Emission Inventories

W. Addy Majewski

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Abstract: Atmospheric pollutants as well as greenhouse gases are emitted from a variety of sources. Emission inventories are maintained to estimate the significance of particular emission sources to the overall emission load. Diesel engines are significant contributors to the overall NOx and PM pollution, while gasoline engines are major contributors to VOC and CO emission inventories.


Emission inventories are developed to inform government’s clean air policies as to the extent of air pollution and the relative significance of various emission sources. Inventories of criteria air pollutants (CAP) are normally maintained by environmental protection authorities. In the United States, the EPA publishes the Air Quality and Emissions Trends report, updated annually, and the National Emissions Inventory (NEI) report updated every three years.

Similarly, emission inventories are developed for GHG emissions. As part of the reporting requirements under the United Nations’ Paris Agreement, countries are required to periodically submit national GHG inventories. Under the auspices of the UN Environment Programme, the data is then compiled into a global GHG emission inventory.

The development of emission inventories is a complex process typically performed using a bottom-up methodology. It involves the identification of emission sources (mobile and stationary), their number and size in each given area, the types of pollutants they produce, and the determination of emission factors for each pollutant. The emission inventory is then obtained by multiplying the emission factors for each pollutant by the respective emission source data, followed by summation of the results for all sources in a given area and time period.

Internal combustion engines produce the overwhelming majority of CAP and GHG emissions from mobile sources—including light- and heavy-duty vehicles, off-road equipment, marine vessels, and aircraft. Additionally, combustion engines produce a small proportion of emissions from stationary sources (such as natural gas and diesel generator sets).

In the United States and other developed countries, mobile sources are typically responsible for more than 50% of NOx emissions, about 20-30% of HC (or volatile organic compounds, VOC) emissions, as much as 60-80% of CO emissions, and a sizable share of PM emissions. Among combustion engines, diesels have been typically responsible for a large proportion of NOx and PM emissions, while gasoline fueled engines are major contributors to VOC and CO inventories.

However, differences between emission inventories are common. These differences depend on the geographical area, traffic intensity, type and age of the vehicle fleet, climate, as well as the methodology used to develop the emission inventory. In California, diesel engines were identified as the key source of PM emissions. Total diesel exhaust PM emissions were estimated to be 56% of the total PM emissions generated by combustion of all fuels in the state. Onroad diesel vehicles were estimated to generate 59% of the total statewide diesel PM emissions, while mobile nonroad diesel engines contributed 36%, and stationary diesel engines 5%. These findings supported the designation of diesel particulates as a toxic air contaminant in California in 1998 [261].

Contrary to the California estimates, not all studies confirm the high proportion of diesel PM in the overall PM inventories. Also in 1998, Colorado State University completed a major air quality investigation known as the Northern Front Range Air Quality Study [317]. Emission inventories for locations in the Denver, Colorado area showed that PM2.5 emissions from diesel engines produced 10% of the total (man-made + natural) PM2.5, while gasoline vehicles contributed 29%. It was found that the majority of gasoline PM emissions was caused by “smoky”, poorly maintained vehicles (about 2.5% of the fleet) and during cold starts. PM emissions from well-maintained gasoline engines operating in hot, stabilized mode amounted only to 3% of the total PM2.5. The low proportion of diesel PM emission was also caused by a small population of diesels, estimated at 4-5% of vehicles on the road.

In the US EPA nationwide inventories, the contribution of diesels to the overall mobile source PM10 emissions has decreased significantly since the 1990s and PM10 emissions from diesel and gasoline engines are comparable (Table 2). This decrease in diesel PM emissions can be attributed to the wide-scale use of particulate filters on diesel engines.