9.5 Locomotives

Categories 936, 1681, and 1722

9.5.1 Introduction

Categories 936, 1681, and 1722 account for emissions from road hauling locomotive operations, switching locomotive operations , and passenger operations, respectively.

Locomotives are an important mode of transportation for the movement of goods in California. The ability of locomotives to haul a large amount of goods and the established network of rails throughout California make locomotive an ideal transportation mechanism for goods movement. This methodology document accounts for emissions from three major types of locomotive operations. The emissions reflected herein include all five criteria pollutants (PM, VOC, NO_x, SO_x, CO).

9.5.2 Methodology

Locomotive emissions data are derived by inventory staff of the state’s chief air quality regulatory agency, the California Air Resources Board (CARB). This significant emissions dataset, sorted by county, is published every few years and is a product of the State Implementation Plan (SIP) emissions document formally known as the California Emissions Projection Analysis Model (CEPAM) inventory³⁵¹. For related sets of categories, such as airport ground support equipment (GSE), ships, structures coatings etc., where independent data collection and derivation of emissions are both cost- and time-prohibitive and likely a redundant effort, BAAQMD staff export the emissions data directly from CEPAM into the District’s Base Year inventory package. This calculation approach and collection of categories is internally termed as “CARB Source Categories”.

The CEPAM provides historical emissions as well as forecasts emissions for major emission source classifications including –

1. on-road mobile sources [from Emissions Factor (EMFAC) model],
2. off-road mobile sources (OFFROAD model), and,
3. stationary and areawide sources - For these major source classifications, CEPAM combines facility level /area source emissions data reported to the California Emissions Inventory Development and Reporting System (CEIDARS) for multiple years by various regional air quality agencies (including the BAAQMD).

The current base year inventory uses the 2016 CEPAM v1.05 to estimate emissions from “CARB Source Categories”. This version of the CEPAM derives emissions from a 2012 base year inventory and contains backcasts and forecasts from year-2000 to year-2035. All applicable regulatory and technological controls are assumed to be built into the CEPAM dataset during CARB staff’s inventory computation work. After the emissions data are exported, the inventory for CARB Source categories is taken through a quality assurance (QA) process.

In the QA process, BAAQMD staff perform a systematic crosswalk between CEPAM’s source category classification (Emission Inventory Code - EICs) and the District’s source category classification (category identification number - cat_ids). Based on the scope of emissions covered, individual EIC or a group of EICs are mapped to a single cat_id. This process also addresses issues when cat_ids have no matching EICs or there are discontinued EICs that need to be investigated. Following this, emissions data are backcasted to year-1990, as well as forecasted to year-2040 using certain mathematical methods, as described in the Trends section. Finally, the emissions trends spanning from year 1990-2040 for each category and pollutant are evaluated, and CARB staff are consulted for explanation of any observed anomalies in trends.

9.5.3 Changes in Methodology

There are no changes in the methodology to estimate emissions in the current base year inventory as compared to the previous base year inventory (year 2011).

9.5.4 Emissions

A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.

9.5.5 Trends

The emissions data from CARB CEPAM inventory only covers year 2000 to 2035. Different approaches were applied to estimate emissions for years 1990 to 1999 and 2036 to 2040 as described below. Please refer to CARB’s CEPAM Inventory Documentation ¹ for more information.

(a) Historical Emissions / History

Three different methods are used for backcasting to project historical emissions prior to year-2000 going back to year-1990, which is the starting year of the current base year inventory (referred to as BY2015). Following preliminary evaluation of the data available for initial years (post-year 2000) in the CEPAM inventory, BAAQMD staff recommends a backcasting approach. The default backcasting approach is defined by a formula here –

\[ \begin{eqnarray} \text{BY2015 emissions for year X for county Y} = \text{BY2011 total emissions for year X} \\ \\ \times \dfrac{\text{BY2015 year-2000 emissions}}{\text{ BY2011 year-2000 emissions}} \\ \\ \times \dfrac{\text{county Y fraction}}{\text{ fractional total}} \end{eqnarray} \]

This calculation is referred to as the BY2011 scaling method (based on the previous base year inventory). The second calculation approach is to derive the linear regression best fit of the year 2000-2008 CARB Sources emissions data for a category, species, and county, and extrapolate that data back in time. If this backcasting results in negative emissions prior to a certain year, the lowest positive emission value is held constant from that particular year back to year-1990. The third possible calculation approach is to use a specific growth profile for the category, provided by the in-house staff expert. These custom growth profiles are often derived from surrogate data provided by another agency or resource, or available activity data that is assumed to scale with the emissions for that specific CARB Source category.

For locomotives, the BY2011 scaling method is used to derive emissions for years 1990 through 1999.

(b) Future Projections / Growth

For CARB Sources, emissions beyond base year to 2035 are forecasted by CARB staff by applying growth profiles to the base year-2012 inventory data (see above in ‘Methodology’ section). Emissions forecasting beyond 2035 was done by holding the year-2035 emissions data constant through and up to year-2040.

(c) Controls

Since 1992, CARB has made updates to the locomotive emission inventory. In 2003, CARB updated the emission inventory by revising growth assumptions. In 2005, CARB updated the inventory to incorporate fuel correction factors, add passenger train data, and Class III locomotives. These changes have made a significant impact on the total inventory.

9.5.6 Uncertainties

For area source emission factors, the main uncertainty in the updated methodology arises from the use of emission factors for certain categories that remain constant over several decades although technological controls may have been applied at facilities. Additionally, the emission factors themselves are seldom verified and validated against measurements, in part due to the difficulty in measuring fugitive emissions from area sources.

9.5.7 Contact

Author: Tan Dinh

Reviewer: Abhinav Guha, Yuan Du

Last Update: November 06, 2023

9.5.8 References & Footnotes

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351. CARB. Criteria Pollutant Emission Inventory Information (CEPAM) . [accessed 2023 Mar 13]. https://ww2.arb.ca.gov/criteria-pollutant-emission-inventory-data↩︎