US EPA: Speciation - Clearinghouse for Inventories and Emissions Factors
Another step in modeling inventory development is speciation. This is the process of disaggregating inventory pollutants into individual chemical species components or groups of species. The need for speciation is determined by the inventory purpose. Inventory applications that require detailed speciation include photochemical modeling, air toxics inventories, chemical mass balance modeling, and visibility modeling.
Depending on the purpose of a particular emissions inventory, the inventory may include TOG, NOx, sulfur oxides (SOx), CO, total suspended particulate matter (TSP), particulate matter less than 10 micrometers in aerodynamic diameter (PM10), or ammonia (NH3). However, modeling inventories may require these emissions to be expressed in terms of other pollutants. Additionally, for some models, NOx emissions may need to be specified as NO and NO2. Also, PM may need to be separated into various fractions, such as PM10 and PM less than 2.5 micrometers in aerodynamic diameter (PM2.5).