Estimating Emissions from Sources of Air Pollution

6.3 Estimating Emissions from Off-Road Mobile Sources

6.3.9 Existing Models for Off-Road Mobile Source Emissions Introduction

In general, VOC, NOx, CO, CO2 and toxic emissions from off-road mobile sources are typically a large portion of the inventory and therefore are important to track. However, the variety of equipment and uses for the equipment make these sources one of the more challenging to accurately monitor. Nevertheless, there are two widely used emissions models in the US that are designed to predict emissions from the complete gamut of off-road sources. At this time there are no mainstream off-road models in-use elsewhere. The two identified models are:

* The EPA’s Non-Road Model

* The CARB’s Off-Road Model

Since the regulation of non-road equipment is in its infancy, the results of these models should produce reasonable results for equipment outside of the United States and California; although, caution should always be exercised when using these country specific models.

Each of these are described briefly below. EPA’s Non-Road Model

The US EPA’s Non-Road Model, originally released in 1998, predicts emissions of HC, CO, CO2, NOx, SOx and PM from all non-road sources (except for commercial marine, locomotive and aircraft) for different states and counties in the United States. The model is used for official SIP inventory development as well as many other uses. The current version of the model (NON-ROAD2005), user’s guide, and documentation can be found at This model uses the emissions factors from the newest NEVES study described in a previous section. The model includes more than 260 specific types of non-road equipment, and further stratifies equipment types by horsepower rating. Fuel types include gasoline, diesel, compressed natural gas (CNG), and liquefied petroleum gas (LPG). The outputs include emissions for a specified area by hour, day, or year. These outputs require the use of activity information that is built into the data files in the model. The model’s interface allows a user to replace the default data with their own data, if they want to change the activity profile or some other input. Alternatively, the user can output emissions only, in terms of emissions by grams/operating hour for a given technology, or emissions in grams per hp-hour by a given technology. This output option is useful if an area simply wants to use the emission rates from the latest model and apply their own activity database. The technologies are referred to by their SCC (Source Classification Code). The technology descriptions of the SCC codes can be found at For example, an Off-highway Vehicle Gasoline, 2-Stroke tractor used in construction and mining has a SCC code of 2260002075. The one drawback of the SCC code classification is that it does not differentiate between controlled and uncontrolled technologies. In the 1990s, the US began regulating and placing controls on off-road equipment, and therefore the average emissions rates reflect the introduction of the various controlled equipment. CARB’s Off-Road Model

The ARB has developed an off-road model, which predicts emissions from the same sources as the EPA’s non-road model. The ARB developed this model specifically for estimating emissions in California from off-road sources. They use it primarily in the development of their official SIP inventory for compliance purposes. The model reports exhaust, evaporative and toxic emissions from detailed off-road sources. The default input data is activity and emissions data specific to California, however, the data files can be edited, both the activity, meteorology, and emission factors, as the user wishes. However, editing the data files must be done outside of the model itself. The model, user guide and supporting documents can be found at

The model uses a typical equation for predicting off-road emissions from a source. This equation is similar but slightly more detailed than the simple equation 6.3.9-1 (seen here and earlier), since it accounts for the load factor. ARB uses the same equation as presented in equation 6.3.9-2 for the detailed emissions estimation of off-road sources.


AvgHp = Maximum rated average horsepower

Load = Load factor

Activity = Annual activity in hours per year (hr/yr)

EF = Emission factor in grams per horsepower-hour (g/bhp-hr)

Pop = Population