Abstract. The HD(CP) 2 Observational Prototype Experiment (HOPE) was performed as a major 2-month field experiment in Jülich, Germany, in April and May 2013, followed by a smaller campaign in Melpitz, Germany, in September 2013. HOPE has been designed to provide an observational dataset for a critical evaluation of the new German community atmospheric icosahedral non-hydrostatic (ICON) model at the scale of the model simulations and further to provide information on land-surface-atmospheric boundary layer exchange, cloud and precipitation processes, as well as sub-grid variability and microphysical properties that are subject to parameterizations. HOPE focuses on the onset of clouds and precipitation in the convective atmospheric boundary layer. This paper summarizes the instrument set-ups, the intensive observation periods, and example results from both campaigns.HOPE-Jülich instrumentation included a radio sounding station, 4 Doppler lidars, 4 Raman lidars (3 of them providePublished by Copernicus Publications on behalf of the European Geosciences Union. temperature, 3 of them water vapour, and all of them particle backscatter data), 1 water vapour differential absorption lidar, 3 cloud radars, 5 microwave radiometers, 3 rain radars, 6 sky imagers, 99 pyranometers, and 5 sun photometers operated at different sites, some of them in synergy. The HOPEMelpitz campaign combined ground-based remote sensing of aerosols and clouds with helicopter-and balloon-based in situ observations in the atmospheric column and at the surface.HOPE provided an unprecedented collection of atmospheric dynamical, thermodynamical, and micro-and macrophysical properties of aerosols, clouds, and precipitation with high spatial and temporal resolution within a cube of approximately 10 × 10 × 10 km 3 . HOPE data will significantly contribute to our understanding of boundary layer dynamics and the formation of clouds and precipitation. The datasets have been made available through a dedicated data portal.First applications of HOPE data for model evaluation have shown a general agreement between observed and modelled boundary layer height, turbulence characteristics, and cloud coverage, but they also point to significant differences that deserve further investigations from both the observational and the modelling perspective.

<p><strong>Abstract.</strong> The "HD(CP)<sup>2</sup> Observational Prototype Experiment" (HOPE) was executed as a major 2-month field experiment in Jülich, Germany, performed in April and May 2013, followed by a smaller campaign in Melpitz, Germany in September 2013. HOPE has been designed to provide a critical evaluation of the new German community atmospheric Icosahedral non-hydrostatic (ICON) model at the scale of the model simulations and further to provide information on land-surface-atmospheric boundary layer exchange, cloud and precipitation processes as well as on sub-grid variability and microphysical properties that are subject to parameterizations. HOPE focuses on the onset of clouds and precipitation in the convective atmospheric boundary layer. The paper summarizes the instrument set-ups, the intensive observation periods as well as example results from both campaigns. <br><br> HOPE-Jülich instrumentation included a radio sounding station, 4 Doppler lidars, 4 Raman lidars (3, 3, and 4 of these provide temperature, water vapor, and particle backscatter data, respectively), 1 water vapour differential absorption lidar, 3 cloud radars, 5 microwave radiometers, 3 rain radars, 6 sky imagers, 99 pyranometers, and 5 Sun photometers operated in synergy at different supersites. The HOPE-Melpitz campaign combined ground-based remote sensing of aerosols and clouds with helicopter- and balloon-based in-situ observations in the atmospheric column and at the surface. <br><br> HOPE provided an unprecedented collection of atmospheric dynamical, thermodynamical, and micro- and macrophysical properties of aerosols, clouds and precipitation with high spatial and temporal resolution within a cube of approximately 10 × 10 × 10 km<sup>3</sup>. HOPE data will significantly contribute to our understanding of boundary layer dynamics and the formation of clouds and precipitation. The datasets are made available through a dedicated data portal.</p>

Abstract. This paper examines the representativeness of ground-based in situ measurements for the planetary boundary layer (PBL) and conducts a closure study between airborne in situ and ground-based lidar measurements up to an altitude of 2300 m. The related measurements were carried out in a field campaign within the framework of the HighDefinition Clouds and Precipitation for Advancing Climate Prediction (HD(CP) 2 ) Observational Prototype Experiment (HOPE) in September 2013 in a rural background area of central Europe.The helicopter-borne probe ACTOS (Airborne Cloud and Turbulence Observation System) provided measurements of the aerosol particle number size distribution (PNSD), the aerosol particle number concentration (PNC), the number concentration of cloud condensation nuclei (CCN-NC), and meteorological atmospheric parameters (e.g., temperature and relative humidity). These measurements were supported by the ground-based 3 + 2 wavelength polarization lidar system Polly XT , which provided profiles of the particle backscatter coefficient (σ bsc ) for three wavelengths (355, 532, and 1064 nm). Particle extinction coefficient (σ ext ) profiles were obtained by using a fixed backscatter-to-extinction ratio (also lidar ratio, LR). A new approach was used to determine profiles of CCN-NC for continental aerosol. The results of this new approach were consistent with the airborne in situ measurements within the uncertainties.In terms of representativeness, the PNSD measurements on the ground showed a good agreement with the measurements provided with ACTOS for lower altitudes. The ground-based measurements of PNC and CCN-NC are representative of the PBL when the PBL is well mixed. Locally isolated new particle formation events on the ground or at the top of the PBL led to vertical variability in the cases presented here and ground-based measurements are not entirely representative of the PBL.Based on Mie theory (Mie, 1908), optical aerosol properties under ambient conditions for different altitudes were determined using the airborne in situ measurements and were compared with the lidar measurements. The investigation of the optical properties shows that on average the airbornebased particle light backscatter coefficient is 50.1 % smaller for 1064 nm, 27.4 % smaller for 532 nm, and 29.5 % smaller for 355 nm than the measurements of the lidar system. These results are quite promising, since in situ measurement-based Mie calculations of the particle light backscattering are scarce and the modeling is quite challenging. In contrast, for the particle light extinction coefficient we found a good agreement. The airborne-based particle light extinction coefficient was just 8.2 % larger for 532 nm and 3 % smaller for 355 nm, for an assumed LR of 55 sr. The particle light extinction coefficient for 1064 nm was derived with a LR of 30 sr. For this wavelength, the airborne-based particle light extinction coefficient is 5.2 % smaller than the lidar measurements. For the first time, the lidar ratio of 30 sr for 1064 nm was det...

Abstract. Measuring vertical profiles of the particle light-absorption coefficient by using absorption photometers may face the challenge of fast changes in relative humidity (RH). These absorption photometers determine the particle light-absorption coefficient due to a change in light attenuation through a particle-loaded filter. The filter material, however, takes up or releases water with changing relative humidity (RH in %), thus influencing the light attenuation. A sophisticated set of laboratory experiments was therefore conducted to investigate the effect of fast RH changes (dRH ∕ dt) on the particle light-absorption coefficient (σabs in Mm−1) derived with two absorption photometers. The RH dependence was examined based on different filter types and filter loadings with respect to loading material and areal loading density. The Single Channel Tricolor Absorption Photometer (STAP) relies on quartz-fiber filter, and the microAeth® MA200 is based on a polytetrafluoroethylene (PTFE) filter band. Furthermore, three cases were investigated: clean filters, filters loaded with black carbon (BC), and filters loaded with ammonium sulfate. The filter areal loading densities (ρ*) ranged from 3.1 to 99.6 mg m−2 in the case of the STAP and ammonium sulfate and 1.2 to 37.6 mg m−2 in the case the MA200. Investigating BC-loaded cases, ρBC* was in the range of 2.9 to 43.0 and 1.1 to 16.3 mg m−2 for the STAP and MA200, respectively. Both instruments revealed opposing responses to relative humidity changes (ΔRH) with different magnitudes. The STAP shows a linear dependence on relative humidity changes. The MA200 is characterized by a distinct exponential recovery after its filter was exposed to relative humidity changes. At a wavelength of 624 nm and for the default 60 s running average output, the STAP reveals an absolute change in σabs per absolute change of RH (Δσabs∕ΔRH) of 0.14 Mm−1 %−1 in the clean case, 0.29 Mm−1 %−1 in the case of BC-loaded filters, and 0.21 Mm−1 %−1 in the case filters loaded with ammonium sulfate. The 60 s running average of the particle light-absorption coefficient at 625 nm measured with the MA200 revealed a response of around −0.4 Mm−1 %−1 for all three cases. Whereas the response of the STAP varies over the different loading materials, in contrast, the MA200 was quite stable. The response was, for the STAP, in the range of 0.17 to 0.24 Mm−1 %−1 and, in the case of ammonium sulfate loading and in the BC-loaded case, 0.17 to 0.62 Mm−1 %−1. In the ammonium sulfate case, the minimum response shown by the MA200 was −0.42 with a maximum of −0.36 Mm−1 %−1 and a minimum of −0.42 and maximum −0.37 Mm−1 %−1 in the case of BC. A linear correction function for the STAP was developed here. It is provided by correlating 1 Hz resolved recalculated particle light-absorption coefficients and RH change rates. The linear response is estimated at 10.08 Mm−1 s−1 %−1. A correction approach for the MA200 is also provided; however, the behavior of the MA200 is more complex. Further research and multi-instrument measurements have to be conducted to fully understand the underlying processes, since the correction approach resulted in different correction parameters across various experiments. However, the exponential recovery after the filter of the MA200 experienced a RH change could be reproduced. However, the given correction approach has to be estimated with other RH sensors as well, since each sensor has a different response time. And, for the given correction approaches, the uncertainties could not be estimated, which was mainly due to the response time of the RH sensor. Therefore, we do not recommend using the given approaches. But they point in the right direction, and despite the imperfections, they are useful for at least estimating the measurement uncertainties due to relative humidity changes. Due to our findings, we recommend using an aerosol dryer upstream of absorption photometers to reduce the RH effect significantly. Furthermore, when absorption photometers are used in vertical measurements, the ascending or descending speed through layers of large relative humidity gradients has to be low to minimize the observed RH effect. But this is simply not possible in some scenarios, especially in unmixed layers or clouds. Additionally, recording the RH of the sample stream allows correcting for the bias during post-processing of the data. This data correction leads to reasonable results, according to the given example in this study.

<p><strong>Abstract.</strong> This study presents vertical profiles up to a height of 2300 m a.s.l. of aerosol microphysical and optical properties and cloud condensation nuclei (CCN). Corresponding data have been measured during a field campaign as part of the High-Definition Clouds and Precipitation for Advancing Climate Prediction (HD(CP)<sup>2</sup>) Observational Prototype Experiments (HOPE), which took place at Melpitz, Germany from September 9 to 29, 2013. <br><br> The helicopter-borne payload ACTOS (Airborne Cloud and Turbulence Observation System) was used to determine the aerosol particle number size distribution (PNSD), the number concentrations of aerosol particles (PNC) and cloud condensation nuclei (CCN) (CCN-NC), the ambient relative humidity (RH), and temperature (<i>T</i>). Simultaneous measurements on ground provided a holistic view on aerosol microphysical properties such as the PNSD, the chemical composition and the CCN-NC. Additional measurements of a 3 + 2 wavelength polarization lidar system (Polly<sup>XT</sup>) provided profiles of the aerosol particle light backscatter coefficient (σ<sub>bsc</sub>) for three wavelengths (355, 532 and 1064 nm). From profiles of σ<sub>bsc</sub> profiles of the aerosol particle light extinction coefficient (σ<sub>ext</sub>) were determined using the extinction-to-backscatter ratio. Furthermore, CCN-NC profiles were estimated on basis of the lidar-measurements. <br><br> Ambient state optical properties of aerosol particles were derived on the basis of airborne in situ measurements of ACTOS (PNSD) and in situ measurements on ground (chemical aerosol characterization) using Mie-theory. <br><br> On the basis of ground-based and airborne measurements, this work investigates the representativeness of ground-based aerosol microphysical properties for the boundary layer for two case-studies. The PNSD measurements on ground showed a good agreement with the measurements provided with ACTOS for lower altitudes. The ground-based measurements of PNC and CCN-NC are representative for the PBL when the PBL is well mixed. Locally isolated new particle formation events on ground or at the top of the PBL led to vertical variability in the here presented cases and ground-based measurements are not representative for the PBL. Furthermore, the lidar-based estimates of CCN-NC profiles were compared with the airborne in situ measurements of ACTOS. This comparison showed good agreements within the uncertainty range. <br><br> Finally, this work provides a closure study between the optical aerosol particle properties in ambient state based on the airborne ACTOS measurements and derived with the lidar measurements. The investigation of the optical properties shows for 14 measurement-points that the airborne-based particle light backscatter coefficient is for 1064 nm 50 % smaller than the measurements of the lidar system, 27.6 % smaller for 532 nm and 29.9 % smaller for 355 nm. These results are quite promising, since in-situ measurement based Mie-calculations of the particle light backscattering are scarce and the modelling is quite challenging. In contradiction for the particle light extinction coefficient retrieved from the airborne in situ measurements were found a good agreement. The airborne-based particle light extinction coefficient was just 7.9 % larger for 532 nm and 3.5 % smaller for 355 nm, for an assumed lidar ratio (LR) of 55 sr. The particle light extinction coefficient for 1064 nm was derived with a LR of 30 sr. For this wavelength, the airborne-based particle light extinction coefficient is 5.2 % smaller than the lidar-measurements. Also, the correlation for the particle light extinction coefficient in combination with Mie-based LR's are in agreement for typical LR's of European background aerosol.</p>

Abstract. A unique data set derived from remote sensing, airborne, and ground-based in situ measurements is presented. This measurement report highlights the known complexity of comparing multiple aerosol optical parameters examined with different approaches considering different states of humidification and atmospheric aerosol concentrations. Mie-theory-based modeled aerosol optical properties are compared with the respective results of airborne and ground-based in situ measurements and remote sensing (lidar and photometer) performed at the rural central European observatory at Melpitz, Germany. Calculated extinction-to-backscatter ratios (lidar ratios) were in the range of previously reported values. However, the lidar ratio is a function of the aerosol type and the relative humidity. The particle lidar ratio (LR) dependence on relative humidity was quantified and followed the trend found in previous studies. We present a fit function for the lidar wavelengths of 355, 532, and 1064 nm with an underlying equation of fLR(RH, γ(λ))=fLR(RH=0,λ)×(1-RH)-γ(λ), with the derived estimates of γ(355 nm) = 0.29 (±0.01), γ(532 nm) = 0.48 (±0.01), and γ(1064 nm) = 0.31 (±0.01) for central European aerosol. This parameterization might be used in the data analysis of elastic-backscatter lidar observations or lidar-ratio-based aerosol typing efforts. Our study shows that the used aerosol model could reproduce the in situ measurements of the aerosol particle light extinction coefficients (measured at dry conditions) within 13 %. Although the model reproduced the in situ measured aerosol particle light absorption coefficients within a reasonable range, we identified many sources for significant uncertainties in the simulations, such as the unknown aerosol mixing state, brown carbon (organic material) fraction, and the unknown aerosol mixing state wavelength-dependent refractive index. The modeled ambient-state aerosol particle light extinction and backscatter coefficients were smaller than the measured ones. However, depending on the prevailing aerosol conditions, an overlap of the uncertainty ranges of both approaches was achieved.

Abstract. Measuring vertical profiles of the particle light absorption coefficient by using absorption photometers may face the challenge of fast changes in relative humidity. These absorption photometers determine the particle light absorption coefficient due to a change in light attenuation through a particle-loaded filter. The filter material, however, takes up or releases water with changing relative humidity (rh in %), influencing thus the light attenuation. A sophisticated set of laboratory experiments was therefore conducted to investigate the effect of fast rh changes (drh/dt) on the particle light absorption coefficient (σabs in Mm−1) derived with two absorption photometers. The rh dependency was examined based on different filter types and filter loadings with respect to loading material and loading areal density. Different filter material was used in the two examined instruments. The Single Channel Tri-Color Absorption Photometer (STAP; Brechtel Manufacturing Inc, 1789 Addison Way, Hayward, CA 94544, USA) relies on quartz-fiber filter (PALL LifeScience, Pallflex Membrane Filters Type E70-2075W) and the microAeth® MA200 (AethLabs, 1640 Valencia St, Suite 2C, San Francisco, CA 94110, USA) is based on a Polytetrafluoroethylene (PTFE) filter band. Furthermore, three cases were investigated: clean filter, filter loaded with black carbon (BC) and filter loaded with ammonium sulfate. The filter loading areal densities (ρ*) ranged from 3.1 to 99.6 mg m−2 in the case of the STAP and ammonium sulfate, 1.2 to 37.6 mg m−2 considering the MA200. Investigating BC loaded cases, ρ*BC was in the range of 2.9 to 43.0 and 1.1 to 16.3 mg m−2 for the STAP and MA200, respectively. In addition, the effect of a silica-bead based diffusion on the rh effect was investigated. Both instruments revealed opposing responses to relative humidity changes (Δrh) with different amplitudes. Whereas the STAP shows a linear dependence to relative humidity changes, the MA200 is characterized by an exponential recovery after its filter was exposed to relative humidity changes. At a wavelength of 624 nm and for the default 60 second average output, the STAP reveals an absolute change in σabs per absolute change of rh (Δσabs/Δrh) of 0.14 Mm−1 %−1 in the clean case, 0.29 Mm−1 %−1 in the case of BC loaded filters, and 0.21 Mm−1 %−1 considering filters loaded with ammonium sulfate. The 60-second running average of the particle light absorption coefficient at 625 nm measured with the MA200 revealed response of around −0.4 Mm−1 %−1 for all three cases. Whereas the response of the STAP varies over the different loading materials in contrast the MA200 was quite stable. The minimum and maximum response was for the STAP 0.17 Mm−1 %−1 and 0.24 Mm−1 %−1 considering ammonium sulfate loading and in the BC loaded case 0.17 Mm−1 %−1 and 0.62 Mm−1 %−1, respectively. The minimum response shown by the MA200 was −0.42 Mm−1 %−1 and −0.36 Mm−1 %−1 at maximum for ammonium sulfate and −0.42 Mm−1 %−1 and −0.37 Mm−1 %−1 in case of BC loading, respectively. Using the aerosol dryer upstream, the STAP did not change the behavior, but the amplitude of the observed effect was reduced by a factor of up to three. A linear correction function for the STAP was developed here. It is provided by correlating recalculated particle light absorption coefficients at 1 Hz time resolution against the change rate of rh. The linear response is estimated with 10.08 Mm−1 s−1 %−1 and can be used to correct for bias induced to rh changes at this time resolution. A correction approach for the MA200 is also provided, however, the behavior of the MA200 is more complex. Further research and multi-instrument measurements have to be conducted to fully understand the underlying processes, since the correction approach resulted in different correction parameters across various experiments. However, the exponential recovery after the filter of the MA200 experienced a rh change could be reproduced. Due to our findings, we recommend to use an aerosol dryer upstream of absorption photometers to reduce the rh effect significantly. Furthermore, when absorption photometers are used in vertical measurements, the ascending or descending speed through layers of large rh gradients has to be low to minimize the observed rh effect. Additionally, recording the rh of the sample stream allows correcting for the bias during post processing of the data. This data correction leads to reasonable results, according the given example in this study.

Air quality measurements usually consist of ground-based instrumentation at fixed locations. However, vertical profiles of pollutants are of interest for understanding processes, distribution, dilution and concentration. Therefore, a multicopter system has been developed to investigate the vertical distribution of the concentration of aerosol particles, black carbon, ozone, nitrogen oxides (NOx) and carbon monoxide and the meteorological parameters of temperature and humidity. This article presents the requirements by different users, the setup of the quadrocopter system, the instrumentation and the results of first applications. The vertical distribution of particulate matter next to a highway was strongly related to atmospheric stratification, with different concentrations below and above the temperature inversion present in the morning. After the qualification phase described in this article, two identically equipped multicopters will be used upwind and downwind of line or diffuse sources such as highways or urban areas to quantify the influence of their emissions on the local air quality.