Ice residual properties in mixed-phase clouds at the high-alpine Jungfraujoch site

2.2.1 Particulate Volume Monitor (PVM-100) and Cloud Droplet Probe

Measurements of liquid water content (LWC) were carried out using a Cloud Droplet Probe (CDP, Droplet Measurement Technologies, USA) [Lance et al., 2010] and two Particulate Volume Monitors (PVM-100, Gerber Scientific Inc., USA) [Gerber, 1991; Wendisch et al., 2002]. The CDP is a single-particle instrument which provides a size distribution in the 2–50 μm diameter size range based on the intensity of scattered light at 4–12° in the forward direction from particles passing through a 658 nm laser beam [Lance et al., 2010]. LWC can subsequently be calculated from the measured size distributions. In contrast to the CDP, the PVM-100 measures light scattered by a particle ensemble. A 780 nm laser illuminates particles passing between the transmitter and receiver arms of the instrument, which house the laser diode and receiver optics, respectively. The light scattered in the forward direction is collected by the receiver optics and transmitted through two spatial filters. One of the spatial filters converts the scattered light with scattering-angle-dependent weighting factors to an output signal proportional to the volume concentration of the particle ensemble, i.e., LWC in the case of a droplet ensemble. The second spatial filter converts the scattered light with scattering-angle-dependent weighting factors to an output signal proportional to the surface area concentration. “In-cloud” periods during CLACE 2013 were defined using a LWC threshold value of 0.02 g m⁻³ from 5 min averages of the CDP and PVM-100 measurements (see Table 1 for a list of all measured cloud events). It should be noted that the presence of ice will lead to LWC overestimation in both the CDP and PVM-100 measurements. In the CDP measured optical diameter (D_opt) size range of 2–50 μm number concentrations of ice crystals are in general very low in MPCs as compared to the liquid droplet concentrations; therefore, the effect of ice presence on LWC inferred by the CDP should be minimal. The effect is likely larger for the PVM-100, as it does not have a size cut, and larger ice particles will thus also contribute to the PVM-100 signal. As we are interested in LWC only as a determinant of cloud presence, overestimation of LWC due to additional light scattering from ice crystals is not considered to be an issue.

2.2.2 Small Ice Detector

The number, size, shape, and phase of hydrometeors were measured using the Small Ice Detector (SID-3) [Kaye et al., 2008; Ulanowski et al., 2014; Vochezer et al., 2016], which acquires light scattering patterns from particles passing through a laser beam with a wavelength of 532 nm. The particles are counted and sized based on the scattered light intensity detected in the range of 40.75–59.25° relative to the forward direction. The shape and phase of cloud particles are deduced from high-resolution scattering patterns which are recorded by an intensified CCD camera in the 8–23° forward direction. The camera exposure trigger threshold can be adjusted. Typically, the trigger threshold was set high enough to avoid acquisition of liquid droplet scattering patterns. However, this also prevents the acquisition of small ice crystal scattering patterns. During several periods only, the trigger threshold was lowered so that measurements of cloud particles in the 2–28 μm size range could be made. During CLACE 2013 the SID-3 was mounted on a rotating platform, which automatically adjusted to shifts in the wind direction. The high-resolution scattering patterns obtained contain detailed information on the phase and habit of individual cloud particles. In order to distinguish the phase of the particles, their azimuthal symmetry is analyzed. This is based on the fact that a spherical droplet generates an airy pattern with perfect azimuthal symmetry, as opposed to ice crystal scattering patterns, which have lower azimuthal symmetry. Further details on the data retrieval procedure and associated uncertainties can be found in Vochezer et al. [2016].

2.3.1 Scanning Mobility Particle Sizers

For the size-resolved measurement of aerosol particles two scanning mobility particle sizers (SMPS) were deployed, one downstream of the total inlet (denoted further on as SMPS TI) and one downstream of the Ice-CVI (denoted further on as SMPS Ice-CVI). SMPS TI covered the mobility diameter (D_mob) range from 16 to 530 nm, while the SMPS Ice-CVI covered a smaller mobility diameter range from 16 to 420 nm. For further details on the setup of the SMPS systems and corrections applied to the SMPS measurements, the reader is referred to section S3 of the supporting information [von der Weiden et al., 2009; Wiedensohler et al., 2012].

2.3.2 Optical Particle Size Spectrometers (UHSAS, Grimm OPSS, WIBS3, and TSI OPS)

The Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) [Cai et al., 2008], the Grimm 1.108 and Grimm 1.129 [Burkart et al., 2010], and the TSI optical particle sizer (OPS) model 3330 all belong to the optical particle size spectrometer (OPSS) family of instruments, and operate on the same principle, whereby the intensity of light scattered by particles passing through a laser beam is measured and converted into an optical diameter. The WIBS3 [Foot et al., 2008; Gabey et al., 2010; Kaye et al., 2005] additionally measures ultraviolet light-induced fluorescence; however, particle sizing in the instrument is conducted using the same method as standard OPSS instruments. As particle fluorescence measurements are not within the scope of this paper and only the particle sizing data are used, the WIBS3 is described alongside the other OPSS instruments.

The Grimm 1.129 (denoted as Grimm 1), Grimm 1.108 (denoted as Grimm 2), WIBS3, and TSI OPS measure aerosol particles in the diameter size ranges of 0.25–32 μm, 0.3–20 μm, 0.8–20 μm, and 0.3–10 μm, respectively, while the UHSAS employs a high-intensity intracavity laser to detect and size particles between 60 and 1000 nm optical diameter. Although size distributions are measured up to 32 μm, all size distributions are shown for sizes up to 5 μm due to the cutoff of the CVI component of the Ice-CVI; interstitial particles above this size can be transmitted through the CVI and cannot be distinguished from true ice residuals, thus leading to potential contamination of the signal. Further information on the setup and calibration of the OPSS instruments, the data corrections applied and the measurement errors are provided in section S4 of the supporting information [Kaye et al., 2005; Foot et al., 2008; Gabey et al., 2010, 2011].

2.3.3 Single Particle Soot Photometers

Number concentration, size distribution, and mixing state of refractory black carbon (rBC) were monitored using single particle soot photometers (SP2) manufactured by Droplet Measurement Technologies (Boulder, CO, USA). Two four-channel SP2s were deployed during the campaign; one sampled the total aerosol, while the second measured the IR downstream of the Ice-CVI inlet. A comprehensive description of the SP2 operation principles is given by Stephens et al. [2003], Schwarz et al. [2006], and Moteki and Kondo [2007]. In brief, absorbing refractory particles passing through a 1064 nm high-fluence Nd:YAG laser beam are brought to their boiling point. The peak of the thermal radiation emitted by the incandescent BC core of a single particle is proportional to the mass of rBC contained in the particle. This dependency was calibrated during multiple calibrations using size-selected fullerene soot (Alpha Aesar; #FS12S011) according to Baumgardner et al. [2012], Gysel et al. [2011], Laborde et al. [2012a], and Moteki et al. [2010]. Measured rBC mass of a single particle is converted to a rBC mass equivalent diameter (D_MEV) using a void-free BC material density of 1800 kg m⁻³ [Moteki et al., 2010]. Detection of light scattered into a solid angle subtended from 13° to 77° and from 103° to 167° with respect to the laser beam axis [Gao et al., 2007] enables optical sizing of the sampled aerosol. The peak of the scattering signal is converted into the partial scattering cross section using a calibration coefficient defined during in situ calibrations carried out with spherical polystyrene latex size standards (Thermo Scientific, formerly Duke Scientific). The optical size is then calculated from the calibrated partial scattering cross section using Mie theory and assuming that the particles are spherical. A refractive index of 1.59 was used for the calibration, while 1.50 was assumed for measurements of the total aerosol and IR. The SP2 design ensures 100% detection efficiency for particles above the lower size limit of detection, which corresponds to a D_MEV=80 nm for rBC cores. As the SP2 is a quantitative, single-particle instrument, there is no number or mass concentration lower limit of detection, making it well suited to IR measurements. The leading edge only (LEO) method introduced by Gao et al. [2007] and slightly adapted by Laborde et al. [2012b] was applied for the optical sizing of all particles in order to avoid interference from particle evaporation within the laser beam for BC-containing particles. The partial scattering cross section derived from the LEO fit was converted to an optical diameter using Mie theory and assuming a homogeneous sphere or a concentric spheres core-shell structure for BC-free and BC-containing particles, respectively. The rBC core, if present, is assumed to have an index of refraction of 2.26 + 1.26i [Moteki et al., 2010], while the coating index of refraction is assumed to be 1.50, for consistency with the standard optical sizing. This, together with the BC core size constrained by the incandescence measurement, fully constrains the total particle size. The LEO-fit optical sizing is cross checked against the standard optical sizing and works in the diameter range from 170 to 440 nm. The coating thickness is defined as the difference between the radius of the total particle and the rBC core mass equivalent radius. In this work the coating thickness is only reported for BC cores having a D_MEV between 170 and 220 nm. Larger core sizes were not included in the results due to insufficient counting statistics. Agreement of the two SP2s between each other for all measured parameters was verified during the campaign based on simultaneous measurements at the total inlet.

3.3.1 Ice Activated Fraction

The campaign average size distributions presented in Figure 6 were used to calculate the ice activated fraction (IAF) as a function of particle diameter, where the particle diameter is either the mobility diameter (SMPS data) or the optical diameter (UHSAS, Grimm OPSS, WIBS3, and TSI OPS data), shown in Figure 7 for each instrument separately. As discussed in section 13, these size distributions are based on nonsimultaneous or only partially simultaneous measurements. The differences between the IAF inferred from different instruments for the same cloud event are thus affected by the variable sampling periods. Nonetheless, the IAF traces show a consistent trend, with the IAF increasing from approximately 10⁻⁴ to 10⁻³ for 100 nm (diameter) particles to values around 0.2 to 0.3 for 1 μm (diameter) particles. We hypothesize that this increase can be explained by a combination of two effects: First, by an increase in the number of active surface sites [Connolly et al., 2009; Niedermeier et al., 2011] with size for particles with similar composition. The relevance of this effect is supported by the study of Archuleta et al. [2005] who found that ice nucleation onset took place at lower relative humidities for larger particles based on continuous flow diffusion chamber measurements of mineral dust surrogates, as well as by Hung et al. [2003] who found that larger hematite cores immersed in aqueous ammonium sulfate droplets induced freezing at lower ice supersaturations than smaller hematite cores. Second, the increase in IAF with size could be due to an increase in the number fraction of particles which are good INP, most notably mineral dust, in the total aerosol as a function of size. This is equivalent to an increase in the density of the particles' active surface sites as a function of particle diameter with respect to the total aerosol population. The IAF trend observed in this study is similar to the results reported by Mertes et al. [2007] during previous Ice-CVI measurements at the Jungfraujoch (see black and orange dotted traces in Figure 7), although the absolute IAF values are generally several factors higher in this study. However, it should be noted that the results from Mertes et al. [2007] are based on a single cloud event of 10 h duration, as compared to several weeks of data presented in this paper, and thus, differences in the absolute values should not be overinterpreted.

In order to provide a means for predicting the number of small (5–20 μm) ice crystals formed, assuming the ambient aerosol size distribution is known, an empirical function has been fit through the measurement-derived IAF traces:

(1)

where D_p is the particle diameter in micrometers.

The fit is shown in Figure 7 as the blue trace. The blue shading spans the range of IAF_fit/5 and IAF_fit·5 (IAF_fit·5 was coerced to 1 at diameters >0.8 μm, where it exceeded the maximum possible IAF of unity). The INP number size distribution can be estimated through multiplication of the ambient aerosol number size distribution (dN/dlogD) with the size-dependent IAF as parametrized in equation 1. The total number of INP can subsequently be obtained by integrating the INP number size distribution. It is important to note that this fit is applicable only for conditions and aerosols similar to those experienced during the measurement periods included in this study (a median cloud temperature T_med=−18.1°C with a first quartile of T_Q1=−21.3°C and a third quartile of T_Q3=−13.1°C).

In order to put the IAF_fit into the context of previous work, we calculate the predicted number concentration of INP estimated from the total aerosol size distribution by means of equation 1 and compare this with the INP number concentration predicted based on the parametrization put forward by DeMott et al. [2010], henceforth referred to as D2010. For the total aerosol size distribution, required for both approaches, the combined UHSAS median size distribution between 0.1 and 0.6 μm and the OPS median size distribution above 0.6 μm are used. As INP are generally considered to be larger than 0.1 μm [Marcolli et al., 2007; Pruppacher and Klett, 1997], we do not consider particles smaller than 0.1 μm to be true INP and do not include such particles in the parametrization. Furthermore, as discussed by Mertes et al. [2007], it is possible that small apparent IR sampled downstream of the Ice-CVI originate from small secondary ice particles. Mertes et al. [2007] showed that the main increase in the activated fraction started at 0.28 μm and argued that particles above this size do not originate from secondary ice and are representative of the original INP. In order to ensure that we limit any potential sampling artifacts from our analysis and to be consistent with the approach taken by DeMott et al. [2010], as the lower size limit, we take 0.5 μm. The resulting number concentration of INP estimated with the parametrization developed in this study is 29 L⁻¹, with the submicrometer fraction (0.5–1 μm) contributing 10 L⁻¹ and the supermicrometer fraction (1–5 μm) contributing 19 L⁻¹. It should be noted that these IR number concentrations, which are obtained by applying the parametrization to the total aerosol size distribution, differ considerably from those directly obtained from the measured IR size distribution (see section 13). The reason for this discrepancy is differences in the IAF measured by different instruments, which results in high uncertainty of the parametrization. Specifically, the parametrization is a fit through measurements from six different instruments (two SMPS systems, UHSAS, WIBS3, Grimm 1, and OPS), while the IR number concentrations presented in section 13 are based only on the median IR size distribution measured by UHSAS and OPS.

Meanwhile, using D2010 with the median cloud temperature T_med and the number concentration of total aerosol larger than 0.5 μm N_>0.5, we obtain INP number concentrations of 0.32 L⁻¹, while INP number concentrations of 0.46 L⁻¹ and 0.15 L⁻¹ are obtained when using T_Q1 and T_Q3, respectively. The INP number concentrations predicted using equation 1 are thus approximately 2 orders of magnitude higher than the INP number concentrations calculated using D2010. In order to obtain a similar INP concentration as predicted using D2010, we need to take a lower size limit of 3.92 μm in our parametrization; this is an unrealistically high value, highlighting the discrepancies between INP measurements using ice nuclei counters and sampling of IR from ambient clouds.

The reason for the large difference in the INP number concentrations predicted by D2010 and the INP number concentrations predicted in this work is not clear. Some interference from small secondary ice collected by the Ice-CVI cannot be excluded. However, as shown in this work, as well as in previous studies [e.g., Kamphus et al., 2010; Mertes et al., 2007], the IR extracted from small ice crystals have physical and chemical properties different to those of the total aerosol, often with enrichment of particle types which, based on theory and laboratory experiments, would be expected to preferentially act as INP (e.g., larger particles and mineral dust). Sampling of secondary ice which has scavenged ambient aerosol particles would result in similar IR and total aerosol size distributions, and thus a lack of size dependency in the IAF, as well as similar chemical compositions of IR and total aerosol particles, in contrast to what we have observed. The difference in the physical and chemical characteristics of IR and the total aerosol, as well as the limited contribution of secondary ice production to the measured ice crystal concentrations, suggests that the ice crystals sampled are of primary ice origin and the IR extracted acted as INP. Consequently, the D2010 parametrization may provide estimates of INP which are substantially too low for the aerosols and cloud conditions probed at the Jungfraujoch during CLACE 2013. INP number concentrations measured at the Jungfraujoch with an ice nuclei counter by Chou et al. [2011] were 1 order of magnitude higher (i.e., a few particles per liter) than predicted using D2010, even though Chou et al. [2011] only investigated deposition nucleation, while the data used to derive the D2010 parametrization were taken at water supersaturated conditions and thus included not only deposition nucleation but also condensation and immersion freezing. The order of magnitude higher INP number concentrations measured by Chou et al. [2011] as compared to D2010 suggest that the discrepancy in concentrations obtained using our parametrization and using D2010 is not only a result of comparing different methods but also that D2010 is not applicable to the Jungfraujoch. Nonetheless, it is clear that further work is needed to close the gap between measurements of ice crystals, IR, and INP in MPCs.

3.3.2 BC Activation Into Ice

The IAF was also calculated for the subset of measurements obtained using the SP2 instruments. In this case, the IAF is calculated based on periods with simultaneous measurements of the total and Ice-CVI SP2s only. As this is a considerably smaller subset than all data included in Figure 7, the results are shown in a separate figure (IAF as a function of optical diameter is shown in Figure 8). In order to verify the SP2-based results, the measurements were compared with the IAF derived from the SMPS for the same period and showed good agreement (the IAF of the sum of BC-free and BC-containing particles measured by the SP2 and the IAF from the SMPS is given by the blue and red traces, respectively, in Figure 8). The IAF obtained from the SP2 instruments was calculated separately for BC-free particles (green trace in Figure 8) and for incandescing BC-containing particles (black trace in Figure 8). While both traces show an increase in the IAF with increasing particle size, the IAF for BC-containing particles is an order of magnitude lower than the IAF for BC-free particles, for a given total particle size.

The IAF was also calculated for BC-containing particles as a function of the rBC mass equivalent diameter (D_MEV) of the BC cores (size distributions of the BC cores in the total aerosol and in the IR are shown in Figure 9 (top), with the resulting IAF displayed in Figure 9 (bottom)). An increase in the IAF from 2·10⁻⁵ for BC cores with D_MEV=0.08 μm to 5·10⁻⁴ for BC cores with D_MEV=0.29 μm is observed. The BC mass size distribution observed at Jungfraujoch peaked at 111 nm (geometric mean), which is similar to previous atmospheric observations [Dahlkötter et al., 2014; Laborde et al., 2013; Moteki et al., 2012; Schwarz et al., 2008]. The total aerosol BC number size distribution peaks below a BC core diameter of 100 nm and decreases with size, reaching a value 1 order of magnitude lower for a core size D_MEV=200 nm and 2 orders of magnitude lower for a core size D_MEV=290 nm. The IR BC number size distribution also peaks below a BC core diameter of 100 nm; however, it decreases much more gradually and is approximately a factor of 2 lower for a core size D_MEV=200 nm and a factor of 4 lower for a core size D_MEV=290 nm. The number-weighted mean activation ratio of BC-containing particles is below 10⁻⁴, i.e., much lower than that of all particles, implying that, on average, BC-containing particles are very poor INP in the probed MPC conditions. Note that some unusual incandescence signals were detected by the SP2, characterized by slower signal rise and decay times as well as different color ratios when compared to incandescence signals from normal BC-containing particles. All unusual incandescence signals, which are most likely a non-BC interference from, e.g., dust or other metal-containing particles, were associated with supermicrometer-sized particles. Therefore, this interference does not at all affect the results presented in Figure 8, which is restricted to particles in the size range below ∼420 nm in diameter. All unusual incandescence signals were filtered for the analysis presented in Figure 9, in which the abscissa is based on the incandescence signal amplitude. Keeping them would have no effect on the total BC core size distribution (blue line in Figure 9 (top)), whereas the BC core size distribution of the ice residual particles (green line in Figure 9 (top)) and the IAF (Figure 9 (bottom)) would increase by less than a factor of 2, thus not changing the basic finding that the IAF at a certain BC core size is on the order of 10⁻⁴.

Based on the SP2 measurements, we conclude that the number of BC-containing particles in IR is very low. Furthermore, BC-containing particles are relatively depleted in the IR, with a number fraction of 1% in the IR population as compared to 12% in the total aerosol, based on SP2 measurements in the optical size range of 170 and 440 nm. Therefore, BC is not considered to play a significant role in nucleating ice at the Jungfraujoch. This result is in agreement with the study of Kamphus et al. [2010], who found no enrichment of BC in IR measured downstream of the Ice-CVI with an Aerosol Time-of-Flight Mass Spectrometer, and the study of Chou et al. [2011], who did not find any correlation between number concentrations of ice nucleating particles and BC mass (both studies were performed at the Jungfraujoch site). However, it is in contrast with the finding of Cozic et al. [2008], who found, in previous CLACE campaigns in 2004 and 2005, an enrichment of the BC mass fraction in submicrometer IR extracted using the Ice-CVI as compared to the submicrometer total aerosol, with BC mass fractions of 27% and 5%, respectively.

In order to understand the difference in the results obtained by Cozic et al. [2008] and those presented in this paper, it is necessary to consider the characteristics and limitations of the instruments used and the data processing procedure followed in the two respective studies. First of all, Cozic et al. [2008] determined BC mass fractions, whereas we report BC number fractions. BC mass fraction and BC number fraction are never perfectly correlated for variable aerosols. Indeed, the shift in the size distributions toward a higher fraction of larger particles in the IR compared to the total aerosol introduces a systematic bias in the direction of the observed discrepancy. However, while this may explain part of the discrepancy between the two studies, it certainly does not make a number-based depletion match with a mass-based enrichment. Possible measurement artifacts are more likely candidates. Cozic et al. [2008] applied a PSAP to quantify the equivalent black carbon (eBC) mass concentration. This is a filter-based technique, which provides a measure of the attenuation coefficient, from which the absorption coefficient is inferred as described by Bond et al. [1999]. The eBC mass concentration is finally inferred by assuming a mass absorption cross section (MAC) for the eBC (and assuming that all absorption is caused by BC). However, eBC mass concentrations inferred from absorption coefficients are tainted with considerable uncertainty due to the assumption to be made on the MAC, cross sensitivity to other absorbing components, such as mineral dust, as well as, to a lesser extent, to artifacts related to multiscattering enhancement within the filter matrix caused by aerosols with a high single scattering albedo.

The MAC value of the BC in the IR might be somewhat larger than the MAC values of the BC in the total aerosol due to differences in the lensing effect [e.g., Schnaiter et al., 2005] if the BC in the IR has thicker coatings, as suggested by the results in section 16. However, this effect is too small to explain the observed discrepancy.

Cozic et al. [2008] attempted to correct the eBC mass determined for the IR for potential interference from mineral dust absorption. However, the dust interference may have been underestimated as detailed in section S8 of the supporting information [Liu et al., 2010].

A further uncertainty in the interpretation of the results obtained by Cozic et al. [2008] is introduced by the fact that the BC mass fraction is calculated from the eBC mass measured for the whole sample divided by the aerosol mass inferred from the SMPS data for the submicrometer fraction only. The fact that the IR size distribution is shifted toward larger sizes compared to the total aerosol causes a larger truncation effect on aerosol mass for the ice residuals compared to the total aerosol, thereby introducing an artificial shift of the BC mass fraction toward larger values.

We cannot quantify which of the above effects play the dominant role in explaining the observed discrepancy. However, the above discussion highlights potential biases and pitfalls when using mass-based or absorption-based methods to assess the heterogeneous ice nucleation ability of BC. As it is the number of ice nucleating particles that is of importance, number-based methods should be favored when analyzing the importance of BC as heterogeneous INP. As such, we consider the SP2 to be a more robust measurement technique for this purpose.