ALADINA – an unmanned research aircraft for observing vertical and horizontal distributions of ultrafine particles within the atmospheric boundary layer

B. Altstädter, A. Platis, B. Wehner, A. Scholtz, A. Lampert, N. Wildmann, M. Hermann, R. Käthner, J. Bange, and H. Baars Institute of Flight Guidance, Technische Universität Braunschweig, Braunschweig, Germany Center for Applied Geosciences, Eberhard Karls Universität Tübingen, Tübingen, Germany Leibniz Institute for Tropospheric Research, Leipzig, Germany Institute of Aerospace Systems, Technische Universität Braunschweig, Braunschweig, Germany

front compartment of the airframe. The CPCs are improved concerning the lower detection threshold and the response time to less than 1.3 s. Each system was characterized in the laboratory and calibrated with test aerosols. The CPCs are operated in this study with two different lower detection threshold diameters of 11 and 18 nm. The amount of ultrafine particles, which is an indicator for new particle formation, is derived from the difference in 15 number concentrations of the two CPCs (∆N). Turbulence and thermodynamic structure of the boundary layer are described by measurements of fast meteorological sensors that are mounted at the aircraft nose. A first demonstration of ALADINA and a feasibility study were conducted in Melpitz near Leipzig, Germany, at the Global Atmosphere Watch (GAW) station of the Leibniz Institute for Tropospheric Research (TROPOS) on two days in Octo- 20 ber 2013. There, various ground-based instruments are installed for long-term atmospheric monitoring. The ground-based infrastructure provides valuable additional background information to embed the flights in the continuous atmospheric context and is used for validation of the airborne results. The development of the boundary layer, derived from backscatter signals of a portable Raman lidar POLLY XT , allows a quick overview of the current vertical 25 structure of atmospheric particles. Ground-based aerosol number concentrations are consistent with the results from flights in heights of a few meters. In addition, a direct comparison of ALADINA aerosol data and ground-based aerosol data, sampling the air at the same 2 location for more than 1 h, shows comparable values within the range of ±20 %. MASC was operated simultaneously with complementary flight patterns. It is equipped with the same meteorological instruments that offer the possibility to determine turbulent fluxes. Therefore, additional information about meteorological conditions was collected in the lowest part of the atmosphere. Vertical profiles up to 1000 m altitude indicate a high variability with dis-5 tinct layers of aerosol, especially for the small particles of a few nanometers in diameter on one particular day. The stratification was almost neutral and two significant aerosol layers were detected with total aerosol number concentrations up to 17000±3400 cm -3 between 180 and 220 m altitude and 14000±2800 cm -3 in the level of 550-650 m. Apart from those layers, the aerosol distribution was well-mixed and reached the total number concentration 10 of less than 8000±1600 cm -3 . During another day, the distribution of the small particles in the lowermost ABL was related to the stratification, with continuously decreasing number concentrations from 16000±3200 cm -3 to a minimum of 4000±800 cm -3 at the top of the inversion in 320 m. Above, the total number concentration was rather constant. In the region of 500 to 600 m altitude, a significant difference of both CPCs was observed. This event 15 occurred during the boundary layer development in the morning and represents a particle burst within the ABL.

Introduction
Atmospheric aerosols play an important role in our climate system (Ramanathan et al., 2001) and for human health (Stieb et al., 2002;Davidson et al., 2005). The size distribu- 20 tion of airborne particles varies between 1x 10 -3 µm and 100 µm in particle diameter (Baltensperger, 1997). A distinction in different modes according to the particle diameter is often used. In this article the term "ultrafine particles" refers to particles in the nucleation mode (diameter < 30 nm). Due to subsequent growth, particles can reach the Aitken mode (diameter 30-100 nm). Larger particles belong to the accumulation mode (diameter 0.1-25 1 µm) and the coarse mode (diameter > 1 µm) (e.g., Kannosto et al., 2008). With increasing radius particles have the potential to act as cloud condensation nuclei (CCN) and to influ-Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ence the climate system indirectly (Spracklen et al., 2008). New particle formation (NPF) is caused by nucleation (gas-to particle conversion) and has been observed at various sites and altitudes of the Earth's system (Wiedensohler et al., 1996;Weber et al., 1997;Kulmala et al., 2004;Jaatinen et al., 2009). Formation mechanisms and growth rates have been investigated using different platforms (ground, ship and aircraft). Generally the origin and 5 history of air masses play a key role in NPF (e.g., Sorribas et al., 2011). However, it is still not clear which meteorological situation or chemical species are most efficient for particle bursts. Homogeneous and heterogeneous nucleation require a sufficient amount of precursor gases . The presence of non-volatile vapours is essential for the growth of freshly formed particles. Binary nucleation of water and sulphuric acid needs con-10 ditions of low temperature, high relative humidities, aerosol concentrations and high amount of sulphuric acid (Doyle, 1961;Raes et. al, 1992;Kulmala et al., 2000).
The atmospheric boundary layer (ABL) is the lowest part of the troposphere and is influenced by exchange of heat, water vapour, trace gases and aerosols with the surface. Its properties determine the formation, growth and processing of airborne particles. In the ABL 15 stable conditions are frequent during night time and often coupled with a strong temperature inversion. Inversions prevent interactions of air masses with the free troposphere (FT) so that various horizontal layers of aerosols can coexist without vertical mixing. Knowledge of the influence of atmospheric boundary layer development and turbulent mixing on particle bursts is still poor. Bigg (1997) and Nilsson et al. (2001) assumed nucleation due to turbulent 20 mixing processes. Nilsson et al. (2001) concluded entrainment and convection as a cause for nucleation. Stratmann et al. (2003) investigated particle burst events in the residual layer (RL) and mixed layer (ML). Moreover, NPF was observed with increasing turbulence in the RL by Wehner et al. (2010). Enhanced concentrations of ultrafine particles in the morning can be trapped vertically due to the stable boundary layer. During daytime convection leads 25 to turbulent mixing and vertical profiles of aerosol concentrations demonstrate a homogeneous distribution. In addition, Siebert et al. (2004) presented new particle formation near the inversion layer of the ABL. Another study investigated new particle formation related to a nocturnal low-level jet (LLJ), measured with the system ACTOS (Airborne Cloud Turbu-4 lence Observation System), installed bellow a tethered-balloon . In this study, NPF events were observed in connection with an increase of SO 2 concentrations in the lower part of the jet. However, more information about sources and formation processes of new particle formation within the atmospheric boundary layer is essential to understand the complex developments of aerosols in the atmosphere and has to be implemented in 5 models (Boy et al., 2006). The vertical aerosol distribution is strongly related to the synoptic conditions, the evolution of the ABL (e.g., Harnisch et al., 2009), the thermodynamic structures and the atmospheric turbulence (Boy et al., 2003). As different processes can be involved in NPF, and subsequent growth out of the gas phase, repeated vertical profiles of the aerosol concentration and size distribution are needed to elucidate the origin 10 of NPF (Hellmuth, 2006). The variability of small particles over large distances has been analysed by networks of ground-based measurement stations (Vana et al., 2004;Wehner et al., 2007) and additionally by aircraft (O'Dowd et al., 2009). In summary, there are many indications that NPF starts in a vertically confined layer around the altitude of the inversion. Nevertheless, the existing data set is still poor and so far a systematic analysis was not 15 possible due to a missing suitable measurement system. Distributions from the surface up to the free troposphere can be measured by in-situ instruments e.g. towers, tethered balloon systems and radiosondes. Manned aircraft measurements offer information about the vertical distribution of aerosols for larger scales and higher altitudes. Airborne measurements investigated the large scale variability of the particle concentrations along air mass 20 trajectories (O'Dowd et al., 2009). Here, in particular, is a lack in detailed measurements of the small-scale vertical and horizontal variability which, however, are recommended for the implementation of new particle formation (NPF) in models (Boy et al., 2006).
Unmanned aerial systems (UAS), also called remotely piloted aircraft systems (RPAS), offer a lot of advantages as research platforms and have become increasingly important 25 in the last decade. The miniaturisation of electronic components allows the implementation of three-dimensional GPS-system (Global Positioning System) and IMU (Inertial Measurement Unit) to record accurate position, attitude, time and speed of the aircraft (van den Kroonenberg et al., 2008). Autopilot control offers the possibility to fly with low cruising speed at constant heights in order to derive turbulent fluxes (e.g., Thomas et al., 2012). Further advantages of small unmanned research aircraft are minimal logistical requirements (e.g. no airport necessary), potentially lower costs compared to manned aircraft, high flexibility, the possibility to investigate atmospheric parameters at small scales and low altitudes, and the potential for application in regions that are too dangerous for manned aircraft. Some appli-5 cations, with regard to sample atmospheric aerosol by a UAS, can be seen e.g. in Clarke et al. (2002), who have tested among other instruments, a "mini OPC" (Met One 237A) for insitu aircraft measurements. Bates et al. (2013) investigated the vertical distribution of atmospheric aerosols in the Arctic (Svalbard, Norway) by means of the UAS Manta. The aerosol package contained in particular a three-wavelength absorption photometer for detecting 10 the optical properties of black carbon (BC). Tropospheric ozone, which is besides its role as greenhouse gas, an important precursor for new particle formation, was investigated by Illingworth et al. (2014). In this study, the variability of ozone concentrations within the ABL was measured by an electrochemical concentration cell (ECC) ozonesonde, equipped on a Skywalker UAS. Corrigan et al. (2008) presented, as part of the extensive MAC (Maldives 15 Autonomous Unmanned Aerial Vehicle Campaign) campaign, vertical profiles of aerosols and BC, measured with the payload installed in the UAS Manta. The aerosol payload contained a total particle counter (TSI Model 3007) with a threshold diameter of 10 nm, an optical particle counter (Met One Model 9722) and a three-wavelength absorption photometer (adapted from Magee Scientific AE-31). The system is similar to ALADINA, but the 20 authors of this presented P360 "ALADINA" study, have a clear focus on the formation of ultrafine particles relating to atmospheric processes. With ALADINA, an efficient and flexible tool is created to study the small-scale horizontal and vertical distribution and variability of aerosols in the size range of 11 nm to 10 µm. Complementary to ground-based monitoring instruments, it provides in-situ measurements of aerosol and meteorological properties at 25 variable altitudes up to 1000 m. ALADINA thus can fill the spatial gap in the atmospheric boundary layer measurements between ground-based instruments, LIDAR (LIght Detection And Ranging) column measurements that usually cover an altitude range above 300 m and measurements at higher altitudes with larger extent provided by manned aircraft.
This article presents the unmanned research aircraft ALADINA and the results of a first application. In Sect. 2, the applied measurement systems and methodology are introduced: The airframe Carolo P360, the ALADINA aerosol payload and its calibration, the meteoro-5 logical sensors and the data acquisition. Further the aircraft MASC is described. In Sect. 3, the measurement site Melpitz and the first field application are discussed in detail. Finally, a summary of the article and a conclusion concerning the new device ALADINA for aerosol research is provided in Sect. 4. The Carolo P360 is a pusher aircraft with a large payload bay (0.35 x 0.31 x 0.19 m) to carry various instrumentation. In the case of ALADINA, (Fig. 1) the P360 is equipped with meteorological sensors and aerosol instrumentation. The aircraft family "Carolo P360" was developed at the Institute of Aerospace Systems at the Technische Universität Braunschweig 15 (Scholtz et al., 2011). It has a wingspan of 3.6 m and a maximum take-off weight of 25 kg. Safe operation is given by wind speed less than 15 m s -1 and below cloud base. The cruising speed is typically 25 m s -1 to 28 m s -1 . An electrical propulsion system allows a flight endurance of approximately 40 minutes. After 20 minutes for exchanging battery packs and saving data, the P360 can be operated again. Flight preparation includes charging the bat- 20 teries, checking the whole system and saving data. A combustion engine is generally compatible with the P360 but is not used for ALADINA in order to reduce vibrations that might disturb the measurements of turbulent fluxes and to make sure that the aerosol sensors are not contaminated with the exhaust from ALADINA. Another advantage of the electrically operated system is keeping the centre of gravity constant. This allows a precise calibration 25 of the angle of attack (van den Kroonenberg et al., 2008) that is necessary for determining 7 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | accurate wind data. The battery system for the electric motor consists of two battery packs with a combined capacity of about 20 Ah. No additional equipment like a winch system or catapult is necessary for take-off. A landing gear, fixed on the lower fuselage, offers an easy handling during take-off and landing on flat surfaces (e.g. grass, asphalt or snow). The airfield requires a dimension of approximately 60 m x 25 m depending on wind conditions and 5 the pilot's experience.

Measurement systems and methodology
The meteorological sensors and an aerosol inlet are installed at the front of aircraft nose (Fig. 2). The miniaturised aerosol instrumentation is located in the modular payload bay (Fig. 3) of the aircraft. Also, the data acquisition of ALADINA is installed in the front part and allows real-time data transfer by a telemetry downlink. The total weight of the payload 10 is < 2.8 kg. Every electric subsystem like propulsion, autopilot combined with a manual control unit and aerosol instrumentation together with meteorology measurement unit, GPS and IMU has its own power supply and can be operated stand-alone. ALADINA can be flown automatically by the autopilot ROCS (Research Onboard Computing System) that is provided by the Institute of Flight Mechanics and Control of the University Stuttgart. More 15 information can be taken from Haala et al. (2011) andWildmann et al. (2014a). This autopilot allows precise measurement flights in the lower ABL over larger distances. ALADINA follows the flight pattern which was sent to the aircraft before take-off by the ground station. Within the telemetry range of 1.5 km, the ground staff are able to follow and monitor the position, attitude and airspeed of the aircraft. Changes of the waypoints and altitudes 20 are possible within that range and during flights. In principle, ALADINA is able to operate at night equipped with the appropriate lighting upon permission of the local civil aviation authorities (CAA). Operation in precipitation is currently not possible since ALADINA is not rainproof concerning the measurement electronics. In addition, operation in clouds is not allowed by the CAA.

Aerosol instrumentation
The aerosol instrumentation (Fig. 3) consists of three particle counters and a stainless steel tube with rounded inlet lips as aerosol inlet (Fig. 2, Number 3). One optical parti-cle counter, OPC, (model GT-526, Met One Instruments Inc., Washington, USA) and two condensation particle counters, CPCs, (model 3007, TSI Inc., St Paul, USA) were miniaturized and calibrated in the laboratory at the Leibniz Institute for Tropospheric Research. Weight reduction of the instruments was realized by removing housings, internal batteries and displays of the OPC and the CPCs, as well as using only one pump for both CPCs. 5 The CPCs were calibrated in the laboratory with silver particles of different sizes (Hermann and Wiedensohler, 2001). The counting efficiency and response time of the newly designed CPCs are displayed in Fig. 4  18 nm and 2 µm (N 18 ). The aerosol volume flow rate through the CPCs was increased from originally 0.10 l min -1 to 0.16 l min -1 in order to decrease the response time of the CPCs. The increased volume flow rates through the CPCs led to an increasing lower threshold diameter. To counter-act this effect and to derive two different lower threshold diameters, temperature differences between the saturator and condenser inside the CPCs were indi-20 vidually increased using software commands. In order to reach even lower threshold diameters, both CPCs were modified again after the test campaign. Now it is possible to detect particles with a diameter of 6 nm from the first CPC. The OPC (Fig. 3, Number 9) runs with its original pump system and an airflow of 2.83 l min -1 . The OPC yields the particle number concentrations in six particle size channels from about 0.3 to 10 µm particle diameter. The 25 ALADINA aerosol inlet is a 36 cm long 3/16" straight stainless steel tube. The sampling line downstream the inlet is made of flexible conductive tubings, in total 24 cm long, 1/4" for the OPC and 28 cm long, 1/4" and 3 mm, for the CPCs. Two metal y-pieces sequentially split the flow between the OPC and the CPCs. The ALADINA aerosol inlet system sampling Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | efficiency is dominated by two major particle processes (Baron and Willeke, 2001): diffusional losses for particles with a few nanometer diameters and aspiration enhancement for particles with a few micrometer diameters. Both can be well estimated using empirical equations given in Baron and Willeke (2001). The ALADINA aerosol inlet is neither isokinetic (TAS = 25.0 and v sampling = 8.8 m s -1 ) nor isoaxial (mostly in the range of ± 20 • ). The 5 limited size and weight prohibited a more complex isokinetic flow system. Effects caused by the non-isoaxial sampling are minimized using rounded inlet lips. Due to the non-isokinetic sampling, particles with several micrometer diameters can be enhanced in the total number concentration by a factor of up to three. However, 1 µm particles experience only an enhancement of 7 %. Particle losses due to diffusion were determined in the laboratory as 10 17 % for 10 nm particles and much smaller for larger particles. Thus, the ALADINA aerosol inlet system has a sampling efficiency in the narrow range of 83 % to 106 % for Aitken and accumulation mode particles (10-1000 nm diameters).

Meteorological sensors and data acquisition
The meteorological sensor package, mounted at the front of the aircraft (Fig. 2), was de- 15 signed and integrated by the Environmental Physics group of the Eberhard-Karls University of Tübingen (EKUT). It contains a full thermodynamic sensor package with an output of the three-dimensional wind vector, barometric pressure, water vapour content and temperature. In order to measure the three-dimensional wind vector, several sensors are involved: The IMU/GPS system IG-500N by SBG systems delivers information about attitude and 20 velocity in the geodetic coordinate system. A five-hole probe which was designed at the Institute of Fluid Dynamics at the Technische Universität Braunschweig measures the airflow angle and true airspeed. Details about the calibration strategy and airflow retrieval from the five-hole probe, as it is performed in the ALADINA system, is given in Wildmann et al. (2014b). The calculation of the wind vector from all single measurements is described 25 in van den Kroonenberg et al. (2008). With this technique, wind fluctuation of up to 30 Hz can be resolved and the total accuracy of the wind speed measurement was estimated to approximately 0.5 m s -1 (van den Kroonenberg et al., 2008). The temperature is simultane-ously measured by two fast response sensors: a thermocouple and a fine wire platinum resistance thermometer which are described in detail in Wildmann et al. (2013). A relative humidity sensor of type P14 Rapid (Wildmann et al., 2014c) by Innovative Sensor Technologies is used as a small and light weight solution to measure the water vapour content of the air. The sensor has a specified time response of 1.5 s. Careful post-processing, as 5 presented in Wildmann et al. (2013), is applied and allows a frequency resolution of up to 3 Hz in the current setup. The central data acquisition system is the airborne meteorological on-board computer (AMOC) that was developed by EKUT in cooperation with the University of Applied Sciences Ostwestfalen-Lippe. All the above described sensors, including the aerosol instrumentation, are connected to AMOC. Input channels are processed and stored 10 to a secure digital memory (SD)-card at a sampling rate of 100 Hz. In addition, data transfer through a 2.4 GHz telemetry downlink allows real-time monitoring of sensor data at a ground station computer with a rate of 1 Hz.

Unmanned research aircraft MASC
For additional information about turbulence within the atmospheric boundary layer and to 15 obtain a three-dimensional picture of atmospheric processes, a second unmanned research aircraft MASC (Multi-purpose Airborne Sensor Carrier) (Wildmann et al., 2014a) was operated during the first test campaign simultaneously with ALADINA. MASC was developed at the EKUT and carries the same meteorological sensors as ALADINA. Depending on battery and payload, MASC has a take-off weight of 5.0-7.0 kg and a wingspan between 2.6 m 20 and 3.4 m. It has payload capabilities of up to 1.5 kg. An electrical propulsion provides a maximum flight endurance of 60 minutes. Launching is realized by a bungee rope on flat surfaces. A safety pilot is responsible for take-off and landing. Once in the air, the aircraft is controlled by autopilot ROCS (see Subsect. 2.1) with a cruising speed of 25 m s -1 . Furthermore, the same data acquisition is used in order to derive meteorological parameters. 25 Turbulent fluxes can be obtained in conditions of quasi stationarity by horizontal flight legs at different altitudes. To obtain statistically relevant data, an average length of at least 1 km is evaluated for each altitude.

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3 First field application

Measurement site and experiment
A feasibility study and test of ALADINA performance were conducted at the research station of TROPOS in Melpitz on 8 and 9 October 2013. The site is settled 41 km north-east of Leipzig, Germany. It is built on a flat meadow and surrounded by agricultural land (Spindler 5 et al., 2013). The surface is smooth and meets the requirements for take-off and landing the aircraft. Air masses are influenced by industrial and agricultural pollution due to the proximity to Leipzig and due to its surrounding of grass, crop land and woods. The measurement site is qualified for networks like European Monitoring and Evaluation Programme (EMEP) and Aerosols, Clouds, and Trace gases Research InfraStructure Network 10 (ACTRIS). Ground based instruments are used for long-term monitoring in Melpitz. In addition to meteorological parameters, gas concentrations (for example O 3 , NO, NO 2 and SO 2 ) and atmospheric aerosols are analysed. Particle number size distributions are measured in a range between 5 nm and 800 nm by a twin scanning mobility particle sizer (TSMPS) system (Jaatinen et al., 2009;Wiedensohler et al., 2012). Larger particles are detected by 15 an aerodynamic particle sizer (APS). Scans are performed every 10 minutes and offer the possibility to observe the temporal development of the atmospheric aerosol. Apart from this, optical instruments detect scattering and absorption coefficients in order to investigate the optical particle properties. The portable 3+2 Raman lidar POLLY XT (Althausen et al., 2009) was operated at the same time in Melpitz. The data set obtained during the field campaign 20 is used in order to describe the boundary layer development. Previous studies reveal that the station offers great potential for detecting formation mechanisms of ultrafine particles. New particle formation has been mostly observed during spring and summer with particle size distributions increasing from sunrise to noon (Birmili and Wiedensohler, 2000;Birmili et al., 2001). Jaatinen et al. (2009) investigated new particle formation events at three different 25 European sites including Melpitz. Long-term measurements for two years (July 2003-June 2005 show nucleation events depending on seasons. Operational data implies that 26 % of all measurement days contain events with new particle formation. Formation rates of up to 4.6 cm -3 s -1 in polluted air masses were detected. Furthermore a mean growth rate of 6.1 nm h -1 was determined. ALADINA was operated during three measurement flights in October 2013. Due to technical problems during the first flight, only the data sets obtained during the second and third flight are analysed here. Flight two lasted from 13:05 UTC to 13:35 UTC on 8 October 5 2013. The measurement period of flight three took place from 08:26 UTC to 08:58 UTC on 9 October 2013. Altogether, it was possible to obtain 14 vertical profiles with varying vertical extent (0-1000 m, 100-500 m or less maximum altitude). During the test campaign, combined flights with the two research aircraft ALADINA and MASC were performed. Each flight consisted of several vertical profiles from the surface up to 1000 m and horizontal legs 10 at various altitudes to investigate layers of enhanced aerosol concentrations and to observe the horizontal distribution of atmospheric aerosols. The best strategy resulted to be vertical profiling of ALADINA, to catch the development of the ABL and to observe the vertical distribution of aerosols, while MASC performed measurements of turbulent fluxes at the interesting altitudes (for example below and above the inversion or at the layers of enhanced 15 aerosol concentration).
In the following, the general synoptic situation and the meteorological parameters, that were recorded at the observatory during the field campaign, are presented so that one may gets an overview of the prevailing weather condition (Subsect. 3.2). Data sets obtained by the lidar POLLY XT and by vertical soundings with the research aircraft ALADINA and MASC, 20 are discussed in the context of the boundary layer development on the two measurement days (Subsect. 3.3). Aerosol variability, depending on diurnal cycles at the ground, is shown by data sets of a TSMPS and finally compared with the vertical profiling of ALADINA in two different case studies (Subsect. 3.4). 25 In this subsection, the weather situation is described in order to obtain information about the contribution of air masses that arrived at the measurement site during the field campaign in October. First, the general weather situation is analysed by the 500 hPa geopotential height, 13

Synoptic situation
derived from the GFS model on 8 October at 12:00 UTC (available publicly). Air masses in Melpitz were influenced by a high pressure system at the surface above eastern Germany. In the 500 hPa geopotential level, a trough led to cold advection in the atmosphere. Due to the presence of the high pressure system in the lower part of the atmosphere, the trough has been weakened. The wave system moved eastwards which can be seen by a compar-5 ison of the general weather situation with the next day. On 9 October at 08:00 UTC, Middle Europe was still influenced by a high pressure system at the surface. The trough, which was developing over the Atlantic Ocean in Northern Europe, has been settled further to Great Britain. All in all, the weather situation in Germany was rather stable. Back trajectories indicate the source and transport of air masses and were generated by using the NOAA Hysplit 10 model (Draxler and Rolph, 2014;Rolph, 2014). In this case, we used the back trajectories in the heights of 500 m, 1000 m and 1500 m in order to describe their origin and long-transport within the ABL. Analysis of back trajectories reveal the origin of air masses over the Atlantic Ocean. Furthermore, air masses were transported south-west over the northern part of Great Britain and western part of Germany before reaching the research site in Melpitz 15 on 9 October at 08:00 UTC. Especially, the back trajectory of 500 m showed a longer residence time of two days over the eastern part of Germany. However, we assume rather clean air with low concentrations of atmospheric aerosols because the air masses were mainly influenced by marine environmental conditions. In order to describe the detailed weather situation on-site, we use the meteorological data that was measured continuously 20 by a weather mast at the research station in 1 m and 6 m height above ground during the flights period. On 8 October (Fig. 6), the temperature range in 1 m level is between 4.9 • C at 22:10 UTC and 18 • C at 13:55 UTC. The relative humidity reaches a maximum value of 99 % in the morning and in the evening at 19:40 UTC. Nevertheless, no rain was measured by the sensor. The pressure decreases continuously from 1017 hPa to 1013 hPa during the 25 day. The main wind direction is north-east but the wind is changing from north to south-west in the afternoon. The maximum wind speed of 3 m s -1 is detected in 6 m above ground at 14:30 UTC. Fluctuations of global radiation values indicate a continuously changing cloud coverage. During the second flight time (13:05-13:35 UTC), the temperature reaches val-ues of 16.5-17.6 • C. The relative humidity varies between 55 % and 59 % with a pressure of 1014 hPa. The wind is weak with a maximum of 1.9 m s -1 and from north-east. The global radiation ranges from 448 W m -2 to 533 W m -2 . Figure 7 shows the situation for the third flight on 9 October. The pressure decreases further from 1013 hPa to 999 hPa during the day. The temperature reaches values up to 17.2 • C at 10:05 UTC and a minimum of 8.8  15 First, the boundary layer development is analysed by the data set of the lidar POLLY XT (Fig. 8 and Fig. 9) that was installed at the same time in Melpitz. Lidar backscatter profiles reveal a boundary layer with pronounced aerosol load indicated by green, yellow and red colours. Figure 8 represents the backscatter signal in the 532 nm range depending on height from 100 m up to 2 km on 8 October 2013. A strong inversion layer was present in 20 the morning, increasing from about 800 m to 1100 m altitude. At this height, clouds were formed which can be identified by white structures in the lidar POLLY XT backscatter profiles. The clouds dissolved during the midday and afternoon. All in all, backscatter signals indicate a well-mixed contribution within the ABL. Figure 10 shows seven vertical profiles of potential temperature (Fig. 10a) and water vapour mixing ratio (Fig. 10b) obtained by verti-25 cal sounding of ALADINA and combined with MASC on 8 October. The first two profiles at 08:59 UTC and 09:04 UTC were flown with MASC and demonstrate a well-mixed ABL up to an altitude of 500 m above ground level. The potential temperature gradient is stable by values of 268.5 K to 287.0 K within the two profiles. Following profiles show still a well-mixed ABL with no significant inversion layer. During ALADINA flights at 13:05 UTC, 13:14 UTC and 13:18 UTC, the ABL topping inversion was higher than the maximum flight altitude of 1000 m above ground level. The last profile of MASC at 15:01 UTC indicates a weak inversion at 300 m. On this day, the water vapour mixing ratio obtained by the profiles of both 5 aircraft varies between 6.6 g kg -1 and 8.4 g kg -1 at the surface and decreasing values with height. On 9 October, the ABL is composed of two different air masses which can be seen by the backscatter signal in Fig. 9. Starting from the surface at night, the layer extends to approximately 400 m above ground level at 08:00 UTC due to solar radiation and therefore 10 given by the exchange of heat fluxes at the surface. During the day, the second layer is still increasing and finally mixed within the ABL at 14:00 UTC in the afternoon. Again, vertical profiles of both UAS are used in order to describe the lower part of the ABL in a range of 0-1000 m altitude in detail (Fig. 11). Three vertical profiles were flown with ALADINA from 08:26 UTC to 08:41 UTC with various vertical extents. Thereafter one vertical profile 15 of MASC ranged from 150 m a.g.l. to 650 m a.g.l. at 08:58 UTC. The water vapour mixing ratio reaches values up to 9 g kg -1 at the surface and the potential temperature a minimum of 288 K at 150 m above ground level. Above 350 m a different layer with other properties, almost neutrally stratified, extends up to 750 m which can be taken from both parameters. 20 During the campaign in October 2013, the aerosol number concentration of particles larger than 10 nm varied between 1500 cm -3 and 35500 cm -3 (Fig. 12, Fig. 13) which is relatively low for continental conditions. But as it was mentioned in Subsect. 3.2, the origin of air masses was over the Atlantic Ocean. Furthermore, air masses were transported only within the last two days over Central Europe so that there was a low potential for pollution. 25 On October 8, the contour plot obtained by the TSMPS data (Fig. 12) shows a significant decrease of the number concentrations between less than 1600 cm -3 and 4470 cm -3 within the developing boundary layer around 07:00 UTC. At 10:00 UTC, an increase of particle number concentrations with a diameter of about 10 nm is observed and meets a maximum of 6000 cm -3 . These small particles are formed due to new particle formation. At 11:00 UTC, a particle burst of aerosols with a diameter range of 100 nm to 200 nm occurs. After 12:00 UTC the picture appears even more variable. NPF stops and starts again but with a broader burst. This is probably due to variations in the global radia-5 tion by increasing coverage of cumulus clouds (see Fig. 6). However, this day is not a classical new particle formation day with a continuous growth of the small particles, they appear as small bursts. These ground-based measurements fit well with the measured profiles of ALADINA. At the ground the mean total aerosol number concentration of the small particles is 10000±2000 cm -3 and constant to a height around 180 m (Fig. 14c). Be-10 tween 180 and 240 m altitude, a layer of significantly enhanced aerosol is observed. The maximum of the CPC1 is N 11 =17000±3400 cm -3 and the CPC2 detects a maximum of N 18 =14500±2900 cm -3 . A second layer of enhanced aerosol load is detected in 550 to 620 m altitude. Above, the total aerosol number concentration is decreasing to the same amount as measured at the ground. All in all, a continuous difference between CPC1 and 15 CPC2 is observed by the vertical distribution of ∆N, meaning there is a certain number of particles in the size range between 11 nm and 18 nm. Nevertheless, the most dominant differences are in the two distinct aerosol layers.

Aerosol distribution within the ABL
On the next day (October 9), no sharp jump of number concentrations of small particles with a diameter of 10 nm can be observed by the TSMPS at the ground (Fig. 13). But Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | the aerosol distribution is rather constant. But in the height of 480 to 620 m, the total aerosol number concentration is rapidly increasing to N 18 =8000±1600 cm -3 and N 11 =11000±2200 cm -3 .
This difference corresponds to ∆N=3000±600 cm -3 , thus to a particle burst and here without any obvious connection to the stratification of the ABL. In the altitude between 620 and 1000 m, the total aerosol number concentration is decreasing again and both CPCs detect 5 the same amount. All in all, a high dependency of atmospheric aerosols on the atmospheric stratification is observed, apart from the particle burst events.
According to the time series of the backscatter lidar POLLY XT (Fig. 8), the ABL altitude increases from about 800 m above ground to about 1100 m during the morning transition on 8 October. The aerosol load appears to be rather well-mixed within the ABL because 10 no strong layers could be identified from the colour scheme. On 9 October, an increase of the ABL altitude from about 1000 m to 1200 m is also observed (Fig. 9). The aerosol distribution shows a layer of clearly enhanced aerosol load at an altitude below 400 m. These features are in agreement with the observations of ALADINA. In Fig. 14d) and Fig.  15d) the profiles of the OPC 0.5 µm channel reveal a uniform distribution of aerosols up 15 to 1000 m on 8 October. In contrast, an enhanced aerosol load in the lowermost 400 m is observed on 9 October. Both days indicate clean air masses due to the small amount of atmospheric particles with a diameter range of larger than 300 nm. However, for the small particles recorded with the CPCs, there is an additional fine layer structure visible that is not caught by the larger wavelengths of the OPC (300 nm, 500 nm and 700 nm). On 20 8 October, layers with enhanced small particle concentration are observed directly above ground, at an altitude of 200 m to 250 m and at 600 m. All in all, a high vertical variability of the small-size aerosol concentration is presented (Fig. 14c). On 9 October 2013, the small-size particles are also enhanced in the lowest 320 m. Pronounced peaks of smallsize aerosol are found at an altitude of 650 m and 780 m. The enhanced aerosol load in 25 the lowest 400 m on 9 October 2013 is directly related to the ABL conditions. The profile of the potential temperature (Fig. 15a) shows a slightly unstable stratification within the lowermost 400 m. At this altitude, a shallow layer of stably stratified air prevents mixing with higher altitudes. Also, the water vapour mixing ratio is considerably enhanced in the lowest 400 m (Fig. 15b). From 320 m to 870 m the thermal profile is neutral and then again stable above. The layer of enhanced small-size aerosol concentration at 780 m coincides with the beginning of the stable stratification. In contrast, the profile of the potential temperature on 8 October reveals an almost neutral, but strongly varying potential temperature (Fig. 14a). This indicates that air masses of different origin might have been sampled during the ascent 5 of ALADINA in spirals. Again, the layers of enhanced small size aerosol concentration (Fig.  14c) coincide with the altitudes where changes in the profile of the potential temperature are observed, enabling or prohibiting mixing processes.

Summary and conclusions
In this study, the new unmanned research aircraft Carolo P360 "ALADINA" is presented. 10 It was developed for atmospheric boundary layer aerosol research and thus equipped with a small and light (< 2.8 kg) aerosol and meteorological payload. Two CPCs (condensation particle counters) with different threshold diameters (11 and 18 nm) offer the possibility to measure total particle number concentrations of freshly formed particles with a response time t 10 %-90 % of 1.3 s. An OPC (optical particle counter) records size distributions of larger 15 particles in six channels between 0.3 and 10 µm. Fast meteorological sensors with a resolution of up to 30 Hz provide turbulence measurements and thus the possibility to connect atmospheric and aerosol dynamics. After sensor characterization in the laboratory and in a wind tunnel, a first test campaign was conducted at the GAW station Melpitz (Germany) in October 2013. Typical flight patterns for identifying the vertical distribution of aerosols 20 were flown by vertical profiles from the ground up to 1000 m altitude. Additionally, MASC provided measurements of meteorological parameters simultaneously. In this paper, data of two flights with different aerosol conditions were analysed. One flight was performed on 8 October 2013 at 13:14 UTC. On this day, new particle formation was observed at the ground by the TSMPS data. A continuous difference of both CPCs, ∆N, represents a low 25 number concentration of freshly formed particles in the prevailing neutrally stratified ABL. In addition, two significant aerosol layers were present with a maximum of 17000±3400 cm -3 in 180 and 220 m altitude and 14000±2800 cm -3 in the level of 550 m to 650 m, in an otherwise well-mixed structure of aerosols in the ABL. During the other flight on 9 October at 08:37 UTC, no particle burst event was detected at the ground, but an internal layer in the atmospheric boundary layer with different aerosol properties was present. From the ground to the inversion at 320 m altitude, the total number concentration was decreasing 5 and reached a minimum above the inversion. At a level between 500 and 600 m altitude, a difference of both CPCs was measured that corresponds to a new particle formation event. The comparison of the ALADINA results with ground-based in-situ aerosol instruments and a lidar POLLY XT gives confidence in the obtained data and in the capability of this new system to record aerosol size distributions with good reliability. After the campaign, one 10 additional original TSI Peltier element was installed to cool the condenser of one CPC and to increase the temperature difference inside the second CPC to achieve two more distinct detection threshold limits (6 nm and 18 nm). The potential of this flexible airborne system for new particle formation investigations will be further explored in future follow-up campaigns at the Melpitz observatory. The further technical and scientific development of ALADINA will 15 accompany the follow-up campaigns. The possibility to optionally implement other aerosol instruments (e.g. an aethalometer) is currently under discussion. Future applications will also include the validation of remote sensing derived aerosol properties. Harnisch, F., Gohm, A., Fix, A., Schnitzhofer, R., Hansel, A., and Neininger, B.: Spatial distribution of aerosols in the Inn Valley atmosphere during wintertime, Meteorol. Atmos. Phys., 103, 1-4, 223-235, 2009. Hermann, M., and Wiedensohler, A.: Counting efficiency of condensation particle counters at lowpressures with illustrative data from the upper troposphere, J. Aerosol Sci. 32, 8, 975-991, 2001. 15 Hellmuth, O.: Columnar modelling of nucleation burst evolution in the convective boundary layer-first results from a feasibility study, Part III: Preliminary results on physicochemical model performance using two "clean air mass" reference scenarios, Atmos. Chem. Phys., 6, 12, 4231-4251, 2006. Illingworth, S.M., Allen, G., Percival, C., Hollingsworth, P. 22 Wiedensohler, A., Birmili, W., Nowak, A., Sonntag, A., Weinhold, K., Merkel, M., Wehner, B., Tuch, T., Pfeifer, S., Fiebig, M., and others: Mobility particle size spectrometers: harmonization of technical standards and data structure to facilitate high quality long-term observations of atmospheric particle number size distributions, Atmos. Meas. Tech., 5, 657-685, 2012. Wildmann, N., Mauz, M., and Bange, J.: Two fast temperature sensors for probing of the atmospheric 5 boundary layer using small remotely piloted aircraft (RPA), Atmos. Meas. Tech., 6, 8, 2101-2113, doi:10.5194/amt-6-2101-2013, 2013     The aerosol payload is installed in the front compartment of the air plane and consists of two condensation particle counters (7 and 8) and one optical particle counter (9) that is mounted underneath (view from the rear). The first CPC1 (7) detects particle number concentrations in a particle diameter range of 11 nm to 2 µm. The second CPC2 (8) measures the particle number concentrations between 18 nm and 2 µm.    5.96*10 4 -10 5 3.55*10 4 -5.96*10 4 2.11*10 4 -3.55*10 4 1.26*10 3 -2.11*10 4 7.56*10 3 -1.26*10 4 4.47*10 3 -7.56*10 3 2.66*10 3 -4.47*10 3 1.59*10 3 -2.66*10 3 9.46*10 2 -1.59*10 3 5.63*10 2 -9.46*10 2 3.36*10 2 -5.63*10 2 2.00*10 2 -3.36*10 2 < 200  5.96*10 4 -10 5 3.55*10 4 -5.96*10 4 2.11*10 4 -3.55*10 4 1.26*10 3 -2.11*10 4 7.56*10 3 -1.26*10 4 4.47*10 3 -7.56*10 3 2.66*10 3 -4.47*10 3 1.59*10 3 -2.66*10 3 9.46*10 2 -1.59*10 3 5.63*10 2 -9.46*10 2 3.36*10 2 -5.63*10 2 2.00*10 2 -3.36*10 2 < 200   Figure 14. Vertical profiles of a) potential temperature θ, b) water vapour mixing ratio q, c) aerosol number concentrations for particle sizes above 11 nm (N 11 , green line), detected with the CPC1 and aerosol number concentrations for particles above 18 nm (N 18 , red line), measured with the CPC2, the difference of both CPCs (∆N, black dots) and d) particle number concentrations for particle sizes above 300 nm (pink line), 500 nm (bright green line) and 700 nm (