Implications for models and measurements of chemical inhomogeneities among cloud droplets

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Abstract
Cloud droplet solutes are expected to vary in both species composition and molarity with droplet size and from droplet to droplet of the same size. Inhomogeneities in droplet composition and molarity on the microscale (spatial dimensions up to ca. 1 m) result from the size-dependent chemical composition of the aerosol particles that act as cloud condensation nuclei (CCN), the inverse dependence of droplet growth rate on droplet radius, and from in-cloud scavenging and chemical reactions. The first two factors alone produce droplet solute molarities that can vary over 4 orders of magnitude for a factor of 2 range of droplet size. This strong size dependence is established very early in the growth of the droplet, and is the starting point for subsequent chemical and physical processes that can enhance or diminish the initial inhomogeneities. Pronounced vertical inhomogeneities in droplet solute molarity are expected as a consequence of the size-dependence of droplet growth, particularly near the base of a cloud. For some droplet sizes, solute molarities can change by 3 orders of magnitude over height intervals of 30 m. Turbulent entrainment of sub-saturated air from outside of the cloud produces additional cloud-scale inhomogeneities by causing some or all of the droplets to evaporate, and by supplying additional CCN and reagent gases to the cloud. The presence of an ice phase in the upper reaches of a cloud is another factor that alters the variability of hydrometeor composition and chemical concentration. Differences in the chemical and physical inputs to a cloud produce different inhomogeneities. For example, a wider range of particle sizes is expected to act as CCN in very clean air or in clouds with high updraft velocities, leading to stronger gradients in solute concentration with droplet size. In many cases, it may be impossible to observe the full range of inhomogeneities present in the cloud. Nevertheless, it is important that experimental and numerical investigations of cloud chemistry recognize that such inhomogeneities are present, and that the inhomogeneities are controlled by both physical and chemical factors. DOI: 10.1034/j.1600-0889.1992.t01-2-00004.x