An air tracer is a chemical that when emitted into an air parcel can be measured as it moves through time and space. Air tracing can be viewed as having two related parts, namely, broadcast and distribution of the tracer (the analyte) in air, and collection and measurement of the tracer from a known volume of the labelled air (the sample).In addition to the physical properties which allow a tracer to persist during the measurement term, an air tracer needs to fulfill additional requirements regarding its practical suitability. These include health and safety, low environmental burden, high availability and good handling in practical use. Not least, the emitted tracer should be recordable with established sensing techniques over a wide concentration range and with high precision. And lastly tracer measurement should be highly-resolved in time, so exposures occurring in different locations and from different sources can be precisely differentiated; and the technology should allow measurements over extended time periods, so that a complete and connected (time-correlated) picture of the variation in temporal and spatial mass concentration can be obtained at each sensor location. (Laussmann 2011)