maintain thermal equilibrium of the frequency-doubling crystals.

Time series under automatic control have been extended for the stationary system to up to 4 d. In this way, numerous atmospheric transport studies could by made, with the first 4 d series leading to the first detection of North American ozone over Europe (Eisele et al., 1999; Trickl et al., 2003).

Since the first measurement series in 1991 the ozone data have been calibrated by using the absorption cross sections from the University of Reims (Daumont et al., 1992; Malicet et al., 1995). The motivation for this is described by Kempfer et al. (1994). Most importantly, the measurements account for the decomposition of ozone during the absorption measurements by precise pressure measurements. The cross sections have measured again and again (e.g. Gorshelev et al., 2014; Serdyuchenko et al., 2014, and references therein), but no improvement has been achieved, except for perhaps the temperature dependence. Very recently, four new cross sections measured between 244 and 254 nm at an uncertainty level of 0.1 % have been provided by Viallon et al. (2015). In view of the choice for our ozone DIALs it is extremely satisfactory that the agreement with the corresponding values in the Reims data is within ±0.06 %.

The temperature dependence as a function of altitude is obtained by interpolation of the cross sections from Reims measured for different temperatures.

For the convenience of data users, the system performance is summarized in Table 4 for the different periods of operation. The uncertainties have been derived from validation exercises, sensitivity studies in low-signal ranges, and noise estimates and reproducibility of the ozone densities during diurnal series of measurements.

The lidar system has been systematically validated (since 2007 on each sounding day) by using the in situ data from the nearby mountain stations Wank (1780 m a.s.l.) and Zugspitze (2962 m a.s.l.) until the measurements at these sites were discontinued (evaluated data are available until 2010). Afterwards, the ozone values of the Schneefernerhaus (UFS) Global Atmosphere Watch station have been used for occasional comparisons (Trickl et al., 2014, 2020). UFS is located on the southern face of Zugspitze at a distance of 9 km from the ozone DIAL at IFU. The gas inlet is at 2670 m. The average ozone mixing ratios are about 1 % lower than those at the summit (Ludwig Ries, personal communication). The lidar data agree similarly well with those from UFS as previously with the Zugspitze ozone.

In addition, a large number of successful comparisons have been made with the Hohenpeißenberg ozonesondes (distance: 38 km); a few examples were given by Eisele et al. (1999). A more extensive comparison is planned for the 2018 data, accompanied by a highly successful comparison with a sonde launched by colleagues from Jülich directly at IMK-IFU in February 2019. The latter side-by-side comparison for mixing ratios of about 50 ppb yielded a rather constant bias of the sonde of 2 to 3 ppb up to 7 km and, above this, a slightly higher variability of the differences.

These comparisons have certain limitations. In the case of the Hohenpeißenberg sondes the air-mass difference matters in certain altitude ranges due to a 48 km distance between the two stations. Under comparable conditions the differences between the profiles have been between 5 % and 10 %.

The lidar has shown a slightly positive bias with respect to the Wank site, mostly not exceeding 5 ppb. This bias is not present during night-time but mostly forms in the morning under warm conditions. It has therefore been ascribed mostly to slope winds (Carnuth and Trickl, 2000, Fig. 5) venting morning-type low-ozone air from the valley up this rather isolated summit that acts like a chimney. Frequently the summertime morning values agree better with the 05:00 CET measurement than with the Wank mixing ratio for the true data acquisition time. Until 2011 some alignment issues occasionally exist that enhanced the uncertainty for distances below 0.5 km. The Wank site has been invaluable for verifying good alignment of the near-field telescope, until 2011 with some resulting problems.

The comparisons with the Zugspitze in situ data have been mostly very convenient. The differences of the mixing ratio have rarely exceeded 2 ppb, with exceptions typically occurring if there is a pronounced ozone gradient around 3000 m. In the absence of an extended comparison since 2012 an example from a 4 d series in May 1999 (Trickl et al., 2003, 2011) is shown in Fig. 13 that exhibits more noise than recent comparisons. The data are compared for two lidar altitudes, 2970 and 2786 m. The lower altitude accounts for the air-mass rise during the final approach towards the high mountain. The results for 2970 m show a few positive departures that result in a positive average difference between the lidar and station of 0.82 ppb (standard deviation: 2.15 ppb). For the lower altitude the “bias” is just 0.34 ppb (standard deviation: 1.61 ppb). These values are all small in comparison with the average Zugspitze mixing ratio, but the sign agrees with the expectation for the 1.8 % bias of the in situ measurements obtained in the recent cross-sectional study by Viallon et al. (2015).

The performance of the mobile system is discussed in Sect. 5.5.

Important species absorbing in the typical wavelength range of ozone DIAL systems are SO2, NO2, and some hydrocarbons. Under the clean-air conditions prevailing at the Alpine site Garmisch-Partenkirchen and in the free troposphere, spectral interference from these constituents should be very rare. As mentioned, the mobile DIAL retrievals for the wavelength pair 266–299 nm are almost insensitive with respect to SO2 and NO2.

Oxygen must also be considered in the wavelength region below 285.66 nm (Krupenie, 1972; Jeunouvrier et al, 1999). The absorption cross sections of O2 in this region (Herzberg bands) are rather low, but absorption cannot be completely neglected due to the high concentration of this molecule. We found some approximate coincidences with non-relevant high rotational levels and an approximate coincidence of the 277.11 nm emission with J=5–7 components of the extremely weak A′→X (2,0) band. 266.12 nm is slightly outside a group of O2 lines. In summary, absorption of the emissions used in the two DIAL systems in oxygen can be neglected, in agreement with the good validation results.

After the first upgrading of the stationary DIAL in 1994 and 1995, the system yielded greatly improved sensitivity and a much larger vertical range up to about 15 km due to the three-wavelength operation. The number of measurements per year grew and time series under automatic control were extended up to 4 d, the first 4 d series being the well-documented one in May 1996 published by Eisele et al. (1999), Stohl et al. (2000), Cristofanelli et al. (2003), and Trickl et al. (2003). However, until 2003 the operation was limited to funded projects and focused research topics. After the second major system upgrading routine measurements were started in 2007. Almost 5000 ozone profiles were obtained from 1991 to February 2019, and numerous examples can be found in our publications (see the Appendix; the most recent one for the period 2007 to 2016 can be found in Trickl et al., 2020).

A summary of the work done is given in Table 3. Uncertainties estimated for the different periods and altitude ranges are specified in Table 4 as a guide for potential data users.

Figure 14 shows the raw backscatter signals (a) uncorrected and (b) with automatic exponential correction. The amplitudes of the corrections grow with the area of the backscatter signal, which is larger for the far-field telescope than for the near-field telescope, and grow with the wavelength due to the decreasing absorption cross section. In the range in which such an exponential wing affects the lidar signal it does not exceed a few times 10-5 of the input voltage range (100 mV). The slightly enhanced noise in channel 6 (313 nm, red curve) is caused by the early-morning daylight roughly 1 h after sunrise.

The introduction of three-wavelength operation made possible an internal quality assurance. Ozone profiles are derived from different wavelength combinations. The observation of mutual deviations in the retrieved densities results in immediate re-examination of the alignment. As mentioned, just two misalignments matter: the overlap of the partial beams emerging from the Raman shifters after recombination and the pointing of the beam emitted into the atmosphere. Minor discrepancies for 292–313 nm due to alignment drifts during extended periods of unattended operation can be conveniently recalibrated by using the 277–313 nm profiles as a reference, which was routinely done in recent years. As mentioned, the 277 nm channel of the large telescope was found to be insensitive to slight misalignments, presumably due to the particularly small focal point in the entrance slit of the spectrograph. In addition, small drifts in laser pointing do not result in a transverse displacement of the spot on the detectors that are placed in the image planes of the principal mirror of the telescope.

One example of a measurement with a perfectly aligned lidar is shown in Fig. 15 (26 October 2015). The figure contains three ozone profiles from both receivers. The three ozone profiles match well in their common overlap regions. Nevertheless, due to low ozone the near-field signal (here 277–313 nm) yields reasonable ozone values up to 2.5 km above the ground (740 m a.s.l.). The range for the same wavelength pair in the large receiver extends up to 6.5 km a.s.l., with moderately elevated ozone. The simultaneously measured ozone value at UFS is lower by just 0.7 ppb. The 292–313 nm ozone profile exhibits less structure than that for 277–313 nm. The absorption cross section for 292 nm is less than one quarter of that for 277 nm, which necessitates smoothing the 292–313 nm ozone over larger intervals (Sect. 5). In the uppermost part of the red curve a 292 nm single-wavelength retrieval was applied that reduces the noise inferred by the 313 nm profile, but it otherwise agrees with the DIAL solution. Such a retrieval is not possible in the presence of aerosol or clouds.

The ozone hump between 3.0 and 4.7 km is caused by a very dry layer (1 % minimum relative humidity at 4.2 km for the Munich radiosonde, roughly 100 km to the north; 1 % is an artificial cut-off in the listings for the RS92 radiosonde; Trickl et al., 2014). 315 h backward trajectories calculated with the HYSPLIT model (Draxler and Hess, 1998; http://ready.arl.noaa.gov/HYSPLIT_traj.php, last access: 19 November 2020), selecting reanalysis meteorological data, suggest a long-range descent from the stratosphere over western Canada. The Munich thermal tropopause for both standard launch times is significantly higher than the onset of the ozone rise. It is well-known, also from our measurements, that the thermal tropopause does not perfectly coincide with the onset of the ozone rise (Hoerling et al., 1991; Pan et al., 2004).

In general, as pointed out in Sect. 4.2, the near-field receiver yields reasonable ozone typically up to at least 2 km above the ground (2.74 km a.s.l). The quality is limited due to the rapid drop of the backscatter signal. The useful range for 277 nm of the far-field receiver is 6.5 to 8 km in winter (40 to 50 ppb). 292 nm is rarely used in the lower troposphere because of the lower sensitivity for ozone and the stronger sensitivity to aerosol (Eisele and Trickl, 2005). However, the 277–292 nm profiles are preferred in the presence of pronounced aerosol structures because of a less critical aerosol correction. The typical range for 292 nm is roughly 3 km above the tropopause, which can vary with the slope of ozone rise. In summer, when ozone in the free troposphere can exceed 100 ppb, sometimes the range is limited to 10 to 11 km and the seasonally higher tropopause is not reached due to the strong loss of radiation.

Due to the short measurement time of just 41 s the reproducibility of the data can be easily verified. In Fig. 16 we show the profiles for three measurements under complex conditions (Saharan dust up to 4 km and a stratospheric air intrusion around 5.7 km) obtained within less than 3 min on 18 June 2013. The intrusion originated at 10 km or more higher the United States at least 13 d backward in time (Trickl et al., 2020). The layer descended to southern Spain and then turned north-eastward towards the Alps, slightly rising. Due to the long travel the minimum relative humidity was as high as 6 %, as measured by both our water vapour DIAL and the Munich radiosonde (Trickl et al., 2020).

Due to elevated ozone mixing ratios (50 to 80 ppb) the radiation loss results in an increase in the short-term variability of the ozone profiles in the upper troposphere, which indicates a level of uncertainty of about ±10 ppb. The noise of the 277–313 nm ozone values strongly increases above 5.5 km, where the data from the 292–313 nm pair are used.

With the latest PMT version (2012) the far-field performance of the lidar during the warm season decreases around noon due to the growing daylight background at 313 nm and the resulting nonlinearity. The 313 nm constant background is largest in the presence of clouds. The signal must be corrected mathematically (Sect. 5) to achieve both a quantitative ozone profile and a reasonable aerosol retrieval with zero aerosol in clean parts of the atmosphere. The DIAL result based on the corrected 313 nm data is then also compared with the 292 nm single-trace ozone retrieval and usually agrees well. These comparisons demonstrate the value of simultaneously evaluating aerosol and O3. For the strongest ozone mixing ratios (exceeding 100 ppb in the middle and upper troposphere) the range of the system may be limited to about 10 km and the stratospheric ozone rise is missed.

The best results are achieved in winter due to low ozone and low solar background. In Fig. 18 we give as an example the measurements on 13 February 2014. The measurements were limited to the morning hours due to the arrival of clouds ahead of a cold front, just before 11:00 CET. The profiles coincide extremely well outside two dry layers (01:00 CET Munich radiosonde, 4 % to 12 % and 6 % RH, respectively) in the lower free troposphere and above 6 km that might be associated with the slightly elevated ozone at 08:00 CET at around 3.8 and 6.1 km, respectively. The tiny peak at 6.1 km at 08:35 CET does not significantly exceed the uncertainty level in that altitude range. However, in addition to the low RH around 01:00 CET the corresponding HYSPLIT trajectories indicate a descent over at least 13 d from high altitudes over the North Pacific for both layers, confirming the idea of stratospheric intrusions. Intrusions with just a low rise in ozone are not rare during the cold season (Trickl et al., 2020). They can be resolved at least in the range covered by the less noisy 277–313 nm wavelength pair.

In Fig. 19 examples of aerosol retrievals of ozone-corrected 313 nm backscatter profiles during the brightest period of the year are shown. A constant backscatter-to-extinction ratio of 0.020 sr-1 was applied. Backscatter coefficients of (1–3)×10-6 m-1 sr-1 are typical of the warm season at this site unless there is a strong Saharan dust or fire event. Here, the air masses originate in Italy and eastern Europe. The top altitude of 5 km resembles that for Saharan dust (Jäger et al., 1988; Papayannis et al., 2008) but was caused by orographic lifting during transport across the Alps almost parallel to the mountains. The free troposphere was free of aerosol on that day, which allows one to visualize the low noise of the lidar, at least during the early hours. Aerosol data from ultraviolet channels are usually strongly influenced by the noise of the strong Rayleigh background.

In the presence of strong aerosol in the planetary boundary layer (PBL), such as in the case of smoke or pronounced Saharan dust, the signal-to-noise ratio is strongly attenuated. High-aerosol events prevail in summer, which adds to lowering the upper-tropospheric performance of the system.

Starting in late 2012, the aerosol backscatter coefficients were archived in the EARLINET database, mostly with a delay of less than 1 d after the measurements.