RETENTION OF ORGANIC AND INORGANIC SUBSTANCES DURING THE PHASE
CHANGE INDUCED BY RIMING
M. SZAKÁLL1, A. JOST2 , K. DIEHL1, S. K. MITRA2 and S. BORRMANN1,2
1
Institute for Atmospheric Physics, University of Mainz, Germany
2
Particle Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
Keywords: riming, retention, mixed-phase clouds, phase change chemical processes.
INTRODUCTION
The largest precipitation particles in mixed phase clouds, graupel and hailstones, grow by accretion of small
supercooled liquid cloud droplets. This microphysical process is the so-called riming, during which a phase
transition from liquid to ice occurs as the supercooled droplets freeze onto the surface of the glaciated
hydrometeor upon contact. Riming is the most effective growth processes in mixed-phase clouds
(Pruppacher and Klett, 2010). Furthermore, riming is also involved in the partitioning of volatile trace
substances of the atmosphere. During the freezing process of the liquid droplets on the ice particles’ surface
trace chemical substances dissolved in the liquid escape from the developing crystal structure. This
partitioning is quantified by the retention coefficient defined as the ratio of the concentration of the substance
in the aqueous phase prior to riming and the concentration of the substance in the ice phase after freezing.
Unless scavenged by precipitation, these substances contained in the ice phase will be released back to the
air by evaporation, e.g., at higher altitudes, if they are carried aloft in convective cloud systems. Thus,
retention during riming plays an important role in the redistribution and transport of atmospheric trace
substances which in turn may influence oxidation processes in the atmosphere. Nevertheless, retention is
one of the main uncertainties in cloud resolving models which simulate the redistribution of trace substances
by convective storms (e.g.: Barth et al., 2007a, Leriche et al., 2013, Bela et al., 2016). Further, the
scavenging of trace substances and subsequent precipitation of them play a significant role in the global
distribution and is an additional source of uncertainty related to the convection parameterizations in global
models (Tost et al., 2010). The dependencies of the retention coefficient could be determined by
systematically studying the ratio of the involved timescales for mass transport (aqueous phase, interfacial,
gas phase) to the freezing time of the droplets (Stuart and Jacobson, 2003, 2004). On the one hand the
retention coefficient was found to be highly dependent on chemical factors such as the effective Henry’s
law constant accounting for solubility and dissociation. On the other hand physical factors such as
temperature, ventilation, droplet size, and the liquid water content might significantly affect the retention
coefficient (Stuart and Jacobson, 2003, 2004, 2006; von Blohn et al., 2011, 2013). Therefore, we present
here the results of wind tunnel experiments carried out to investigate the retention coefficients of watersoluble inorganic and organic substances during this phase change under conditions close to the ones
prevailing in the mixed phase zone of convective clouds.
METHODS
In the Mainz vertical wind tunnel atmospheric particles of different sizes can be freely suspended at their
terminal velocities in a vertical air stream (Diehl et al., 2011). Two vacuum pumps are maintaining the
continuous air flow through the tunnel. Wind tunnel speeds up to 40 m/s are possible so that hydrometeors
of different sizes from a few micrometers up to several centimeters can be investigated. Thus, ventilation as
well as heat and mass transfers concerning hydrometeors are similar to those in the real atmosphere. The
wind tunnel air can be cooled down to -30 °C, which ensures the investigation of microphysical and chemical
processes under mid to upper tropospheric conditions in mixed phase clouds. A cloud of supercooled
droplets containing a single chemical component to be investigated was produced upstream of the rime
collector with the means of sprayers.
The conditions in the wind tunnel during the riming experiments correspond to those under which riming is
most effective in the atmosphere (Pruppacher and Klett, 2010); that is, the temperature ranged from -16 to
-7°C, the liquid water contents varied between 0.7 and 1.7 g/m3, and vertical wind speeds from 2 to 3 m/s
depending on the rime collector type. The captively-floated ice particles had a constant diameter of 8 mm
and the quasi-floated snowflakes had diameters between 8 and 12 mm. These conditions correspond to the
dry growth regime, i.e. the surface temperature of the collectors were below 0°C during riming. The
substances were investigated individually with liquid phase concentrations ranging from 30 to 100 μmol/l
and pH values ranging from 3.3 to 5.3.
RESULTS AND DISCUSSION
Fig. 1 shows a representative example of the measurements. Depicted are the retention coefficients of sulfur
dioxide as function of temperature. As indicated by the linear regression the parameterized retention
coefficient shows a negative dependency of the temperature. We have experimentally confirmed in the
present wind tunnel study that for most of the investigated substances the retention coefficient can properly
be estimated by means of the effective Henry’s law constant. Thus, the retention coefficients of the
substances increase with increasing solubility or – the other way around – they decrease with increasing
volatility. Consequently, very soluble substances like hydrochloric acid and nitric acid show retention
coefficients close to 1. In contrary, as predicted also by the timescale analysis of Stuart and Jacobson (2003,
2004), more volatile substances – such as sulfur dioxide, for instance – show low retention coefficients. The
retention coefficients of such substances are also dependent on ambient temperature and ventilation, i.e.
enhancement of mass and heat transfer due to flow around the collector (see Fig. 1). We found that the
dependency on the effective Henry’s law constant can only be confirmed for substances for which aqueous
phase processes occur quickly compared to mass transport processes. For substances which are limited by
aqueous phase reactions, such as ammonia in the presence of CO2 (Hannemann, 1995), and formaldehyde,
the estimation of the retention coefficient by the effective Henry’s law constant fails. In these cases, one has
to account for the appropriate timescales for aqueous phase reactions when describing retention. This
becomes especially important during freezing of a ventilated spread cloud droplet of 10 μm (typical
timescale of 1 ms) in radius. The mean values of the measured retention coefficients range from 0.2 for
sulfur dioxide up to 1.0 for nitric acid and hydrochloric acid, respectively. These high values validate
retention during riming as an efficient scavenging process for water-soluble trace substances. These findings
were confirmed by the investigation of organic species such as formic acid, acetic acid, oxalic acid, and
formaldehyde. From the whole dataset a reliable relationship between the retention coefficients and the
effective Henry’s law constant was obtained.
In conclusion one can say that the derived parameterization can be applied for all substances whose aqueous
phase reaction kinetics are fast compared to the involved mass transfer timescales (see Stuart and Jacobson,
2003, 2004, 2006). However, the cases of ammonia and formaldehyde emphasizes the importance of
aqueous phase reaction kinetics when describing retention. Particularly, real cloud droplets are highly nonidealized mixtures of all kind of substances, which might interact with each other. Although the developed
relationship of the retention coefficients on the effective Henry’s law constant is a large step towards a better
representation of retention in cloud resolving models, the actual nature of the process is still not well
understood. Beside the chemical point of view, retention contains many uncertainties arising from the
solidification process. In particular, the interaction between the molecules and the developing ice is complex
and depends on various parameters which by themselves are poorly understood, e.g. trapping of molecules
in liquid volumes during crystallization, segregation of the molecules from ice, the formation of an ice shell
along the surface of the droplets (which effectively hinders further degassing), and spreading of the droplets
upon impact on the rime collectors. Finally, retention describes a combination of coupled processes, which
need further laboratory and modeling efforts to better characterize them, also for wet-growth riming
conditions such as the growth of hail.
Figure 1: Measured retention coefficient of SO2 as function of ambient temperature. The solid lines
represent the linear regression of the data and the 95% confidence bands. From von Blohn et al., 2013
with changes.
ACKNOWLEDGEMENTS
This work was supported by the German Research Foundation (Deutsche Forschungsgemeinschaft) under
grant MI 483/6-1.
REFERENCES
Barth, M. C., Kim, S.-W., Skamarock, W. C., Stuart, A. L., Pickering, K. E., andOtt, L. E. (2007a).
Simulations of the redistribution of formaldehyde, formic acid, and peroxides in the 10 july 1996
stratospheric-tropospheric experiment: Radiation, aerosols, and ozone deep convection storm.
Journal of Geophysical Research: Atmospheres, 112, D13.
Bela, M. M., Barth, M. C., Toon, O. B., Fried, A., Homeyer, C. R., Morrison, H., Cummings, K. A., Li, Y.,
Pickering, K. E., Allen, D. J., Yang, Q., Wennberg, P. O., Crounse, J. D., St. Clair, J. M., Teng, A.
P., O’Sullivan, D., Huey, L. G., Chen, D., Liu, X., Blake, D. R., Blake, N. J., Apel, E. C., Hornbrook,
R. S., Flocke, F., Campos, T., and Diskin, G. (2016). Wet scavenging of soluble gases in DC3 deep
convective storms using WRF-chem simulations and aircraft observations. Journal of Geophysical
Research: Atmospheres, 121(8), 102-120.
Diehl, K., S. K. Mitra, M. Szakáll, N. v. Blohn, S. Borrmann, and H.R. Pruppacher, (2011) The mainz
vertical wind tunnel facility: a review of 25 years of laboratory experiments on cloud physics and
chemistry in Wind Tunnels: Aerodynamics, Models, and Experiments, ed. Pereira JD. (Nova Science
Publishers, Inc.), 69–92.
Hannemann, A. (1995). Eine experimentelle und theoretische Untersuchung zu gekoppelten Aufnahme von
SO2, NH3 und CO2 in Wassertropfen. PhD Thesis, Institute for Physics of the Atmosphere, University
of Mainz, Germany.
Leriche, M., Pinty, J.-P., Mari, C., and Gazen, D. (2013). A cloud chemistry module for the 3-D cloud resolving mesoscale model meso-nh with application to idealized cases. Geoscientific Model
Development, 6(4), 1275–1298.
Pruppacher, H. R. and Klett, J. D. (2010). Microphysics of clouds and precipitation. Springer, 2 edition.
Stuart, A. and Jacobson, M. (2003). A timescale investigation of volatile chemical retention during
hydrometeor freezing: Nonrime freezing and dry growth riming without spreading. Journal of
Geophysical Research, 108(D6), 4178.
Stuart, A. and Jacobson, M. (2004). Chemical retention during dry growth riming. Journal of Geophysical
Research, 109, D07305.
Stuart, A. L. and Jacobson, M. Z. (2006). A numerical model of the partitioning of trace chemical solutes
during drop freezing. Journal of Atmospheric Chemistry, 53(1), 13–42.
Tost, H., Lawrence, M. G., Brühl, C., Jöckel, P., Team, T. G., and Team, T. S.-O.-D. (2010). Uncertainties
in atmospheric chemistry modelling due to convection parameterisations and subsequent scavenging.
Atmospheric Chemistry and Physics, 10(4):1931–1951.
von Blohn, N., Diehl, K., Mitra, S. K., and Borrmann, S. (2011). Wind tunnel experiments on the retention
of trace gases during riming: nitric acid, hydrochloric acid, and hydrogen peroxide. Atmospheric
Chemisty and Physics, 11, 11569–11579.
von Blohn, N., Diehl, K., Nölscher, A., Jost, A., Mitra, S. K., and Borrmann, S. (2013). The retention of
ammonia and sulfur dioxide during riming of ice particles and dendritic snow flakes: laboratory
experiments in the mainz vertical wind tunnel. Journal of Atmospheric Chemistry, 70, 131–150.