August 10, 2012

MEMORANDUM

To: EPA-HQ-OAR-2010-0799

Subject: Mobile Source Contribution to U.S. GHGs in 2010

A. Overview

On an annual basis, the U.S. Environmental Protection Agency develops a national 

inventory of the U.S. primary anthropogenic sources and sinks of GHG emissions. The 

report is submitted to the United Nations per the U.S. commitment to develop a 

comprehensive inventory of GHG emissions under the United Nations Framework 

Convention on Climate Change (UNFCCC). The report estimates GHG emissions for all 

sectors of the U.S. economy, with the major sectors represented being industrial, 

transportation, residential, commercial, and agriculture. 

This memo will focus on the emissions estimated in the GHG Inventory report published 

by EPA on April 15, 2012, “Inventory of U.S. Greenhouse Gas Emissions and Sinks: 

1990-2010”.

1 Of particular interest are the values estimated in the report for the 

transportation sector as well as mobile source emissions that are attributed to other 

economic sectors. This memo will attempt to answer the following questions:

1) Has transportation been the fastest-growing sector in terms of U.S. GHG 

emissions since 1990?

2) What percentage of transportation and mobile source GHG totals did light-duty 

vehicles comprise?

3) What percentage of U.S. national GHG totals did transportation sources and 

mobile sources comprise? 

4) What percentage of Clean Air Act Section 202(a) GHG totals did light-duty 

vehicles comprise? 

5) What percentage of U.S. national GHG totals did Clean Air Act Section 202(a) 

sources comprise? 

6) For light-duty vehicles, what percentage of overall GHG emissions did CO2

comprise?

B. Definitions

The GHG Inventory contains specific definitions for the terms “transportation” and “nontransportation mobile”. Transportation includes all onroad vehicles, aviation, marine, and

rail, along with other smaller sources of emissions. Non-transportation mobile sources

include any vehicle or equipment not used on the traditional road system, but excluding 

aircraft, rail and watercraft.

2

This category includes snowmobiles, golf carts, riding lawn 

mowers, agricultural equipment, and trucks used for off-road purposes, among others. 
The reason for the distinction is that, in standard Inventory convention, nontransportation mobile sources are not part of the official national estimates for the 

transportation sector. Non-transportation mobile source emissions are instead included in 

the totals for their respective sectors; e.g., agricultural equipment is part of agriculture 

sector emissions, construction equipment is part of industrial sector emissions, etc. Table 

1 below indicates Inventory convention for which modes are considered to be in the 

transportation sector and which are non-transportation mobile.

For regulatory purposes, it is often of interest for EPA to represent mobile source 

emissions. For the purposes of this memo, mobile source emissions as a whole refers to 

the aggregate of transportation emissions and non-transportation mobile emissions.

Table 1: Modes contained in Transportation and Non-transportation mobile sectors

Mobile Source Emissions

Transportation Non-Transportation Mobile

Passenger Cars Agricultural Equipment

Light-Duty Trucks Construction and Mining Equipment

Medium- and Heavy-Duty Trucks Lawn and Garden Equipment

Buses Logging Equipment

Motorcycles Recreational Equipment

Aviation

Marine

Rail

Pipelines and Lubricantsa

C. Key Questions to be Addressed

1) Has the Transportation sector been the fastest-growing sector in terms of GHG 

emissions since 1990?

Emissions by economic sector are presented in two ways in the GHG Inventory: by 

distributing electricity-related emissions over the economic sectors or by disaggregating 

the electricity-related emissions and presenting electricity as an economic sector of its 

own. Commonly, EPA presents transportation sector emissions with the electricityrelated emissions included, In order to be consistent, this memo will present the sectors 

with electricity-related emissions distributed.

While the transportation sector has experienced the largest absolute increase in GHG 

emissions since 1990, it has not the sector with the largest increase on a percentage basis. 

As a result, it is not correct to state that the transportation sector is the fastest-growing 

sector in terms of GHG emissions since 1990. Rather, it is more appropriate to state that

the transportation sector is the source of the largest absolute increase in GHG emissions 

since 1990.

 

a Only CO2 emissions from pipelines and lubricants are estimated as part of transportation sector totals.
Table 2: U.S. Greenhouse Gas Emissions Allocated to Economic Sector with ElectricityRelated Emissions Distributed (Tg CO2e)

3

Economic Sector 1990 2010 Absolute 

Change

Percentage 

Change

Industry 2237.7 2019.0 -218.7 -9.8%

Transportation 1548.3 1838.6 290.3 18.7%

Residential 953.2 1226.6 273.4 28.7%

Commercial 939.4 1171.0 231.6 24.7%

Agriculture 462.9 521.1 58.2 12.6%

U.S. Territories 33.7 45.4 11.7 34.7%

Total Emissions 6175.2 6821.8 646.6 10.5%

2) What percentage of transportation and mobile source GHG totals did light-duty

vehicles comprise?

3) What percentage of U.S. national GHG totals did transportation sources and mobile 

sources comprise? 

Table 3: 2010 Transportation and Mobile Source Emissions

Transportation Sources

GHG 

Emissions 

(Tg CO2e)

% of 

Domestic 

Transport

% of 

Domestic 

Mobile

% of 

Domestic 

Total

Light-Duty Vehicles 1134.3 61.7% 55.5% 16.6%

Passenger Cars 787.9 42.9% 38.6% 11.5%

Light-Duty Trucks 346.4 18.8% 17.0% 5.1%

Motorcycles 3.8 0.2% 0.2% 0.1%

Buses 16.5 0.9% 0.8% 0.2%

Medium- and Heavy-Duty Trucks 402.3 21.9% 19.7% 5.9%

Aviation Total 143.9 7.8% 7.0% 2.1%

Commercial Aviation 115.2 6.3% 5.6% 1.7%

General Aviation 16.1 0.9% 0.8% 0.2%

Military Aircraft 12.6 0.7% 0.6% 0.2%

Marine Total 43.3 2.4% 2.1% 0.6%

Recreational Boats 16.8 0.9% 0.8% 0.2%

Ships 26.5 1.4% 1.3% 0.4%

Rail 46.3 2.5% 2.3% 0.7%

Pipelines 38.8 2.1% 1.9% 0.6%

Lubricants 9.5 0.5% 0.5% 0.1%

Transportation Total 1838.6 100.0% 90.0% 27.0%

Agricultural Equipment 48.2 2.4% 0.7%

Construction and Mining 

Equipment 73.6 3.6% 1.1%

Other Non-Transportation Mobile 82.5 4.0% 1.2%
Non

-Transportation Mobile Total 204.3 10.0% 3.0%

Mobile Total 2042.9 100.0% 29.9%

Total GHG emissions 6821.8 100.0%
4) What percentage of Clean Air Act Section 202(a) GHG totals did light-duty vehicles 

comprise? 

5) What percentage of U.S. national GHG totals did Clean Air Act Section 202(a) 

sources comprise?

The sources covered under Section 202(a) of the Clean Air Act are onroad sources, as 

listed in Table 4 below: cars; light trucks; motorcycles; buses; and heavy-duty trucks. 

However, the Section 202(a) total differs slightly from the onroad total from the GHG 

Inventory because hydrofluorocarbon (HFC) emissions associated with refrigerated 

transport are not considered a 202(a) source. The specific difference between the onroad 

and 202(a) totals is attributed to medium- and heavy-duty trucks, where 11.6 Tg CO2e are 

associated with refrigerated transport.

Table 4: 2010 Section 202(a) Emissions

Transportation Sources

GHG 

Emissions 

(Tg CO2e)

% of 

Section 

202a

% of Total 

GHG 

Emissions

Light-Duty Vehicles 1090.2 70.6% 16.0%

Passenger Cars 769.3 49.8% 11.3%

Light-Duty Trucks 320.9 20.8% 4.7%

Motorcycles 3.8 0.2% 0.1%

Buses 16.1 1.0% 0.2%

Medium- and Heavy-Duty Trucks 390.6 25.3% 5.7%

Cooling from Sec. 202a sources 44.5 2.9% 0.7%

Section 202a Totals 1545.2 100.0% 22.7%

GHG Inventory On-Road Totals 1556.8 22.8%

Total domestic GHG emissions 6821.8 100.0%
6) What percentage of GHG emissions did CO2 comprise for light-duty vehicles?

Table 5: CO2 as a percentage of GHG emissions by mode

Transportation Sources

CO2 

Emissions 

(Tg)

GHG 

Emissions 

(Tg CO2e)

CO2 as 

percentage of 

GHG emissions

Light-Duty Vehicles 1073.5 1134.3 94.6%

Passenger Cars 757.5 787.9 96.1%

Light-Duty Trucks 316.0 346.4 91.2%

Motorcycles 3.7 3.8 98.3%

Buses 16.0 16.5 97.3%

Medium & Heavy-Duty 

Trucks 389.3 402.3 96.8%

Aviation Total 142.4 143.9 99.0%

Commercial Aviation 114.0 115.2 99.0%

General Aviation 15.9 16.1 98.9%

Military Aircraft 12.5 12.6 99.0%

Marine Total 42.6 43.3 98.4%

Recreational Boats 16.6 16.8 99.2%

Ships 26.0 26.5 98.0%

Rail 43.5 46.3 94.1%

Pipelines 38.8 38.8 100.0%

Lubricants 9.5 9.5 100.0%

Transportation Total 1759.5 1838.6 95.7%

 

1 U.S. Environmental Protection Agency. 2012. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 

1990–2010. EPA 430-R-12-001. Available at 

http://www.epa.gov/climatechange/Downloads/ghgemissions/US-GHG-Inventory-2012-Main-Text.pdf

2 U.S. Environmental Protection Agency. 2012. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 

1990–2010. Annex 3. p. A-32. EPA 430-R-12-001. 

http://www.epa.gov/climatechange/Downloads/ghgemissions/US-GHG-Inventory-2012-Annex-3-

Additional-Source-or-Sink-Categories.pdf

3 U.S. Environmental Protection Agency. 2012. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 

1990–2010. pp. 2-19 – 2-20. EPA 430-R-12-001. Available at 

http://www.epa.gov/climatechange/Downloads/ghgemissions/US-GHG-Inventory-2012-Main-Text.pdf