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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-207
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
13 Mar 2017
Review status
A revision of this discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
Chemistry of Riming: The Retention of Organic and Inorganic Atmospheric Trace Constituents
Alexander Jost1,2, Miklós Szakáll1, Karoline Diehl1, Subir K. Mitra2, and Stephan Borrmann1,2 1Institute for Atmospheric Physics, University of Mainz, 55099 Mainz, Germany
2Particle Chemistry Department, Max Planck Institute for Chemistry, 55218 Mainz, Germany
Abstract. During free fall in clouds ice hydrometeors such as snowflakes and ice particles grow effectively by riming, i.e., the accretion of supercooled droplets. Volatile atmospheric trace constituents dissolved in the supercooled droplets may remain in ice during freezing or may be released back to the gas phase. This process is quantified by retention coefficients. Once in the ice phase the trace constituents may be vertically redistributed by scavenging and subsequent precipitation or by evaporation of these ice hydrometeors at high altitudes. Retention coefficients of the most dominant carboxylic acids and aldehydes found in cloud water were investigated in the Mainz vertical wind tunnel under dry growth (surface temperature < 0 °C) riming conditions which are typically prevailing in the mixed phase zone of convective clouds (i.e., temperatures from −16 to −7 °C and a liquid water content of 0.9 ± 0.2 g cm−3). The mean retention coefficients of formic and acetic acids are found to be 0.68 ± 0.09 and 0.63 ± 0.19. Oxalic and malonic acids as well as formaldehyde show mean retention coefficients of 0.97 ± 0.06, 0.98 ± 0.08 and 0.97 ± 0.11, respectively. Application of a semi-empirical model on the present and earlier wind tunnel measurements reveals that retention coefficients can be well interpreted by the effective Henry's law constant accounting for solubility and dissociation. A parameterization for the retention coefficients has been derived for substances whose aqueous phase kinetics are fast compared to mass transport timescales. For other cases, the semi-empirical model in combination with a kinetic approach is suited to determine the retention coefficients. These may be implemented in high resolution cloud models.

Citation: Jost, A., Szakáll, M., Diehl, K., Mitra, S. K., and Borrmann, S.: Chemistry of Riming: The Retention of Organic and Inorganic Atmospheric Trace Constituents, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-207, in review, 2017.
Alexander Jost et al.
Alexander Jost et al.
Alexander Jost et al.

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Short summary
During riming of graupel and hail, soluble chemical trace constituents contained in the liquid droplets could be retained while freezing onto the glaciated particle, or released back to the air potentially at other altitudes as retained. Quantification of retention constitutes a major uncertainty in numerical models for atmospheric chemistry and improvements hinge upon experimental determination of retention for carboxylic acids, aldehydes, SO2, H2O2, NH3, and others, as presented in this paper.
During riming of graupel and hail, soluble chemical trace constituents contained in the liquid...
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