unmanaged ecosystems filled with a multitude of species, some of which have life cycles that span decades. These approaches also fail to address the indirect effects of air pollution on, for example, soils and aquatic ecosystems. As a result, the effects of pollutants on specific agricultural crops are generally better defined than the long-term effects of pollutants on unmanaged ecosystems, such as forests, grasslands, wetlands, lakes, and estuaries. One potential technique that might be useful for examining long-term effects of air pollutants on intact ecosystems has thus far been applied to studies of the long-term ecological effects of increased atmospheric carbon dioxide (CO2) and is referred to as free air CO2 enrichment (FACE) studies. In a FACE facility, scientists can increase the concentration of a trace gas, such as CO2, in a controlled way in the air surrounding an intact ecosystem and measure plant and soil responses to the altered conditions over years to decades in accordance with the long life span of trees (see Figure 2-13 and Delucia et al. 1999). The FACE systems are not without fault but offer an alternative approach to enable long-term experimental exposure to air pollutants and evaluation of effects on whole ecosystems and their components under otherwise natural environmental conditions.