During January and February 2014, the Ice NUcleation Process Investigation And Quantification (INUPIAQ) project was conducted at the High-Altitude Research Station Jungfraujoch (JFJ; 3580 m a.s.l. – above sea level; 46.55∘ N, 7.98∘ E) in Switzerland to investigate the influence of a range of aerosol types on ice crystal number concentration alongside secondary ice processes in natural supercooled clouds. The facility is situated on a mountain ridge in between the peaks of the Jungfrau and Mönch with the Great Aletsch glacier, the largest in the Alps, to the south and is well away from major anthropogenic pollution sources. The JFJ site is enveloped by cloud for approximately 37 % of the time making it ideal for studying cloud–aerosol interactions, with the site residing in the free troposphere for most of the time during the wintertime .

Fluorescent aerosol number–size distributions were measured using a Wideband Integrated Bioaerosol Spectrometer version 4 (WIBS-4; University of Hertfordshire) on a single particle basis and designed primarily for identifying bio-fluorphores. A full technical description can be found in , while various applications and analysis approaches including monitoring at high-altitude sites can be found in , , and . A brief description of the instrument is now given. The WIBS-4 spectrometer exploits the principle of ultraviolet light-induced fluorescence where a particle of interest is excited with UV radiation and the resultant fluorescence is detected, with fluorescence being an indicator that the particle may be biological. In the WIBS-4 aerosol is drawn into the sample volume and illuminated by a 635 nm laser and the resultant forward scattered light is used to determine the particle size and shape using a quadrant detector . Side scattered light is collected and sequentially triggers two xenon flash lamps, filtered to excite the sampled particle at 280 and 370 nm. The first lamp is pulsed and the resultant fluorescence is collected, filtered and passed to two fluorescence detectors. The detectors are filtered to measure fluorescence over two detection bands (320–400 and 410–650 nm), which are then recorded. The second flash lamp is then triggered and the fluorescence detected by the second band is recorded. The whole process takes approximately 25 µs and the instrument has a maximum particle analysis rate of 125 particles s-1. This provides three measurements of particle fluorescence over two excitation wavelengths, particle size, and an approximation of particle shape, all on a single particle basis . The excitation and detection wavelengths have been selected to conform to auto-fluorescence bands of common components of biological materials (e.g. proteins, tryptophan and Nicotinamide adenine dinucleotide (NADH); the latter is related to cell metabolism) such that they can be discriminated from non-biological, non-fluorescent particles . Due to detector sensitivity and background fluorescence within the WIBS-4 optical chamber, the fluorescence of aerosol with diameters Dp < 0.8 µm cannot be accurately measured, and the counting efficiency decreases . Therefore, the analysis presented here is limited to aerosols with diameters greater than 0.8 µm, unless otherwise stated. Whilst WIBS-4 instruments have many advantages over traditional methods, ultraviolet – light-induced fluorescence (UV-LIF) spectrometers, limitations include difficulties in discriminating different classes of biological particles unambiguously and fluorescent non-biological aerosols must be identified. Fluorescence of some mineral dusts was examined by , who characterised their weak fluorescence properties allowing them to be generally discriminated from common PBAP using UV-LIF. In this study we use a new hierarchical agglomerative data processing method for WIBS-4 UV-LIF measurements to discriminate between particle types and the methods used are described in Sect. 4. A detailed discussion of this can be found in .

The WIBS-4 sampled from a total inlet (TI), which is now described. The TI samples all particles with Dp < 40 µm and for wind speeds < 20 m s-1. The sampled air is first heated to +20 ∘C, evaporating droplets and ice crystals such that their residuals are sampled along with any interstitial aerosol .

A custom built scanning mobility particle sizer (SMPS) has sampled continuously from the TI since 2008. It consists of a differential mobility analyser (DMA; TSI 3071) and a condensation particle counter (CPC; TSI 3775) and it measures the aerosol size distribution between 20 and 600 nm in diameter with 6 min time resolution . This was used to determine the origin of the sampled air masses, using the concentration of particles larger then 90 nm in diameter as described in Sect. 3.1 and .

Comprehensive cloud microphysics measurements were made at the site and are described in . In this study, cloud droplet and ice crystal number concentrations were measured, respectively, with a Cloud Droplet Probe (CDP-100; Droplet Measurement Technologies; DMT), described by , and a 3-View Cloud Particle Imager (3V-CPI). The CDP-100 is an optical scattering spectrometer able to size particles in the range 2 < Dp < 50 µm, whilst the 3V-CPI is an integrated 2-D Stereo (2DS) LED imaging spectrometer and Cloud Particle Imaging (CPI), charge-coupled device imaging spectrometer with resolutions of 10 and 2.3 µm, respectively . These are capable of measuring ice particle size distributions between 10 and 1280 µm and able to discriminate particle habit (based on shape analysis) for particles greater than approximately 25–30 µm. Details of the analysis techniques used for these instruments are provided in and .

An overview of the meteorological conditions at the JFJ site over the background period 6–18 February is provided in Fig. . Average temperatures of -11.3 ± 4.3 and -14.6 ± 3.3 ∘C were reported for out-of-cloud and in-cloud periods, respectively, with wind speeds of 5.2 ± 3.3 m s-1. Daily Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) back trajectory analysis (Fig. ) showed the majority of air masses to have passed over the Atlantic Ocean in the preceding 72 h during this period. Analysis of wind speed and direction shows the highest concentrations of fluorescent aerosols occur when the wind is coming from the south-east for wind speeds in excess of 15 m s-1, i.e. coincident with flow up from the Aletsch glacier.

We use the approach of to determine the origin of the sampled air masses so that boundary layer influenced air masses can be excluded from analysis; here we use the concentration of particles larger than 90 nm in diameter (N90) as described in to distinguish periods of free tropospheric conditions from those influenced by planetary boundary layer (PBL). They found that N90 = 40 cm-3 was a good approximation to describe free tropospheric background aerosol across all seasons, with periods influenced by the PBL resulting in N90 concentrations of several hundred to 1000 cm-3. These values were found to be lower in winter, so we use N90 < 30 cm-3 to be representative of background FT conditions and N90 < 50 cm-3 to be representative of FT-like conditions during the sampling period as described in . A time series of the SMPS N90 concentration for the analysis period is presented in Fig.  where the background FT condition of N90 < 30 cm-3 is met 66.2 % of the time and FT-like conditions where N90 < 50 cm-3 is met 88.4 % of the time. Periods with N90 > 50 cm-3, such as the extended period between 09:00 CET 15 February–09:00 CET 16 February, are excluded from analysis.

To assess the background conditions during the sampling period, we have compared the aerosol data collected during the campaign to long-term measurements made during February at the site between 2009 and 2014. Figure  shows median, 25th percentile, and 75th percentile SMPS and Optical Particle Counter (OPC) size-resolved concentration measurements made during the month of February from 2009 to 2014, which we compare to the campaign median SMPS, OPC, and WIBS non-fluorescent and fluorescent size-resolved concentrations, where the SMPS reports mobility diameter and the OPC and WIBS report optical diameter. It can be seen that the campaign measurements typically lie within the range of the 25th percentile and median values of the long-term measurements during February at the site, suggesting that the measurement period can be considered to be representative of the typical background aerosol concentration at the Jungfraujoch during wintertime.

The average out-of-cloud total coarse aerosol, NAll, and total fluorescent aerosol concentrations, NFl, measured by the WIBS-4 were 30.6 ± 19.3 and 6.3 ± 5.7 L-1, respectively, for the period 6–18 February, as shown in Fig.  (5 min averages).

To investigate the potential interaction of fluorescent aerosol with clouds, we have studied the fluorescent aerosol concentration fraction (NFl/NAll) over different temperature regimes for out-of-cloud, mixed-phase, and glaciated conditions as summarised in Fig. . Here we define out of cloud as all periods where the total water content (TWC) is less than 0.01 g m-3, mixed phase as all periods where the TWC ≥ 0.01 g m-3 and ice mass fraction (IMF) is less than 0.9, and glaciated as all periods where TWC ≥ 0.01 g m-3 and IMF ≥ 0.9.

To test the statistical significance of these results we have performed a one-way analysis of variance (ANOVA) analysis on subsets of the data, which we now describe; first we assessed the influence of temperature separately for in cloud (TWC ≥ 0.01 g m-3) and out-of-cloud (TWC < 0.01 g m-3) conditions where it can be seen in Fig.  that in each case the fluorescent fraction decreases with decreasing temperature. The ANOVA analysis returns small p values (4 × 10-6 and 1 × 10-4 for the out-of-cloud and in-cloud cases, respectively), which indicates that the means are statistically significantly different; however, the spread in values are large; next we assessed the influence of the presence of cloud on fluorescent fraction at each temperature by comparing the out-of-cloud and in-cloud cases for each temperature regime. This shows that the fluorescent fraction is generally increased in clouds (Fig. , top panels) with p values indicating that the means are significantly different (p < 0.05); finally we assessed the influence of cloud type on the fluorescent fraction for each temperature regime as shown in Fig. . Here it can be seen that the fluorescent fractions are generally greater in mixed-phase conditions than in glaciated conditions.

In summary it can be seen across all temperature regimes that the average in-cloud fluorescent aerosol fractions were slightly greater than for out-of-cloud conditions with the largest increase occurring during mixed-phase conditions. The observed increase in the fluorescent aerosol fraction in mixed-phase conditions is generally a result of a reduction in the non-fluorescent aerosol concentration relative to the corresponding out-of-cloud cases, rather than an enhancement in the fluorescent aerosol concentration. One possible explanation for this is that non-fluorescent aerosol has been removed via Cloud Condensation Nuclei (CCN) activation and lost in precipitating raindrops in mixed-phase clouds as this is not pronounced in the glaciated cases; however, caution must be applied when interpreting the results of this general approach as the differences in fluorescent aerosol fraction may be caused by differences in the sampled air masses for each case.

Figure  shows the fluorescent aerosol fraction for cloud events persisting for a minimum of 30 min in duration with mean, minimum, and maximum observed average fluorescent aerosol fractions of 0.27 ± 0.12, 0.05, and 0.49, respectively, over 31 separate cloud events. It can be seen that many of the clouds feature large variations in fluorescent aerosol fraction, while others have relatively little variation, which may be an effect of sampling several different air masses during a single cloud event. The correlation between mean and median fluorescent aerosol fraction and the following meteorological and cloud microphysical parameters were investigated: IMF, TWC, ice water content (IWC), liquid water content (LWC), ice and droplet concentrations, temperature, and wind speed and direction. A scatter plot of the mean (black +) and median (red diamonds) values for each cloud event is shown in Fig. , along with the corresponding r2 value where no significant correlation between parameters is observed. No apparent trend is observed between the fluorescent aerosol fractions and contemporaneous mean meteorological or cloud microphysical parameters, suggesting that particle fluorescence does not impact cloud evolution or formation.

The majority of cloud events occur in the -15 ∘C ≤ T < -10 ∘C regime: Fig.  shows the average fluorescent and non-fluorescent particle size distributions for out-of-cloud, mixed-phase, and glaciated conditions in this temperature regime. In each case the single mode of the distribution occurs at 0.58 µm; however, the counting efficiency for particles Dp < 0.8 µm is low , so the true mode is likely to be much smaller when measured with, e.g., an SMPS as indicated in Fig. .

Figure  shows the size dependence of the fluorescent aerosol fraction for the three studied temperature regimes for out-of-cloud, mixed-phase, and glaciated conditions. In each case, it was observed that the fluorescent aerosol fraction increases with size, with approximately 80 % of 10 µm particles being fluorescent in nature, with the fluorescent aerosol fraction decreasing to approximately 20 % for 1 µm particles. Caution must be applied when interpreting the sub-micron fluorescent aerosol fraction due to the reduced fluorescent counting efficiency for particles Dp < 0.8 µm , which may lead to an underestimation of the fluorescent aerosol fraction at small sizes. For clarity and ease of comparison only the mean ratios for each case are presented here. Individual plots for each case showing the mean and standard deviation of the fluorescent ratio are provided as a Supplement.