Estimating Emissions from Sources of Air Pollution

6.1 Development of an Emissions Inventory

6.1.3 Compounds to Consider for an Emissions Inventory
There are literally thousands of types of pollutants that are released into the atmosphere. Most of these compounds are different types of organic molecules, but there are also a large number of inorganic compounds to consider. Because of the large variety of pollutants, only those pollutants that are common in urban and rural atmospheres are typically considered for inclusion in air quality management programs and thus emission inventories. Even with this limitation there are a very large number of potential compounds that should be considered for inclusion in the emissions inventory for a region. The term criteria pollutant is a term used often in the United States to identify those pollutants that are so pervasive in the atmosphere that specific air quality standards have been established for them. There are a number of pollutants that are so closely linked to the criteria pollutants that they are included within this category. An example is NO (nitrogen oxide), which is the prime pollutant released from most combustion processes but converts to NO2 in the atmosphere. The second class of pollutants of importance is toxic air pollutants. Of course, criteria pollutants are toxic, but this class of pollutants refers to the non-criteria pollutants that can cause cancer and/or other significant specific acute and chronic health impacts in the small quantities that can occur in the atmosphere. The definition of a toxic air pollutant is not very clear, thus, U.S. law specifically lists 188 compounds which are designated as toxic air pollutants ( Global climate change (global warming) pollutants are those pollutants that can contribute to global climate change. Stratospheric ozone-depleting substances are those compounds that destroy the stratospheric ozone that acts as a shield for ultraviolet radiation. These substances are classified as Class I and Class II ozone-depleting substances ( Finally, there are a number of other important compounds that play key roles in atmospheric reactions or are considered toxic, although they did not make the original U.S. EPA list of 188 toxic air pollutants. Table 6.1.3-1 below lists the compounds that are most commonly considered for an emissions inventory for each category of pollutants.
6.1.3-1 Compounds Commonly Considered for Inclusion in an Emissions Inventory

The location and ultimate purpose of the inventory determines which compounds are to be actually included.

VOC and Particulate Matter do not represent single substances. In the case of particulate matter, the different types of particles in the air are rarely considered. There are exceptions to this statement. Some notable examples of particles that are considered independently are lead particles, asbestos particles, and diesel particles. These particles have been found to have specific health effects that warrant their independent consideration. However, with the exception of these few types of particulate matter, typically only the size of the particulates appears to be important for most health considerations. Thus, particulate matter is designated as total suspended particulate matter (TSP), particulate matter smaller than 10 microns in size (PM10), and particulate matter smaller than 2.5 microns in size (PM2.5). TSP typically refers to all particles from about 50 microns in diameter on down. These are the particle sizes that can remain suspended in the air for a significant amount of time. As discussed in Chapter 3, PM2.5 particles can penetrate deep into the lungs. The PM10 particles, although larger, can also evade many of the body’s defensive mechanisms and impact the respiratory system. Emission inventories designed to address particulate problems normally classify emissions into the three particulate size categories.

VOC is a concern since some of the VOC compounds are toxic, but these toxic compounds are normally treated specifically in the inventory as a toxic air pollutant. VOC is also a concern because it is one of the key ingredients in the formation of ozone. However, not all VOCs contribute to the formation of ozone in an equal manner. Thus, it is common to classify VOCs by their reactivity to form ozone. In order to do this, the types of VOC emitted by different sources are an important consideration. Emission inventories designed to address ozone problems make allowances for classifying VOC compounds into different reactivity categories.