1. A chemical or thermodynamic property of the flow that is conserved during advection. It can be used to track air-parcel movement and to identify the origins of air masses. Examples are absolute humidity, equivalent potential temperature, radioactivity, and CCN composition. 2. Any substance in the atmosphere that can be used to track the history of an air mass. It can be chemical or radioactive in nature. The main requirement for a tracer is that its lifetime be substantially longer than the transport process under study. An example of an inert chemical tracer is SF₆, which is often released during a field experiment and measured at a later time to assess the extent of dilution of the air mass. Chemicals such as methane (CH₄) and nitrous oxide (N₂O), which are released at the earth's surface and destroyed slowly in the atmosphere, can be used to infer vertical rates of transport. CO released in the boundary layer can be used to trace transport in convection. Radioactive tracers such as ¹⁴C and ⁹⁰Sr have been used to test models of stratospheric circulation. Certain atmospheric gases have also been used as tracers in ocean waters, for example, the chlorofluorocarbons.