The format of an emission inventory can vary considerably based on the intended use of the inventory. For general purposes, in the case of inventories that are intended for simple policy analysis or roll-back modeling (See Chapter 7, Section 1), the emissions are typically classified into broad categories. On the other hand, for the case of complex multidimensional modeling using atmospheric chemistry or more advanced policy and regulatory analysis, then the inventory database must include information on the release location and temperature, stack height, plume velocity, hydrocarbon types, along with linkages to considerations such as jobs produced, electricity used, and other important economic and environmental factors. Obviously this latter inventory format is much more data intensive and thus more complex compared to the basic roll-back type of inventory.

Most policy making and regulatory analyses are carried out using the total amount of emissions in different categories of source emission rather than the results from air quality modeling. This is because the results from the air quality modeling are so complex that they do not easily facilitate public discussions of emissions impacts and health. It is common to define the tons of emissions that a location can receive in total and still meet air quality standards. This is often called “carrying capacity.” Emissions are reduced (or rolled back) during regulatory analysis based on different regulatory assumptions to reach the regions carrying capacity. While this approach is a significant oversimplification of the issues, it works well in the policy arena when working with non-technical policy makers. The air quality management experts must ensure that the use of simplified emissions characterizations such as carrying capacity is producing reasonable policy decisions.

This leads to a final point that must be considered. As sources are regulated, there are secondary impacts. For example, electricity production might be significantly increased or decreased as a result of a certain policy choice. If this is the case, then this change in demand needs to be understood and accounted for in any policy or regulatory analysis and even in the modeling analysis since electricity production normally produces environmental impacts itself. Similarly, the creation or reduction in jobs due to the policy or regulatory choice can be a useful output for the policy makers. Thus, it is valuable in the design of an inventory to include the appropriate links or related information for the emission sources within the inventory that can facilitate these broader analyses. This is not an easy task, but to the extent that it can be achieved, it is a valuable addition to any emissions inventory database.