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ACTRIS TNA Activity Report

Depolyment of the aerosol Raman lidar PollyXT during BEACC 2014 (POLLY-XT at BEACC).

Ronny Engelmann

Introduction and motivation The transport of biogenic aerosol from the surface into the boundary layer is driven by turbulent mixing, whereas the mixing-layer height (MLH) determines the level to which air in contact with the surface reaches (Emeis et al., 2008). A Doppler lidar, a Micro-pulse lidar and a HSRL provided by AMF, will measure the vertical profile of aerosol (layer boundaries, optical thickness) during BEACC at Hyytiälä. In combination with the surface and tower measurements, aerosol fluxes from the surface through the boundary layer can be derived. In addition, radiative-transfer calculations (RTC) will be performed within BEACC to understand the energy balance of the whole atmospheric column. Aerosol vertical profiles are strongly needed as input data for such RTCs. Also processes other than local biogenic emissions can be responsible for the observed aerosols above Hyytiälä. For example, smoke from wildfires in Russia may be transported into the region of interest. Quality assured lidar systems such as the POLLY-XT (FMI) and the HSRL (AMF) are needed in order to identify such long-range transported aerosols at higher altitudes. TROPOS and FMI developed the portable Raman lidar POLLY-XT since 2004 (Althausen et al., 2009). This multi-wavelength lidar (3 backscatter, 2 extinction, depolarization, and water vapor) was now be deployed during BAECC 2014 in Hyytiälä with the help of the applicant within this TNA. Scientific objectives The main focus of this BAECC sub-project funded by ACTRIS TNA was the installation of the FMI-Kuopio lidar PollyXT with special care on quality-assured measurements: The measurements during BAECC concentrated on boundary-layer aerosols. Special care had to be taken in order to measure aerosol profiles with lidar close to the ground. Certain quality assurance (QA) tests developed in EARLINET (Pappalardo et al., 2014; Freudenthaler, 2009; Freudenthaler et al., 2015) were performed to ensure proper operation of the lidar during the campaign. The additional benefit of Polly-XT is the capability of particle typing which is achieved by multi-wavelength measurements. In contrast to the single-wavelength lidar systems deployed by AMF the size and the shape of the aerosols can be derived. By using the wavelength-dependent information microphysical inversion can be performed in order to obtain particle volume (and mass) concentrations which are of importance for the measurement of aerosol fluxes. Additionally, the microphysical inversion can deliver effective radius, refractive index and single scattering albedo of the (optically active, d > 100 nm) aerosols. Reason for choosing station Since the main project of BAECC was already performed at the SMR station the location for the PollyXT lidar setup was predetermined. Method and experimental set-up The following text is taken from the actual work report and contains the performed work and the quality-assurance test results: 31.Mar.: The lidar arrived on 31.3 from Kuopio and was placed next to the ARM mobile facility (AMF) and was placed on a wooden platform at the south western corner of the AMF. Power and heating were already turned on in order to heat up the system to operating temperature. Outside temperatures were slightly below 0°C.

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Figure 1: Lidar PollyXT_fmi placed next to the FMI (grafitty) container on a wooden platform.

The first task was related to the automatic hatch. It hadn’t been used for a long time in Kuopio and was not closing anymore. Apparently, a safety end-limit switch was reached which stopped the normal operation. This issue could be resolved quite quickly. Then, the laser functioning and adjustment was addressed. The original laser head IH101 together with the older power supply IP260 started lasing at a pump voltage of 1250V which is 150V higher than typical. Also the laser adjustment through the second and third harmonic generators (SHG and THG) was not optimal. So it was decided that the entire transmitter plate was dismounted from the optical board so that it could be inspected in a safer environment, first in the FMI container, the next day in a workshop area in the office building of Hyytiälä station. Since SHG and THG were used for a long time and showed some degrading they were replaced. Replacement of the THG required shortening/cutting of the adjustment knob which was easily achieved by a metal saw. Also the flash lamps of the laser IH101 had been running for 84 Mio shots and were replaced. Since it was quite late by then we decided to continue the next day inside the office building. 1. Apr.: In the workshop the original and a new laser were tested. The laser IH101 and IP260 only reached energy of only 4.4 W at a pumping voltage of 1500 V. (3.5 W at VS,1450) Both lasers suffered from overheating of the laser heads. This is usually a sign of aged pump chambers. Finally the decision was made to operate the new power supply IP332 together with the old laser head IH101. At 1450V an infrared power of ~6.0 W could be achieved while operating at 49°C, slightly below the shutdown temperature of the laser head. The new laser head and the old power supply should go for maintenance at Continuum. Then, the laser was assembled together with the new SHG/THG on the transmitter plate. Adjustment was performed and conversion efficiency was maximized (L/4 plate turning and angular placement of the crystals). In the next step, the laser unit and the power supply was reinstalled into the PollyXT system. During that, the beam expander was permanently shifted slightly upwards (5cm) to obtain a bit more space for the laser assembly. After that the cabled were connected again and the SHG/THG crystals heated. Both were set to a value at the temperature controllers of display of 12 mV (44.6°C). Then the overlap adjustment was performed by use of the camera image and by adjustment of the steering prisms. After that the first measurement in Hyytiälä started. All signals looked reasonable except the one from the 1064-nm channel. The maximum count rate was 0.05 Mcps with a neutral density filter (NDF) of 1.0. The high voltage was checked at the voltage monitor output of the HV power supply and was found to be close to 2kV (9.6V monitor voltage and ~1.3V monitor current whereas 10V correspond to the maximum 2kV and maximum current of the HVPS, respectively). The conclusion was the PMT itself is degraded and should be

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replaced in the near future. The NDF of 1.0 was completely removed and signal maximum now is at 5-8 Mcps. We also removed an NDF of 0.5 (from 1.0+1.0 to 1.0+0.5) at the depolarization channel 532s. The aircraft safety radar was installed on/in the FMI container, but yet not functioning properly. Further checks were planned for the next day. The measurement continued during the evening and since the conditions seemed favorable at night we performed a telecover test starting from 2049 UTC in the sequence of N-E-S-W following the standard EARLINET telecover notation. The measurements for the individual segments were about 10 min each. The lidar then continued measurement during the night.

Figure 2: Results of the Telecover test for all individual lidar signals performed on 1 April 2014. Shown are the measurements at

the individual sectors normalized to the measurement of the total primary mirror.

Figure 3: Measurement of the overlap function on 1 April 2014.

2. Apr.: Aircraft Safety Radar: Since the aircraft safety radar was not working we opened the electronic box and checked the circuitry. We changed the control connectors at the USB6009 Box. The shutter status is now on RADAR_INPUT=Dev1/port0/line4 and

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the “shutter overwrite” is now connected to Dev1/port1/line0. The status port is “low” when the shutter is open, e.g., the Aircraft radar doesn’t see a target. It is “high” if the shutter is closed. The “overwrite” port can be set to OUTPUT and put to “high” if the shutter should be open at all times. Additionally to the mechanical shutter we implemented the old routine in the software to shut down the laser (SH,0) if an aircraft arrives. In this way double redundancy is given. 10 sec after the last target was detected by the radar the laser is turned on again (SH,1). In this way also a logging on aircraft sightings is given in the measurement routine. 1064 issue: The measurement count rates at the 1064 channel were extremely low: 2 Mcps at signal maximum with totally removed NDF. The detection channel in the P7882 was swapped shortly to another input in order to see that the P7882 is not the issue. Then first with a high impedance oscilloscope (40 Mhz) the PMT pulses were investigated. The result was reasonable on the 10 ns time scale the typical photon pulses could be observed. But at time scales of 100 us large pulses were overlaid. If this was a problem of the high impedance input of the oscilloscope could not be ruled out. Then, we attached the PMT to the analog detection card (20 MHz) and set the input to 50 Ohm. Again, nice pulses > 10ths of mV could be observed and the “slow” spikes were no more visible. So from the oscilloscopes point of view the pulses were ok. However, since the count rates during the measurement were low we removed the PMT cooler and inspected the PMT and its location inside the cooler. The cooler was switched of and warmed up to about 5 °C. Then we opened the cooler removed the PMT. Everything looked fine as well, except a little water and some condensation on the outside of the vacuum window which maybe occurred only after disconnection from the power supply. The PMT was put back into the cooler and the system and back into operation. The count rates are still low which only lets the conclusion of an aged PMT. (maybe an aged socket…)

Figure 4: Inside view of the TE cooler for the 1064-nm PMT. Little condensation was found

at the vacuum window in the front. Everything was dried and reassembled afterwards.

During the measurements at night heavy signal oscillations (9-10 per hour) could be observed in the 355-nm data. Somehow the air ventilation must have an influence at the third-harmonic conversion crystal. Thus we set the internal heating to continuous mode. Also a Styrofoam cover was placed in front of the entire transmitter side. Measurements after these modifications show that the oscillations are mostly suppressed. In the evening we performed a zero-bin measurement. A white foam block with the fiber inside was put in the beam path before the beam expander. In this way the stray light was minimized and enough laser light could be captured be the fiber. The laser cover plate was closed. The other end of the fiber was hold by hand at the top of the receiving telescope. The plastic-fiber cable and the 1-m pieces were not long enough to obtain light in the 3rd height bin. So the long fiber was cut until we measured the same amounts of counts in the first and the second height bin, respectively. A check of the measurement file showed also similar counts in the first and second bin, so in the middle of 15 and 45 m height. Later the fiber delay was measured in a lab and then the first bins central altitude could be determined.

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Figure 5: Time delay within the optical fiber for the zero-bin measurement.

The time delay within the fiber was found to be 330 ns which correspond to an altitude of 49.5 m for the altitude between the first and second height bin. That means the center of the first bin is at 34.5 m height. 3. Apr.: Depolarization dependent receiver efficiency measurement: We applied the polarized lamp setup at the PollyXT. A fiber and a rotating mount with a Glan-Taylor polarizer was mounted on the optical board on the upper part of the telescope. Depol_check.exe was modified to operate with the P7882 cards. These cards can be operated with a software trigger mode, so no extra trigger signal was needed. The discriminator levels were kept from the settings.txt and were TRIGGERLEVEL=1.150 -0.033 -0.000 -0.010 -0.015 +0.009 -0.016 -0.020 -0.020

#Start Ch1 Ch2 Ch3 ..., in volts

The ND-filters were decreased in order to obtain enough light. It was found that the light beam of the fiber-lamp setup was not well collimated. So the light spot at the focal plane was about 4 cm in diameter and the light levels were relatively low.

Figure 6: The image shows the installation of the light fiber and the

polarization rotation mount inside the receiver telescope.

The measurement was performed with 1° resolution of the polarizer and the P7882 cards were set to 8192 height bins, 4000 sweeps and a dwell-time of 200 ns per channel. The channels order is 355, 532s, 387, 532, 607, 1064, 407, empty. Afterwards a measurement with the lamps shutter set to off was performed with the same settings but with an angular resolution of 30°. This background signal was constant (independent of the rotation angle) and subtracted from the measurements.

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Figure 7: Polarization dependent receiver efficiency measured with an artificial light source.

Pulse height statistics: With the same setup of the artificial lamp the pulse-height-statistics was determined. The software (pulshoehe.exe) was started and the results are shown below in comparison to a measurement of the same lidar after its upgrade in Leipzig 2012. It was found that the Pulse-height distributions (PHD) did not change in comparison to the measurements in Leipzig 2012 for all channels except 1064. The count rates were lower, but only due to the fact that lower light levels were used for the measurements. However the 1064-nm channel showed a much different PHD compared to earlier measurements. Because of the results the discriminator level was set to +0.005 now. However, the measurement strongly suggests that the PMT has to be replaced. 4. Apr.: The last day consisted of final preparations for the automated measurements, instructions of the local operators, and cleaning up the workplace.

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Figure 8: Pulse height distribution. Left, measurement from 2012, Right Measurement from Hyytiälä 2014. Top shows the pulses measured at each discriminator level, bottom shows the derivative, i.e., the pulse-height distribution.

Preliminary results and conclusions The lidar Polly-XT was successfully installed at Hyytiälä. The lidar was carefully aligned and the proposed QA tests (by EARLINET standards) have been performed during the week of the TNA. After several maintenance

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tasks were performed the lidar is in good condition and passed the QA tests. However it was found that the laser energy is slightly lower than expected. Therefore, the laser will be replaced by a newly serviced one sometime in the next one or two weeks. It was also found that the near-infrared lidar channel was aged, but measurements were still possibly. However, replacement will also take place within the next service. The measurements continue successfully since the installation. The first spectacular observation was a strong Saharan-dust outbreak over Europe that started already during the installation of the system in Hyytiälä. A few days after the installation period a plume of the dust even reached Finland and could be observed with the lidar. While dust often occurs in the Mediterranean countries observing Saharan dust at such high latitudes in Europe is a rare phenomenon. Outcome and future studies This project was closely linked to the project of Lucia Mona (SACS on BAECC, Synergistic approach for Aerosol Characterization Studies during BAECC campaign) which focuses on the lidar data evaluation during BAECC. References Althausen, D. et al.: Portable Raman Lidar PollyXT for Automated Profiling of Aerosol Backscatter, Extinction, and Depolarization. J. Atmos. Oceanic Technol., 26, 2366–2378, 2009. Emeis, S. et al.: Surface-based remote sensing of the mixing-layer height – a review. Meteorologische Zeitschrift, 17, 621-630, 2008. Freudenthaler, V.: Lidar Rayleigh-fit criteria. EARLINET-ASOS 7th Workshop, 2009. http://nbn-resolving.de/urn/resolver.pl?urn=nbn:de:bvb:19-epub-12970-6 Freudenthaler,V. et al.: EARLINET lidar quality assurance tools. AMTD, 2015, in preparation Pappalardo, G. et al.: EARLINET: towards an advanced sustainable European aerosol lidar network, Atmos.Meas.Tech., 7, 2389-2409, doi:10.5194/amt-7-2389-2014,2014.