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Dusting off decades-old insight about atmospheric ice nucleators



Over the past year or so we have witnessed an impressive opulence of work describing the pool of ice nucleators in the atmosphere – with special focus on those having activity at temperatures warmer than that of the dominant mineral particles in the atmosphere – and about techniques to assess this (1–13). Results from this work are contributing to a more comprehensive understanding of the diversity of ice nucleators in the atmosphere over various spatial and temporal scales.  They are also moving forward the notion that the “importance” of any type of ice nucleator is not necessarily the direct consequence of concentrations (i.e. numbers / m3), but also involves the interplay of inherent ice nucleation activity, stability, and being at the right place at the right time.

Work conducted several decades ago on ice nucleators from the environment deserves to be brought to the forefront in the context of this evolving vision of atmospheric ice nucleators. Russ Schnell, who over his career has been associated with NCAR, NOAA and the University of Colorado – Boulder among other institutes, consecrated much of his research to understanding the nature of atmospheric ice nucleators.  Inspired by the parallels he saw between the current wave of research mentioned above and earlier work in the 1970’s and 80’s, he has contributed the following paragraphs.  The reports about this work are in conference proceedings and are not easily accessible.  Therefore, you will find links to scans of these documents embedded in the text.

Among the work from the 70’s and 80’s that is pertinent to current research efforts, there are the following:


1) ” A new technique for measuring atmospheric ice nuclei active at temperatures from -20 C to approaching 0 C, with results” (Schnell, 1979) (pdf) presents a technique and results very similar those recently reported about measurements at the Jungfraujoch Observatory (4).  From Schnell 1979, Figure 2,  it may be observed that following a cold front in late August (no freezing temperatures, no snow, just wind and a frontal passage), IN active at just warmer than -2 C were detected at concentrations of 1 IN/liter active at -6 C. In November and December with snow on the ground, air immediately behind cold fronts in air masses that had been snowing along their trajectory prior to reaching Boulder, there were few IN.  The weeks following passage of the fronts, the  IN concentrations built up slowly.  In Figure 1 in this report, it is apparent that IN concentrations measured below a temperature inversion were much more active and in higher concentration than those measured 300 meters higher in altitude which was above the inversion. This suggests that the IN source was probably local, and in this case from vegetation as the sampling tower was located in the center of about 3,000 acres of grass and grain fields.

Overall, this paper suggests that the filter drop-freezing combination technique is a sensitive, simple, cheap and effective means to measure IN concentrations in the atmosphere. Furthermore, it shows that there are warm temperature IN in the atmosphere just as they are in decayed vegetation at the surface in proximity to the atmospheric measurements.


2) “Kaolin and a biogenic ice nucleant: some nucleation and identification studies” (Schnell, 1977) (pdf1, pdf2) presented a range of measurements showing that active IN associated with a kaolin sample were in direct proportion to the organic content of the kaolin and could be removed by heating (Figures 1 and 2).  The paper also showed that the active IN in decayed tree leaf litter could be efficiently transferred to kaolin through water transfer, and then removed by heating.  Taken together, this suggests the possibility that the warmer range IN activity attributed to soil samples is of biological origin.  This would also be the case for dusts from the Sahara that originate in playas (dry lake beds and washes) that have higher biological content from vegetation that grew around and was washed into the playas.  There is little dust transported from the Sahara that is of the large grained variety from sand dunes. This work has particular application to recent papers such as those focusing on characterizing ice nuclei from ground-based measurements in the eastern Mediterranean region (2)  among many others.


3)  “Ice nucleation studies on bacteria aerosols” (Schnell et al., 1981) (pdf) shows that aerosolized IN bacteria can be captured and tested for IN activity using a filter based technique.  The aerosolized bacteria captured on the filters exhibited IN activity similar to their IN activity in water droplets tested prior to being aerosolized but that the aerosolized IN bacteria  lost activity rapidly in the relatively dry air in the aerosol tent (Figure 1).  Furthermore, the IN activity of the  Pseudomonas syringae decreases on filters rapidly over the first few hours and then much more slowly over the next 24 (Figure 2).  In some cases IN activity at -8 C was still detectable on the filters after a month of storage at room temperature.  The work on the survival and INA of bacterial aerosols in a cloud simulation chamber (14) reported somewhat analogous results but the chamber temperature and RH were different.


4)  “Ice nucleus measurement intercomparisons using three systems and three natural ice nucleants” (Schnell et al., 1982) (pdf) shows that the warmest IN in the atmosphere are the rarest but when a larger air sample is taken, more of the warm IN are captured (Figure1).  This is obviously intuitive and the data support this contention. In Figure 3, it can be seen that three different ways to measure IN (Filter-Drop-Freezing: FDF; Diffusion Chamber: DIFF; and NCAR Counter: NCAR) produce comparable results if the natural IN in a well decayed,  well aerosolized leaf  litter sample are the IN source. This paper adds support to the feasibility of measuring atmospheric IN concentrations with the Filter Drop Freezing technique in other studies on the subject.


Literature cited

You can find these and other references in the data base on biological ice nucleators and land-atmosphere feedbacks at:

  1. Atkinson JD, Murray BJ, Woodhouse MT, Whale TF, Baustian KJ, Carslaw KS, Dobbie S, O’Sullivan D, Malkin TL. 2013. The importance of feldspar for ice nucleation by mineral dust in mixed-phase clouds. Nature 498:355–8.
  2. Ardon-Dryer K, Levin Z. 2014. Ground-based measurements of immersion freezing in the eastern Mediterranean. Atmos. Chem. Phys. 14:5217–5231.
  3. Budke C, Koop T. 2015. BINARY: an optical freezing array for assessing temperature and time dependence of heterogeneous ice nucleation. Atmos. Meas. Tech. 8:689–703.
  4. Conen F, Rodriguez S, Hüglin C, Henne S, Herrmann E, Bukowiecki N, Alewell C. 2015. Atmospheric ice nuclei at the high-altitude observatory Jungfraujoch, Switzerland. Tellus B 67:25014,
  5. Conen F, Leifeld J. 2014. A new facet of soil organic matter. Agric. Ecosyst. Environ. 185:186–187.
  6. Hader JD, Wright TP, Petters MD. 2014. Contribution of pollen to atmospheric ice nuclei concentrations. Atmos. Chem. Phys. 14:5433–5449.
  7. Hiranuma N, et al. 2014. A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of seventeen ice nucleation measurement techniques. Atmos. Chem. Phys. Discuss. 14:22045–22116.
  8. O’Sullivan D, Murray BJ, Ross JF, Whale TF, Price HC, Atkinson JD, Umo NS, Webb ME. 2015. The relevance of nanoscale biological fragments for ice nucleation in clouds. Sci. Rep. 5:doi:10.1038/srep08082.
  9. O’Sullivan D, Murray BJ, Malkin TL, Whale TF, Umo NS, Atkinson JD, Price HC, Baustian KJ, Browse J, Webb ME. 2014. Ice nucleation by fertile soil dusts: relative importance of mineral and biogenic components. Atmos. Chem. Phys. 14:1853–1867.
  10. Stopelli E, Conen F, Zimmermann L, Alewell C, Morris CE. 2014. Freezing nucleation apparatus puts new slant on study of biological ice nucleators in precipitation. Atmos. Meas. Tech. 7:129–134.
  11. Tobo Y, DeMott PJ, Hill TCJ, Prenni a. J, Swoboda-Colberg NG, Franc GD, Kreidenweis SM. 2014. Organic matter matters for ice nuclei of agricultural soil origin. Atmos. Chem. Phys. Discuss. 14:9705–9728.
  12. Wex H, Augustin-Bauditz S, Boose Y, Budke C, Curtius J, Diehl K, Dreyer A, Frank F, Hartmann S, Hiranuma N, Jantsch E, Kanji ZA, Kiselev A, Koop T, Möhler O, Niedermeier D, Nillius B, Rösch M, Rose D, Schmidt C, Steinke I, Stratmann F. 2014. Intercomparing different devices for the investigation of ice nucleating particles using Snomax® as test substance. Atmos. Chem. Phys. Discuss. 14:22321–22384.
  13. Whale TF, Murray BJ, O’Sullivan D, Umo NS, Baustian KJ, Atkinson JD, Morris GJ. 2014. A technique for quantifying heterogeneous ice nucleation in microlitre supercooled water droplets. Atmos. Meas. Tech. Discuss. 7:9509–9536.
  14. Amato P, Joly M, Schaupp C, Attard E, Möhler O, Morris CE, Brunet Y., Delort A-M. 2015. Survival and ice nucleation activity of bacteria as aerosols in a cloud simulation chamber. Atmos. Chem. Phys. Discuss. 15:4055–4082.

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