A timescale investigation of volatile chemical retention during hydrometeor freezing: Nonrime freezing and dry growth riming without spreading

[1] Partitioning of volatile chemicals among the gas, liquid, and solid phases during the conversion of liquid water to ice in clouds can impact the poststorm distributions of chemicals in the troposphere and in precipitation. In this paper, we use a timescale-based methodology to determine the key physical parameters involved in retention and derive the dependence of retention on these parameters for nonrime freezing and rime freezing when droplet spreading is minimal. We calculate a dimensionless retention indicator for SO2, H2O2, NH3, and HNO3, for a variety of conditions relevant to natural clouds. We find that solute properties, particularly the effective Henry’s constant, likely have a large impact on retention. Chemicals with very high effective Henry’s constants (e.g., HNO3) will likely be retained completely under all conditions. For chemicals with lower effective Henry’s constants, freezing conditions (including pH, temperature, magnitude of the hydrometeor velocity in air, and drop size) will likely have significant impacts on retention, while air pressure has only a small effect. The dependence on velocity and drop size depends on the limiting mass transport regime and is nonmonotonic due to the competing effects of ventilation on heat and mass transport. The formation of a complete or partial ice shell likely also affects retention significantly. Comparison of our results with available experimental data provides possible explanations of the trends and apparent disagreement found in the studies. The theory-based analysis and methodology presented in this paper can be used to improve experimental design and parameterization of retention in cloud models.