The Fifth International Workshop on Ice Nucleation phase 2 (FIN-02): laboratory intercomparison of ice nucleation measurements
Authors: DeMott Paul J. , Möhler Ottmar , Cziczo Daniel J. , Hiranuma Naruki , Petters Markus D. , Petters Sarah S. , Belosi Franco , Bingemer Heinz G. , Brooks Sarah D. , Budke Carsten , Burkert-Kohn Monika , Collier Kristen N. , Danielczok Anja , Eppers Oliver , Felgitsch Laura , Garimella Sarvesh , Grothe Hinrich , Herenz Paul , Hill Thomas C. J. , Höhler Kristina , Kanji Zamin A. , Kiselev Alexei , Koop Thomas , Kristensen Thomas B. , Krüger Konstantin , Kulkarni Gourihar , Levin Ezra J. T. , Murray Benjamin J. , Nicosia Alessia , O'Sullivan Daniel , Peckhaus Andreas , Polen Michael J. , Price Hannah C. , Reicher Naama , Rothenberg Daniel A. , Rudich Yinon , Santachiara Gianni , Schiebel Thea , Schrod Jann , Seifried Teresa M. , Stratmann Frank , Sullivan Ryan C. , Suski Kaitlyn J. , Szakáll Miklós , Taylor Hans P. , Ullrich Romy , Vergara-Temprado Jesus , Wagner Robert , Whale Thomas F. , Weber Daniel , Welti André , Wilson Theodore W. , Wolf Martin J. , Zenker Jake 
Published: November 2018

Abstract. The second phase of the Fifth International Ice Nucleation Workshop (FIN-02)
involved the gathering of a large number of researchers at the Karlsruhe
Institute of Technology's Aerosol Interactions and Dynamics of the Atmosphere
(AIDA) facility to promote characterization and understanding of ice
nucleation measurements made by a variety of methods used worldwide.
Compared to the previous workshop in 2007, participation was doubled,
reflecting a vibrant research area. Experimental methods involved sampling of
aerosol particles by direct processing ice nucleation measuring systems from
the same volume of air in separate experiments using different ice nucleating
particle (INP) types, and collections of aerosol particle samples onto
filters or into liquid for sharing amongst measurement techniques that
post-process these samples. In this manner, any errors introduced by
differences in generation methods when samples are shared across laboratories
were mitigated. Furthermore, as much as possible, aerosol particle size
distribution was controlled so that the size limitations of different methods
were minimized. The results presented here use data from the workshop to
assess the comparability of immersion freezing measurement methods activating
INPs in bulk suspensions, methods that activate INPs in condensation and/or
immersion freezing modes as single particles on a substrate, continuous flow
diffusion chambers (CFDCs) directly sampling and processing particles well
above water saturation to maximize immersion and subsequent freezing of
aerosol particles, and expansion cloud chamber simulations in which liquid
cloud droplets were first activated on aerosol particles prior to freezing.
The AIDA expansion chamber measurements are expected to be the closest
representation to INP activation in atmospheric cloud parcels in these
comparisons, due to exposing particles freely to adiabatic cooling. The different particle types used as INPs included the minerals illite NX and
potassium feldspar (K-feldspar), two natural soil dusts representative of arable sandy loam
(Argentina) and highly erodible sandy dryland (Tunisia) soils, respectively,
and a bacterial INP (Snomax®). Considered
together, the agreement among post-processed immersion freezing measurements
of the numbers and fractions of particles active at different temperatures
following bulk collection of particles into liquid was excellent, with
possible temperature uncertainties inferred to be a key factor in determining
INP uncertainties. Collection onto filters for rinsing versus directly into
liquid in impingers made little difference. For methods that activated
collected single particles on a substrate at a controlled humidity at or
above water saturation, agreement with immersion freezing methods was good in
most cases, but was biased low in a few others for reasons that have not been
resolved, but could relate to water vapor competition effects. Amongst
CFDC-style instruments, various factors requiring (variable) higher
supersaturations to achieve equivalent immersion freezing activation dominate
the uncertainty between these measurements, and for comparison with bulk
immersion freezing methods. When operated above water saturation to include
assessment of immersion freezing, CFDC measurements often measured at or
above the upper bound of immersion freezing device measurements, but often
underestimated INP concentration in comparison to an immersion freezing
method that first activates all particles into liquid droplets prior to
cooling (the PIMCA-PINC device, or Portable Immersion Mode Cooling chAmber–Portable Ice Nucleation Chamber), and typically slightly underestimated INP
number concentrations in comparison to cloud parcel expansions in the AIDA
chamber; this can be largely mitigated when it is possible to raise the
relative humidity to sufficiently high values in the CFDCs, although this is
not always possible operationally. Correspondence of measurements of INPs among direct sampling and
post-processing systems varied depending on the INP type. Agreement was best
for Snomax® particles in the temperature regime
colder than −10 ∘C, where their ice nucleation activity is nearly
maximized and changes very little with temperature. At temperatures warmer than
−10 ∘C, Snomax® INP measurements (all
via freezing of suspensions) demonstrated discrepancies consistent with
previous reports of the instability of its protein aggregates that appear to
make it less suitable as a calibration INP at these temperatures. For
Argentinian soil dust particles, there was excellent agreement across all
measurement methods; measures ranged within 1 order of magnitude for INP
number concentrations, active fractions and calculated active site densities
over a 25 to 30 ∘C range and 5 to 8 orders of corresponding
magnitude change in number concentrations. This was also the case for all
temperatures warmer than −25 ∘C in Tunisian dust experiments. In
contrast, discrepancies in measurements of INP concentrations or active site
densities that exceeded 2 orders of magnitude across a broad range of temperature
measurements found at temperatures warmer than −25 ∘C in a previous study were
replicated for illite NX. Discrepancies also exceeded 2 orders of magnitude at
temperatures of −20 to −25 ∘C for potassium feldspar (K-feldspar), but these coincided
with the range of temperatures at which INP concentrations increase rapidly at
approximately an order of magnitude per 2 ∘C cooling for
K-feldspar. These few discrepancies did not outweigh the overall positive outcomes of the
workshop activity, nor the future utility of this data set or future similar
efforts for resolving remaining measurement issues. Measurements of the same
materials were repeatable over the time of the workshop and demonstrated
strong consistency with prior studies, as reflected by agreement of data
broadly with parameterizations of different specific or general (e.g., soil
dust) aerosol types. The divergent measurements of the INP activity of illite
NX by direct versus post-processing methods were not repeated for other
particle types, and the Snomax® data
demonstrated that, at least for a biological INP type, there is no expected
measurement bias between bulk collection and direct immediately processed
freezing methods to as warm as −10 ∘C. Since particle size ranges
were limited for this workshop, it can be expected that for atmospheric
populations of INPs, measurement discrepancies will appear due to the
different capabilities of methods for sampling the full aerosol size
distribution, or due to limitations on achieving sufficient water
supersaturations to fully capture immersion freezing in direct processing
instruments. Overall, this workshop presents an improved picture of present
capabilities for measuring INPs than in past workshops, and provides
direction toward addressing remaining measurement issues.