The HCCT 2010 experiment took place between 13 September and 25 October 2010, at the mountain ridge Thüringer Wald in central Germany. The same measurement sites were used as during two previous experiments (FEBUKO, 2001, 2002; Herrmann et al., 2005): (1) an upwind site (Goldlauter, 10∘ 45′20′′ E, 50∘38′28′′ N; 605 m a.s.l.), (2) a summit site (Schmücke, 10∘46′15′′ E, 50∘39′19′′ N; 937 m a.s.l.), and (3) a downwind site (Gehlberg, 10∘47′32′′ E, 50∘40′21′′ N; 732 m a.s.l.). A map of the surroundings of the measurement site along with a table giving the population number of the cities within a radius of approximately 50 km around the site can be found in the Supplement (Fig. S1 and Table S1). At the summit site Schmücke, two particle inlets were installed facing south-west in separate windows at the height of 15 m of a three-story building that hosts a field station of the German Environmental Protection Agency (Umweltbundesamt). The aerosol inlet was used to sample aerosol particles with aerodynamic diameters (daero) smaller than 5 µm under cloud free conditions. During cloudy periods the CVI (Mertes et al., 2005b) was additionally deployed to sample cloud droplets with diameters larger than 5 µm. The single particle aerosol mass spectrometer ALABAMA (Aircraft-based Laser Ablation Aerosol Mass Spectrometer) was operated while manually alternating between these two inlets. We did not attempt to detect interstitial aerosol because the ALABAMA size range (starting at 150 nm; see below) does not permit detection of small, unactivated particles.

A detailed description of the mass spectrometer ALABAMA can be found in Brands et al. (2011). The particles enter the vacuum chamber via a Liu-type aerodynamic lens (Liu et al., 1995a, b) and are focused to a narrow particle beam. Due to the pressure drop the particles are accelerated when exiting the aerodynamic lens. The final particle velocity depends on their size, shape and density. The particles are detected by the scattered light of two orthogonal continuous wave Nd:VO4 laser beams (λ = 532 nm). The optical detection of the particles limits the particle size range for smaller particles to about 150–200 nm. The particle size given as the vacuum aerodynamic diameter (dva) can be derived from the time difference between the two scattering signals by means of calibration with particles of known size. In addition the time difference is used to calculate the time at which a particle will arrive at the ionisation region of the mass spectrometer. There the particle is evaporated and ionised in one step by laser ablation with a pulsed Nd:YAG laser (λ= 266 nm). The resulting positive and negative ions are detected by a bipolar time-of-flight mass spectrometer. The laser ablation method is a qualitative method, such that it is not possible to relate the peak height to a mass concentration of a certain compound (e.g. Middlebrook et al., 2003).

Figure 1 shows the measurement set-up and additionally operated instruments at the summit site Schmücke. Besides the ALABAMA, an optical particle counter (OPC; Grimm, model 1.109, time resolution 6 s) as well as a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS; Aerodyne Research Inc., Drewnick et al., 2005) were run simultaneously. Furthermore a high-resolution time-of-flight AMS (HR-ToF-AMS; Aerodyne Research Inc., DeCarlo et al., 2006) and a multi-angle absorption photometer (MAAP; Thermo Scientific, model 5012, time resolution 1 min) were operated continuously at the aerosol inlet. The MAAP determines the mass concentration of equivalent black carbon (EBC; Petzold et al., 2013) based on the absorption of particles sampled on a filter. The results from the C-ToF-AMS and HR-ToF-AMS measurements are presented in an accompanying paper (Schneider et al., 2016). Outside of the building, a particle volume monitor (PVM; Gerber Scientific Inc., model 100, time resolution 1 min) for investigation of the liquid water content (LWC) and a weather station (Davis Vantage Pro) for meteorological parameters were installed. Additionally, Caltech active strand cloud water collectors (one stage, three stage, and five stage) were mounted. The pH value as well as the content of organic compounds of cloud water were analysed by offline methods at the Leibniz Institute for Tropospheric Research. Furthermore, aliphatic amines were analysed from filtrated cloud water samples (0.45 µm syringe filters, Acrodisc 13, Pall, Dreieich, Germany) using an ion chromatography method adopted from Facchini et al. (2008). Details of the method are given elsewhere (van Pinxteren et al., 2015).

During the campaign, a measurement period was considered as a full cloud event (FCE) if the following criteria were fulfilled: LWC of the summit site cloud above 0.1 gm-3, wind direction from the south-west (200–250∘ sector), wind speed at the Schmücke site between 2 and 12 ms-1, no fog at the two valley sites, no precipitation at any site, and air temperature above 0 ∘C. In the course of the data analysis, only those FCEs were chosen that fulfilled connected flow conditions, which were inferred using cross-correlations and coefficient of divergence (COD) for O3, particle number concentration in the Aitken mode (49 nm) and in the accumulation mode (217 nm). For details see Tilgner et al. (2014). Overall, 14 FCEs were identified and evaluated (Table 1). In this study we present the data from all cloud residue measurements behind the CVI, but the official FCEs are additionally analysed separately, thereby facilitating comparison with other data from HCCT-2010. FCEs showing insufficient number of mass spectra for statistical evaluations (FCE2.1, FCE4.1, FCE5.1, and FCE 26.2) are not considered in the following data analysis. Detailed information on cloud type and meteorological conditions of the individual FCEs can be found in the Supplement to Tilgner et al. (2014), a brief description of the cloud conditions is included in Table 1.

Back trajectories for the air masses encountered during HCCT-2010 were calculated using HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory; Draxler and Rolph, 2012). The air mass origin was determined 96 h before arriving at Mt. Schmücke with a time resolution of 1 h. The coordinates of the Schmücke at a height of 500 m above the ground were used as an endpoint in the model. 500 m were chosen because the model orography cannot resolve a small-scale mountain range like the Thüringer Wald with sufficient detail. The back trajectories for the whole HCCT-2010 campaign can be found in the Supplement (Fig. S2). Back trajectories for the FCE are discussed in Sect. 3.3.

During the whole HCCT-2010 campaign the ALABAMA sampled over 286 000 single particle mass spectra. The mass spectra were distinguished between out-of-cloud aerosol and cloud residual particles according to the inlets and the LWC. For the analysis of out-of-cloud aerosol only measurement periods with an LWC > 0.05 g m-3 were considered (402 h), while data sampled behind the CVI were only examined for an LWC > 0.1 g m-3 (228, 106 h during FCEs). Upon inspection of the data set, a certain number of mass spectra were found to contain ions of only one polarity. The appearance of such monopolar mass spectra was also observed in other single particle mass spectrometer measurements (Bein et al., 2005; Sodeman et al., 2005; Shields et al., 2007; Pratt et al., 2010), resulting either from technical issues (e.g. tuning of high voltages) or from ion formation of only one polarity (Sodeman et al., 2005). Negative ion mass spectra provide better information on secondary organic and inorganic compounds (nitrate and sulfate) and thereby also on the mixing state of the particles. However, in order to constrain the analysis to a consistent data set, monopolar mass spectra were excluded and only mass spectra of both polarities were considered for the data analysis presented here. Out of the remaining 177 752 bipolar single particle mass spectra 164 595 were obtained while sampling out-of-cloud aerosol and 13 157 while sampling cloud residues (out of these 4400 were obtained during FCEs).

Figure 3 shows the size-resolved particle composition for all particles (not separated for out-of-cloud aerosol and cloud residues). The relative fraction of all particles in the specific size class is given in order to eliminate the size-dependent detection efficiency of the ALABAMA (Brands et al., 2011). The total number of analysed particles per size bin is given by the grey line. The maximum of the analysed particles lies in the size range between 500 and 550 nm, due to the best detection and ablation efficiency of the ALABAMA in this size range. The particle types shown in Fig. 3 refer only to the results obtained by the fuzzy c-means clustering; thus, the particle types “Fe, V”, “Ni”, and “Pb” are contained in the type “others”. The most abundant particle types are “org, K”, “biomass burning”, and “soot” (see also Table 4). The particle types “diesel exhaust” as well as “soot, org” only appear significantly at small vacuum aerodynamic diameters between 200 and 450 nm. This indicates that these particles were recently emitted and had no time grow by condensation or coagulation. In contrast, the particle types “org”, “amines”, and “soot” are observed only with diameters larger than 450 nm. This agrees with the observation that all analysed soot particles were internally mixed with nitrate and sulfate (see Sect. 3.4) and indicates that these soot particles were aged and have been processed and coated. Particles of the type “biomass burning” and “org, K” are found in all size classes, although the type “org, K” has a clear maximum between 400 and 500 nm.

Figure 4 shows the number of detected particles as a function of the local wind direction at the Schmücke. Panel a gives the standard wind rose for the whole time period. The dominating wind direction was south-west, with about 50 % probability for wind directions between 200 and 270∘. This direction corresponds to the requirements for cloud events. The absolute number of detected particles is given in panel b, showing that the majority of the detected particles were measured when the wind came from south-west. However, as shown in panel c, per unit of time more particles were detected when the local wind direction was between 0 and 90∘. In these directions lie several larger cities (Erfurt, Weimar, Jena; see map in Fig. S1) such that in general a higher pollution level may be expected.

Several particle types show a distinct diurnal pattern, indicating a source with a specific emission pattern. The fact that the emission pattern is detectable at the measurement site suggests that the source is not too far away, such that the diurnal pattern is not smoothed by different air mass transport velocities and different wind directions. An example is shown in Fig. 5. The figure shows the complete time series for the particle type “diesel exhaust” (upper panel) and the averaged diurnal pattern (lower panel). The diurnal pattern shows the increased occurrence of particles of this type between 09:00 and 24:00 LT and a decrease of this particle type during the night. This indicates the contribution of traffic emissions from within 1 or 2 hours from the measurement site (local traffic typically starts around 07:00 in the morning). All clusters contained in each particle type were inspected for such a diurnal trend. From this the amount of particle influences by local or regional sources were obtained. Table 4 shows the relative abundance of the particle types during HCCT-2010 along with the percentage of clusters showing diurnal variations and those not showing a diurnal trend. In total, about 63 % of the analysed particles belong to clusters indicating a diurnal variation. This finding implies that the aerosol composition during HCCT-2010 is mainly influenced by local and regional sources.

One of the main objectives of this study was the analysis of cloud residues and the comparison to the aerosol composition under cloud-free conditions. Figure 6 shows the average aerosol particle composition for all out-of-cloud aerosol particles and all cloud residues measured during HCCT-2010, not restricted to the full cloud events. It has to be noted that measurements of cloud residues and out-of-cloud aerosol can by definition not be made simultaneously, such that differences in the meteorological condition influences such a comparison. In the following we will compare the relative abundance of the individual particle types between cloud residues and out-of-cloud aerosol.

In the following the selected FCEs (see Table 1) will be analysed in more detail. These cloud events represent a subset of all cloud measurements and are referring to certain conditions that were given in detail in Tilgner et al. (2014). The composition of the cloud residual particles measured during the individual FCE during HCCT-2010 are shown in Fig. 9. Also given are the number of analysed mass spectra and the averaged mass concentration of equivalent black carbon measured in the interstitial aerosol during the events. Only FCE with sufficient (> 100) number of mass spectra are considered. These individual events show large event-to-event variability, especially in the fraction of particles of the type “soot”. Four FCE are characterised by a markedly higher fraction of “soot” particles, namely FCE 7.1, 11.2, 11.3, and 13.3, all of them between 24 September and 6 October. These events are also characterised by a high absolute mass concentration of EBC, especially FCE 11.2 and FCE 13.3 with about 300 ng m-3 of EBC. To further investigate this finding, the origin of the air masses encountered during these FCE is inspected by means of back trajectories. Figure 10 displays the 96 h back trajectories, separated for the “low-soot FCE” (left) and the “high-soot FCE” (right). It becomes clear that the air masses encountered during the “low-soot FCE” arrive mainly from west/north-west and have spent a considerable amount of the 96 h prior to the measurement over the Atlantic Ocean (see also Fig. S1 in the Supplement to Tilgner et al., 2014). These air masses had less opportunity to accumulate pollution particles over the continent. In contrast, the “high-soot” air masses arrive more from south/south-west and have travelled slower, therefore having spent more of the previous 96 h over land, leading to higher accumulation of anthropogenic emitted particles like soot. An exceptional case is FCE 22.0 (included in the left graph of Fig. 10), where the air masses arrive from east/north-east, although the local wind direction was south-west (as a prerequisite for an FCE). This air mass contained the highest fraction of biomass burning particles of all FCE and the highest number of mass spectra per event and time (1561 spectra in only 7.2 h, see Table 1). It has to be noted here that the full cloud events may not be representative for the general situation for clouds at the Schmücke, because of the selected data set. The composition data for the FCE are based on 4400 mass spectra, while for the composition of all cloud residues (Fig. 6) more than 13 000 mass spectra were analysed. For example, the required prerequisite that the local wind direction for an FCE has to be south-west, air masses of all FCE pass the city of Suhl located south-west of the Schmücke, which may lead to a higher relative amount of anthropogenic particles and a higher influence of local and regional emissions in the full cloud events compared to the general case. On the other hand, the analysis of the diurnal cycles in Sect. 3.1 has shown that in general local and regional sources have a high influence on the aerosol particle abundance.

As mentioned before, all identified particle types indicate internal mixtures with nitrate, sulfate, or both species. Therefore, the clustering algorithm cannot provide information about the mixing state of the particles from out-of-cloud to inside of the cloud. Therefore, the mixing state of the particles with nitrate and sulfate was investigated by means of the characteristic marker peaks m/z-62 (NO3-) and m/z-97 (HSO4-).

To compare cloud residues and out-of-cloud aerosol, we selected air masses with comparable origins based on HYSPLIT back trajectories for in-cloud and out-of-cloud conditions. As an additional criterion it was required that the local wind direction at the Schmücke was constant. The listed events “I” and “II” in Table 5 fulfilled these criteria. These events differ slightly from the defined FCEs because the criteria for the FCEs were not taken into account here. The cloud sampling phase of event “I” corresponds mostly to FCE1.1, while that of event “II” is a part of FCE24.0. During event “I” the air masses for in-cloud and out-of-cloud conditions both arrived from France, while air masses for both conditions during event “II” passed over England.

The characteristic marker peaks m/z-62 and m/z-97 in the single particle mass spectra of the events “I” and “II” show that only less than 1 % of the out-of-cloud aerosol particles contained neither nitrate nor sulfate for both events. Thus, 99 % of the out-of-cloud aerosol particles were already internally mixed with secondary inorganic compounds before passing the cloud. Such a high percentage of particles being internally mixed with secondary inorganic compounds were also found by single particle mass spectrometry during other studies in California (Cahill et al., 2012) or Harrow (Jeong et al., 2011). However, we observed an increase of the particle fraction containing nitrate in the cloud residues compared to the out-of-cloud aerosol, and the same finding holds for sulfate (Fig. 11, left). This can be explained by a more detailed analysis of the mixing state of the particles, distinguishing between particles containing only nitrate, only sulfate or both nitrate and sulfate (Fig. 11, right). Particles internally mixed only with sulfate (i.e. containing no nitrate) represent in general a minor fraction (< 3 %). For both analysed events the percentage of particles containing only nitrate and only sulfate was smaller in case of cloud residues compared to the out-of-cloud aerosol while the percentage of particles with nitrate and sulfate was increased. This observed increase indicates a sulfate addition to those particles that contained only nitrate, but also a nitrate addition to those particles that contained only sulfate. In the cloud residues, particles not containing nitrate can almost not be found. It must be noted that relative percentages of out-of-cloud aerosol particles internally mixed with secondary inorganics added for both events are less than the 99 % inferred above, due to the different threshold values that had to be used for the definition whether a peak is present in a mass spectrum or not (see Supplement, Sect. S3). This addition of sulfate and nitrate (and possibly also of organic components as amines, see Sect. 3.2.2) by cloud processing can also be observed in a size shift of the analysed particles. Figure 12 shows the size histograms of the particles analysed by ALABAMA during event I and II. In both cases a shift in the histogram to larger sizes is observed. The activation of CCN to cloud droplets is usually occurring at much smaller sizes (activation diameters upwind of the Schmücke have been observed to range between 123 and 194 nm; Henning et al., 2014), such that the observed size shift by ALABAMA can very likely be attributed to the uptake of gaseous species by the cloud droplets, leading to a size increase of the residual particles.

These two case studies demonstrate the change of the mixing state of the particles by chemical processes inside the cloud liquid phase. Similar observations were found earlier in numerous studies (e.g. Laj et al., 1997b; Sellegri et al., 2003; Brüggemann et al., 2005; McFiggans et al., 2006; Hayden et al., 2008; Zelenyuk et al., 2010). The enrichment of nitrate was also observed by simultaneous measurements with an AMS providing evidence of an increased mass concentration of nitrate in cloud residues compared to interstitial and out-of-cloud aerosol (Schneider et al., 2016). Such an enhancement of nitrate in cloud droplets can be explained by the uptake of gaseous nitric acid into the cloud droplets (Tilgner et al., 2005; Hayden et al., 2008). Enrichment of sulfate in cloud droplets can occur via different pathways. Besides the uptake of gaseous H2SO4 and the scavenging of ultrafine particulate matter also the uptake of SO2 with subsequent oxidation plays a role (Harris et al., 2014). Furthermore Harris et al. (2013) could prove that besides the known SO2 oxidation by H2O2 also the oxidation with O2 catalysed by transition metals plays a significant role in sulfate production. Despite the low number concentration of activated mineral dust particles in general, it became apparent that SO2 oxidation was mainly catalysed by dissolved transition metals during HCCT-2010. As it was shown in this study, transition metals were also detected by the ALABAMA (see Table 2).

Furthermore, the aerosol hygroscopicity was investigated in the same field experiment before and after cloud formation at the valley sites. In agreement with the described results, the hygroscopicity of the particles was found to be increased after passing the cloud (up to 50 %, see Henning et al. (2014). By means of the above described processes, water-soluble material is enriched inside the particles while being processed by the cloud. After evaporation of the cloud the water-soluble material it is likely to remain in the particles, thereby increasing their hygroscopicity. This process will occur in all cloud droplets formed from all CCN sizes, and therefore also influence the CCN properties of aerosol particles smaller than analysed here. For small aerosol particles that are in the size range of the activation diameter for a specific supersaturation the chemical composition plays an important role for the activation.