If the tracer interacts with the hydrologic cycle the preprocessor directive #define TRACERS_WATER should be set in the rundeck in addition to #define TRACERS_ON. This allows for a separate tracer budget in all water reservoirs (cloud liquid water, precipitation, sea ice, lakes, ground water, glaciers and oceans (if a non-trivial ocean model is used)). There can be complicated interactions in the clouds between the in air and in water tracers, and any tracer amount in precip is followed through it's subsequent travels. These tracers are useful for soluble gases and water tracers themselves (isotopes, age, source).