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(California's Inspection/Maintenance program) inventory, and 82% of the point sources emit 5000 tons or more annually; 43% of the NOx inventory is generated by mobile sources (EPA, 1989a). Volatile organic compounds, in contrast, are less well characterized from both a real-time emissions and ambient monitoring perspective.

This chapter provides an overview of the anthropogenic emissions inventory: how it is compiled, what the. major contributing sources are, and where uncertainties lie. There is a similar overview of the inventory of biogenic emissions, and finally a review of efforts to evaluate the accuracy of emissions inventories.

Compilation of Emissions Inventories

In 1971, the U.S. Environmental Protection Agency (EPA) established the National Emissions Data System (NEDS) on sources of airborne pollutants. This system was to summarize annual cumulative estimates of source emissions by air quality control region, by state, and nationwide for the Clean Air Act's five criteria pollutants: particulate matter, sulfur oxides, nitrogen oxides, VOCs, and carbon monoxide. At that time the developers did not envision the evolving demands on emissions inventories that have become common with the advent of increasingly sophisticated air quality models. The original intent to compile annual national trends in the emissions of VOCs, NOx, SO2, CO, and particulate matter has been expanded and amended by the need for chemical speciation of VOCs, consideration of additional chemical species, more detailed information on spatial and temporal patterns of inventoried species, and techniques to project trends in emissions.

An estimate of emissions of a pollutant from a source is based on a technique that uses ''emission factors,'' which are based on source-specific emission measurements as a function of activity level (e.g., amount of annual production at an industrial facility) with regard to each source. For example, suppose one wants to sample a power plant's emissions of SO2 or NOx at the stack. The plant's boiler design and its Btu (British thermal unit) consumption rate are known. The sulfur and nitrogen content of fuel burned can be used to calculate an emissions factor of x kilograms (kg) of SO2 or NOx emitted per y megagrams (Mg, or metric tons) of fuel consumed.

EPA has compiled emission factors for a variety of sources and activity levels (such as production or consumption), reporting the results since 1972 in "AP-42 Compilation of Air Pollutant Emission Factors," for which supplements are issued regularly (the most recent was published in 1985) (EPA, 1985). Emission factors currently in use are developed from only a limited

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