A modeling study of the effects of ice-phase microphysical processes on trace chemical removal efficiencies

Abstract

A simple cloud-chemistry model has been developed to study the effects of specific microphysical processes of precipitation formation and scavenging on the efficiency with which clouds remove trace chemicals form the atmosphere. The model is designed to let an isolated precipitation particle fall through the mixed-phase zone of a steady-state background cloud derived from the ascent of a Lagrangian parcel in which given mixing ratios of SO 2, CO 2 and NH 3 are in equilibrium with the aqueous phase. The precipitation particle is allowed to acquire water mass by both vapor deposition and accretion of supercooled cloud water and solute content by gaseous entrapment in rime ice, as parameterized from prior laboratory experiments. The concept of a relative removal efficiency has been employed to identify the important microphysical fractionation processes within the cloud. The model calculations show that the removal efficiencies of the chemical constituents in the cloud nuclei and soluble trace gases are controlled largely by the accretional growth process. Even with modest solubilities, a trace gas such as SO 2 is estimated to be removed much more efficiently than is water vapor via cold-cloud precipitation because of substantial entrapment of the gas during rime-ice formation.