Isotopes in atmospheric moisture

Abstract

The Earth's atmosphere plays an important role in the global hydrologic cycle. It provides a dynamic link between major reservoirs of water on Earth, i.e. the global ocean, large continental ice sheets as well as soil moisture and other water reservoirs on continents such as surface waters, lakes and water in the biosphere (e.g. leafwater). Atmospheric water vapour is the basic vehicle through which this link is accomplished. The amount of water vapour stored in the atmosphere is negligible when compared for instance with the world ocean or continental ice sheets, the corresponding ratios being approximately 10 -5 and 4x10-4, respectively (Berner and Berner, 1987). Although being trace constituent of the global atmosphere, responsible for only a tiny fraction of its total mass, atmospheric moisture plays a crucial role in the global ecosystem behaviour. It is a "substrate" from which precipitation is formed. Atmospheric water vapour is the most important greenhouse gas responsible for approximately 60% of the natural greenhouse effect which amounts to ca. 32°C. It is the major source of hydroxyl free radicals vital for efficient "cleaning" of the atmosphere. Finally, atmospheric moisture serves as an important regulator of heat fluxes in the atmosphere, greatly reducing thermal gradients between low and high latitudes. In contrast to many other atmospheric constituents, distribution of water vapour in the lower atmosphere is highly inhomogeneous. Its concentration expressed as mixing ratio (kg H2O per kg of dry air) decreases from approximately 10-3 in the lower troposphere close to the planetary boundary level to around 3×10-6 in the lower stratosphere and then increases again to approximately 10-4 at the altitude of 30 km. Close to the Earth's surface, its content varies between approximately 1.5×10 -2 over tropical oceans to 5×10-3 over dry regions in the interior of continents and less than 10-3 over ice sheets at high latitudes. While distribution of water vapour in the troposphere is primarily controlled by thermal gradients imposing phase changes (evaporation-condensation-freezing, sublimation-resublimation), both in horizontal and vertical directions, photochemical reactions such as oxidation of methane remain the major controlling mechanism responsible for the distribution of water vapour in the stratosphere. The mass of water vapour in the contemporary atmosphere is estimated to be around 1.3×1016 kg, of which about 85% occurs in the lower troposphere, below an altitude of about 5 km. In the past the atmospheric load of moisture most probably fluctuated with changing climate, being lower during glacial periods. The atmosphere is only partially saturated with water, with globally averaged relative humidity of the lower troposphere being around 85%. This apparent lack of saturation results from the existing thermal structure and large-scale circulation patterns of the troposphere. The mean turnover time of water vapour in the global atmosphere is around 10 days with respect to the net evaporation or precipitation fluxes. If the gross exchange fluxes during the evaporation process are considered, the mean turnover time of water molecules in the atmosphere drops to approximately 2 days. It is therefore evident that the reservoir of atmospheric moisture is very dynamic, quickly responding to changes of external conditions such as the flux of energy reaching the Earth's surface and/or changes of temperature lapse rate in the lower troposphere. From the overall perspective of the global hydrologic cycle, the isotopes directly incorporated in water molecules ( 2H, 18O, 3H) are of primary interest. However, the atmosphere is also a space where a number of radioisotopes of other elements, both naturally produced and man-made, are present. The most important for hydrologic applications are 14C, 36Cl, 39Ar, 81Kr and 85Kr. They enter the hydrologic cycle via dissolution in rainfall or in surface water bodies. In the case of 14C, its pathway into the water cycle also includes the biosphere. These radioisotopes serve as time indicators, providing important information about dynamics of water movement in various compartments of the hydrologic cycle. © 2005 IEA.