Physical Influences of Altitude on the Chemical Properties of Clouds and of Water Deposited from the Troposphere

Abstract

The physical structure of the atmosphere dictates that its temperature usually decreases as altitude increases, often at a rate approaching the adiabatic limit of ca. -1C per 100 m. This requires consideration of the temperature dependencies of both chemical and physical processes of importance to cloud and deposition chemistry. It also requires recognition of the effects of the natural thermal layering of the atmosphere on the vertical distribution of trace substances and on systematic differences of long distance transport and scavenging with altitude. Among the fundamental temperature dependencies are the vapor pressure of water (and its role in determining the liquid water content of clouds and precipitation amount), the presence or absence of the ice phase, and deposition of supercooled water as occult precipitation by riming. Additional temperature dependencies of the chemical equilibrium constants and Henry’s Law coefficients should cause marked altitude dependencies of pH. The sign of this depends on the species in control of the equilibrium. Altitude dependencies of air composition are not as readily described with physical and chemical theory, but can often be described effectively by a two layer system of the planetary boundary layer (PBL) and the free troposphere. Gases and fine particles (sub um) are seen to behave similarly with coarse particles being limited mainly to the PBL. Long distance transport is likely to be much more efficient at high altitudes, but only for the substances present there. Finally, the influences of orography and climate are involved as important determinants of the local workings of the hydrologic cycle. Data will be presented demonstrating the central features of these altitude dependencies.