In areas that do not comply with the health-based National Ambient Air Quality Standards (NAAQS), the Clean Air Act (CAA) requires the preparation of a State Implementation Plan (SIP) to demonstrate how the area will come into compliance with NAAQS. Part of the process of developing a SIP involves the creation of an emission inventory. Emission inventories are also developed as part of the CAA requirement to demonstrate rate of progress toward NAAQS attainment. In addition, where port growth will affect an areas attainment status or rate of progress, emission inventories are sometimes done for environmental impact statements (EIS) as part of the National Environmental Policy Act (NEPA). The emission inventory is a quantification of all emissions of criteria pollutants that occur within a designated area by their source. Emissions sources are categorized broadly as mobile sources, point sources (e.g., a refinery), and area sources. Mobile sources are further categorized as on-road sources and non-road sources. Port emissions are generated by marine vessels and by land-based sources at ports. Marine emissions come primarily from diesel engines operating on ocean going vessels, tugs and tows, dredges, and other vessels operating within a port area. Land-based emission sources include ground equipment such as yard tractors, cranes, container handlers, and forklifts as well as heavy-duty trucks and locomotives operating within a port area. The port emissions of greatest concern are nitrogen oxides (NOx) and particulate matter (PM).