Estimating Emissions from Sources of Air Pollution

6.1 Development of an Emissions Inventory

6.1.4 Emission Source Categories Introduction

Sources of emissions in an inventory are typically divided into broad categories to facilitate the development of relevant information about these sources. These categories are selected so that the approaches to determining the emissions in a specific category are similar. Policy and regulatory analysis needs also impact the establishment of source categories. The typical categories used in emission inventories are listed below:

1. On-Road Mobile Sources

2. Off-Road Mobile Sources

3. Stationary Sources

4. Area Sources (Stationary Area Sources)

5. Natural (Biogenic) Sources

These source categories are directly and/or indirectly linked to one another. For example, off-road sources can be linked to a specific stationary source. Area sources can result in increased on-road mobile emissions as vehicles are attracted to the area sources. Care must be taken as to how particular sources are classified and how they are linked or associated with each other in order to support policy and regulatory analysis as well as air quality modeling. The following subsections discuss the typical emission inventory categories and how they might be included in an emissions inventory. On-Road Mobile Sources

On-road mobile includes any sources that are normally intended to operate on public roadways. This includes passenger cars, trucks, buses, motorcycles, and 3-wheel vehicles. On-road mobile sources are the most consistently defined classification of emissions; however, there can be some confusion as to the appropriate classification of mobile sources in some limited cases. If a truck is used exclusively on the grounds of an airport or other restricted facility should it be treated as part of the on-road emissions inventory or should it be treated as part of the facility where it operates in effect as a stationary source fugitive emission? Regulatory requirements might be the best determiner of this allocation problem. However, modeling requirements are also an important consideration. If the vehicle never leaves the facility, it might be best to treat it as part of the facility emissions or as an off-road emissions source as discussed below. This allows better modeling and better policy analysis.

On-road mobile emissions are typically the largest source of emissions in an urban area. These emissions are also complex to calculate due to the large number and variety of vehicles involved and the impact of driving patterns on the emissions. Thus, some considerable effort has gone into developing emission factors and software to make emission estimates for on-road mobile sources. Section 6.2 discusses the calculation of emissions from on-road mobile sources. Off-Road Mobile

Off-road mobile sources are the most recent classification of sources to receive significant attention. This classification of sources includes airplanes, construction equipment, ships, and trains. Construction equipment typically operates at one location for a few days to a few months and then is moved to another location. Airplanes, as far as localized emission impacts (not global impacts), operate only at airports. Thus, construction equipment in many ways should be treated as an area source for modeling purposes, but airplanes can be treated more as a stationary source for analysis purposes. Ships and trains impact air quality more like on-road mobile sources; although the shipping routes and rail lines are more restricted than those available to on-road mobile sources.

A confusing classification issue for off-road mobile sources is also how to handle the peripheral emissions associated with these off-road mobile sources. For example, the ground handling equipment at an airport can produce considerable emissions and can, in some cases, be very similar to on-road equipment. Similarly, the cranes that unload and load ships and trains and the trains that simply move railcars around a rail-yard but never leave the yard are classification questions. For policy development purposes it is normally good to understand the total emissions associated with a single activity such as an airport or rail yard. Therefore, it is recommended that all emission sources associated with a fixed site be treated as part of the source where they operate with links to the off-road mobile source category. Section 6.3 discusses approaches to estimating emissions from area sources. Point Sources

Point sources are defined as non-mobile sources of emissions that are large enough to be recorded in the emissions inventory individually. The designation as a point source is typically based upon the total of emissions that occur from a contiguous polluting facility rather than emissions from individual emission points at the facility. Thus, when a “point source” is referred to, it refers to the overall contiguous site occupied by the facility including all emission points that occur at the site. However, the individual emission points at a site are often catalogued individually along with the various emission parameters associated with each emission point to support air quality modeling and regulatory analysis. Since the definition of a facility as a “point source” can carry with it significant regulatory requirements, the actual definition can become complex.

There is no clear definition of how large or small a source might be to be classified as a point source. This is typically left up to the local situation such as information availability, regulatory requirements, and often the severity of the air pollution problem being considered. The United States government sets 10 tons per year, 50 tons per year, and 100 tons per year as important regulatory mileposts for common air pollutants. Thus an emitting facility comprised of all sources at a contiguous site are often classified as point source if its emissions equal or exceed in total one of these mileposts. However, in the South Coast Air Quality Management District in Southern California, which has one of the United States’ worst air pollution problems, facilities with total emissions as small as one ton per year are classified as point sources.

It is worth repeating that the determination of the facilities that are classified as a point source is not a simple task and can depend upon regulations that exist in a particular jurisdiction, air pollution levels in the region, and other factors. A clear definition of what constitutes a point source will have to be made before an emissions inventory can be properly classified.

As has been noted, all sources at a contiguous site that contribute to the industrial process at the site are considered to be part of the emissions from the stationary source of interest. However, emissions are released to the atmosphere in different ways. Some emissions will go through a chimney or stack while others leak through vents and are fugitive in nature.

Thus, emissions within a point source are often sub-classified into one of two categories. One category includes emissions that occur through a stack, which can be called a stack source, and the second are fugitive in nature and are typically designated as fugitive sources. Specifically for the purposes of this manual, stack sources are those sources that are ducted to and emit through a stack of some sort. Fugitive emissions are those emissions that are released through building air vents, through doors, or through other non-stack release points. These fugitive emissions can also be called “area” emissions, but for purposes of this discussion, they will be called fugitive sources and the use of the term area source will be reserved for other types of sources discussed later. This breakdown of emissions is driven to a great degree by the measurement technique as well as the needs for modeling. Emissions through stacks normally have an upward velocity and are hotter than the ambient air, and are often times directly measured continuously or at regular intervals. Fugitive emissions on the other hand typically do not have an upward velocity and are often close to ambient temperatures, and cannot be easily quantified or measured. Thus stack and fugitive sources disperse into the atmosphere differently and thus the need for differentiation. There is often times more uncertainty in the amount of emissions from the fugitive sources than the stack emissions.

There can be exceptions to the inclusion of every emission point at a facility as part of the point source. For example, a small natural gas water heater in the management offices at a point source might be overlooked in the stationary source inventory and be picked up as an area source (area sources are discussed in the next paragraph) if a top down approach is being used to estimate emissions for small sources using natural gas and there is no easy way to separate the natural gas used by the small water heater from the general regional natural gas use values. Otherwise, emissions will be double counted. However, if natural gas use can be determined by user and allocated to specific point sources and subtracted from the regional natural gas use to avoid double counting, then it is our recommendation that all emissions that occur at a point source be included as part of the emissions from that source. Of course, it would be useful in the small water heater example just discussed to provide a link to the water heater category so that better regulatory analysis can be carried out when rules are written for water heaters and similar sources. Finally, if possible, it is best to define as many businesses as possible in the point source inventory as can be handled by local staffing and data availability in order to have the best possible database to support effective air quality modeling and regulatory and policy analysis.

Emission estimates for point sources are typically made based on actual emission measurements made at the major emission points and/or on emission factors developed for this purpose. Section 6.4 discusses the emission calculation process for stationary sources. Emission related factors along with specific locations are needed to support air quality modeling. Area Sources (Stationary Area Sources)

Stationary area sources are those sources that do not move, but are too small or too spread out to be classified as a point source as discussed in the previous subsection. An example might be residential water heaters or a dirt roadway. As discussed in the opening paragraphs, there is no clear definition as to when sources are classified as a stationary area source compared to a classification as a point source. Considerable confusion can arise in comparing inventories from different locations because of this problem. As an example, in most locations, Dry Cleaners are considered to be an area source, but in Los Angeles, they are classified as a point source and each Dry Cleaner is cataloged in the point source inventory. Normally, domestic (household) sources such as heating systems or water heaters are classified as stationary area sources. Refueling emissions from service stations are also often classified as stationary area emissions; although, in California, a number of local air quality management agencies include them individually in the point source inventory.

It is most common for stationary area sources to be referred to as “area sources” without the added term “stationary.” This is the approach that will be used in the rest of this section. It is understood that on-road mobile and other such sources are also types of area sources, but they are referred to by different names.

General sources of emissions such as paved and unpaved roads are also normally classified as area sources even though the emissions are related to mobile sources, they occur from a source that is fixed in location.

It is best to minimize the classification of sources into the area source inventory classification because only generalized location and generalized release parameters can be applied to this category reducing the effectiveness of any associated modeling. Information limitations and database capabilities are the prime determining factors concerning when sources are designated as an area source. In the extreme case, every household water heater and heating system in an urban area could be catalogued as a point source, but this would run into possibly millions of sources to track yet they contribute only a little to the total emissions in a region.

Area source emissions are typically estimated using emission factors and calculated based on some independent variables such as land area or regional fuel use, or population, or some similar generalized estimation criteria. Section 6.5 discusses the estimation of area source emissions. Natural (Biogenic) Sources

There are many natural sources of emissions including wind blown dust, particles from wildfires, nitrogen dioxide from thunderstorms, ammonia and hydrocarbons from the decay of organic matter, and hydrocarbons from living plants. Since these emissions are normally uncontrollable, they are not treated as part of the “anthropogenic” (i.e. human caused) emissions inventory. However, they do contribute to the air pollution problem and must be considered in any analysis of the air pollution problem in a region.

One approach to addressing the natural sources of emissions is to treat them as boundary conditions for purposes of modeling. This is a common approach in all but two cases since policy and regulatory considerations are irrelevant relative to these emissions. The two exceptions are ammonia and hydrocarbons. Both ammonia and hydrocarbons participate in reactions with anthropogenic emissions. The ammonia will react with gaseous nitrogen oxides and sulfur oxides to produce nitrate and sulfate particulates. In some urban areas the nitrates and sulfates can be the predominant source of fine particulate matter during high air pollution episodes. Hydrocarbons react with gaseous nitrogen oxides to produce ozone, which is one of the primary pollutants in urban areas. In addition, regions with significant agriculture can have associated biogenic emissions. These emissions are typically viewed as anthropogenic since they result from human activities.

Ammonia from vegetation decay and hydrocarbons from vegetation decay and from living plants are typically estimated using emission factors based on land area, density, and type of vegetation. Different species of vegetation produce different amounts of hydrocarbons. Thus, there can be different emission factors for different plant or tree types.