Atmosphere–Hydrosphere–Geosphere–Biosphere Linkages

The Earth’s systems are linked through physical, chemical, and biological processes. Each of the environmental systems consists of the three basic chemical phases of matter: gases, liquids, and solids. However, while chemists normally study homogeneous, well-mixed systems, these are very rare in the natural systems. Environmental chemistry is not the chemistry of well-mixed beakers, nor does it involve reactions of large amounts of chemical species. Rather, it is the chemistry of open systems that include transport and transformation of often trace chemical species over a range of scales and time frames. But although environmental systems are very large open systems, they interact with each other in an attempt to reach equilibrium states. In many cases it may take long time periods for the systems to reach equilibrium, but we can still use the principles of chemical kinetics and equilibrium to make approximations that can be very useful to understand their behavior. For example, the reactive lifetimes of chemicals in the atmosphere and hydrosphere, and/or their transport properties, can be evaluated using vapor pressures and solubility constants.

Water is one of the key linking species between all of the environmental systems, promoting the transport of chemical species between them. Our atmosphere consists of both unreactive gases, such as nitrogen and argon, as well as reactive gases, such as oxygen, carbon dioxide, and trace gases. Some gases have a low water solubility and others have a high water solubility, which allows them to interact with the hydrosphere, including fresh and salt surface waters, clouds, and precipitation in the form of rain, snow, hail, and sleet. Clouds themselves are not pure water but are made up of large aqueous droplets that can contain trace particles as well as soluble chemicals. These water droplets can act as important chemical reactors due to water’s ability to act as a universal solvent. If the water droplets become large and numerous enough, clouds can produce a wide variety of precipitation, recycling the water and its chemicals back to the land in the form of fresh water. These surface waters can act to solubilize compounds from the geosphere and ultimately transport them to lakes and seas by way of rivers and other hydro-logical systems. These hydrological systems are strongly linked to the geosphere and biosphere, and the feedback between them is key in determining many factors relating to the currently recognized problems of climate change and air and water quality degradation.

The natural systems can interact with anthropogenic pollution, leading to synergistic effects that sometimes have positive outcomes and sometimes have very negative outcomes. This leads to a complex chemistry, which impacts many if not all of the natural systems. An example of this type of complexity is the release of mercury vapor (Hg ) into the environment from the combustion of coal or smelter operations, as demonstrated in Figure below.

Once released into the lower atmosphere as a gas, mercury can equilibrate with cloud water due to its small water solubility. The cloud water can contain significant amounts of hydrogen peroxide, which is formed as a trace gas in the atmosphere from free radical chemical reactions that lead to the formation of the hydroperoxyl radical (HO2) followed by the reaction of HO2 with oxygen. Although the concentrations of hydrogen peroxide in air are small (low ppb), the cloud water concentrations can be around a hundred micromolar, since hydrogen peroxide is extremely water soluble. This hydrogen peroxide concentration can effectively oxidize the Hg to HgO. That same cloud water can contain organic acids and compounds containing hydroxyl groups, formed from the oxidation of anthropogenic and natural organic compounds. These oxidized organics can act as chelating agents for the oxidized forms of elemental mercury. Once deposited to the surface in rain water, this organically complex mercury is more bioavailable in the hydrosphere and geosphere and thus can accumulate in organisms in the biosphere. Once the mercury is in the biosphere, it can be converted into organomercury compounds that have much higher toxicity than elemental mercury.

The transformation and stabilities of these mercury compounds are very dependent on the chemistry of the environmental systems. They are more stable in reducing environments, such as in lake sediments or anoxic lake or ocean waters, where they can accumulate. But they can be recycled back to elemental mercury and revolatilized back into the air if they are introduced into an oxidizing environment. Once it enters the environment, the mercury levels eventually increase in all of the environmental systems due to its complex environmental chemical behavior. It does not exist simply in one environmental compartment, but is in equilibrium with all of the various compartments: atmosphere, hydrosphere, geosphere, and biosphere.

The complex environmental chemistry of mercury in the atmosphere and its connection to agricultural and biomass burning. Elemental mercury (Hg∘) can react with H2O2 to form Hg2+.Mercury’s complexation with organomercury compounds in clouds and wet aerosols leads to increasing deposition in lakes, and subsequent bioaccumulation in fish through the food chain.

As we develop new energy technologies, we introduce similar chemicals in the environment and they will also undergo transport, transformation, and deposition, and will become incorporated into the atmosphere–hydrosphere–geosphere–biosphere system, dependent upon the chemistry of each species. New materials, such as novel polymers or nanomaterials, will also undergo similar chemical processes. The fundamental properties of these materials – including photochemical stability, susceptibility to key oxidants in the atmosphere and hydrosphere, ability to be reduced in anoxic environments, solubility and volatility, as well as organic or inorganic complexation – need to be known in order to evaluate their environmental fate and impacts. These are some of the types of chemical reactions and properties that we will be examining in detail in the following chapters.

As we begin to examine the environmental chemistries of each of the environmental systems, it is important to remember that these systems are strongly connected, with the atmosphere being the most mobile system, followed by the hydrosphere. There is truly one atmosphere across the globe, even though it is not well mixed on short time scales. This leads to observations of clear skies, strong storms and flooding, as well as droughts, occurring over the same areas as a function of time. But these short-term physical processes, which we call weather, are linked to climate over longer time scales and both are linked to the chemistry and physics of the atmosphere and hydrosphere, and are strongly moderated by the geosphere and bio-sphere. Thus, these environmental systems are all linked and these linkages are at the heart of their chemistry. A review of our current understanding of the fundamental chemical and physical principles of these systems will be accomplished by starting with the atmosphere and moving down and into the hydrosphere, geosphere, and biosphere, noting the important con-nections between them along the way. The goal of environmental chemistry is to make use of this knowledge to predict the behavior of natural and polluted systems, and to minimize the environmental impacts of anthropogenic activities in order to allow for the development of a sustainable, sound, and safe Earth system for future generations.