Comprehensive top-down activity data on off-road vehicles are often unavailable, and where this is the case statistical surveys will be necessary to estimate the share of transport fuel used by off-road vehicles. Survey design is discussed in Chapter 2 of Vol.1 (Approaches to Data Collection). The surveys should be at the level of disaggregation indicated in Table 3.3.1 to make use of the default emission factor data, and be more detailed for the higher tiers. For the Tier 3 approach, modelling tools are available to estimate the amount of fuel consumed by each subcategory of equipment. Box 3.3.1 provides further information on using the NONROAD emissions model. This model may also be developed to incorporate country-specific modifications (see Box 3.3.2 for the Canadian experience).
Nonroad emission model (usepa)
NONROAD 2005 is a mathematical model developed by the USEPA and may be used to estimate and forecast emissions from the non-road (off-road) transportation sectors. The model itself and all available supporting documentation are accessible on the EPA's website (http://www.epa.gov/otaq/nonrdmdl.htm). This model estimates emissions for six exhaust gases: hydrocarbons (HC), NOX, carbon monoxide (CO), carbon dioxide (CO2), sulphur oxides (SOX), and particulate matter (PM). The user selects among five different types for reporting HC — as total hydrocarbons (THC), total organic gases (TOG), non-methane organic gases (NMOG), non-methane hydrocarbons (NMHC), and volatile organic compounds (VOC).
Generally, this model can perform a bottom-up estimation of emissions from the defined sources using equipment specific parameters such as: (i) engine populations; (ii) annual hours of use; (iii) power rating (horsepower); (iv) load factor (percent load or duty cycle), and (v) brake-specific fuel consumption (fuel consumed per horsepower-hour). The function will calculate the amount of fuel consumed by each subcategory of equipment. Subsequently, sub-sector (technology/fuel)-specific emission factors may then be applied to develop the emission estimate. The model is sensitive to the chosen parameters but may be used to apportion emissions estimates developed using a top-down approach.
It is not uncommon for the bottom-up approach using this model to deviate from a similar top-down result by a factor of 2 (100%) and therefore users are cautioned to review documentation for areas where this gap may be reduced through careful adjustment of their own inputs. Consequently, users must have some understanding of the population and fuel/technology make-up of the region being evaluated. However, reasonable adjustments can be established based upon: national manufacturing levels; importation/export records; estimated lifespan and scrappage functions. Scrappage functions attempt to define the attrition rate of equipment and may help illustrate present populations based upon historic equipment inventories (see Box 3.2.3 of Section 3.2 of this volume).
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