Estimating Emissions from Sources of Air Pollution

6.3 Estimating Emissions from Off-Road Mobile Sources

6.3.3 An Enhanced Emissions Inventory Approach
Once a basic inventory (as described in the previous section) is obtained, certain categories may be enhanced to improve the accuracy of the emissions of important categories. The inventory can be enhanced in several ways: more detailed population data, which includes engine size, more detailed activity data, in terms of temporal or spatial distribution, and more detailed information on activity data, in terms of the duty cycle, and improved emission rates as a function of engine size and duty cycle. Usually, these improvements are done across the board since they are dependent upon the other. For example, it does not make sense to collect activity data on the duty cycle of a equipment if your emission rates are too generic to account for changes in the duty cycle. Therefore, to predict the detailed emissions inventory, the required inputs are usually collected all together, and the simple equation can be expanded into the following:

Population: The population by engine size, (POP, SZ) (units, kw)

Activity: The hours of use by season (HRS) (hours/inventory period)

The duty cycle (or load factor) (LF) (unitless)

Emissions: The in-use emission rate (ER) (grams/kw-hr) as a function of duty cycle

The inventory can then be calculated for a particular technology as:

Tailpipe Emissions = POP*HRS*HP*LF*ER

In a simple first cut analysis, only tailpipe emissions from off road sources are usually considered. However, in reality, the emissions from off-road sources come from various parts of the equipment: exhaust emissions from the engine tailpipe, evaporative or fugitive emissions from refueling activities, fuel tanks and engines, and dust or emissions emitted from its use (figure 6.3.3-1). These emissions not only change as a function of engine type and fuel type, engine operation, but also terrain and duty cycle. As one can imagine, the dust emissions will be different from a tractor moving gravel versus a tractor moving fine silt. The exhaust emissions will be very different from a engine that is operating at steady state (one speed), such as a tractor pulling a till, than a engine that is operating in transient conditions, such as a tractor scooping, moving, and dumping a load. The evaporative emissions will be different from a gasoline powered trimmer that is being operated during the summer than during the winter. The number of times of operation of a lawnmower will be different in the summer and the winter, since the growth rates of the grass vary with season and rain and snowfall. With such variability in emissions outcomes, the off-road emissions inventory is one of the most difficult and highly uncertain areas of the inventory. However, even with this uncertainty, a reasonable first cut estimate of emissions from these sources can be obtained by using existing data on emissions factors and duty cycles, and crude population estimates. Then, by determining the most significant offenders, more effort can be focused on these items to refine the inventory.

Other Emissions = POP*HRS*OER

6.3.3-1 Different sources of Air Emissions from off-road, in-use equipment
where OER are other emission rates such as road dust, evaporative emissions, etc. of concern

The main difference between this enhanced approach and the simplified approach is that the simplified approach does not include the effects of different operating cycles and engine sizes, control technologies, and possibly variations in fuel types. Typically, emissions are determined on a city-wide or region-wide basis in terms of a day or year. However, emissions can be disaggregated by hour of day and grid (several km in size) for the purpose of air quality modeling. As described in the following section, these details can change the emissions result sometimes by an order of magnitude or more. So, when possible, the enhanced emissions inventory is recommended, especially for large sources. The gathering of the population, activity, and emission rates are described in the next sections.