U.S. EPA Short-lived Climate
Forcer Emissions and Methods
U.S. Environmental Protection Agency
Office of Air and Radiation
11 October 2021
Follow up questions: Rob Pinder (pinder.robert@epa.gov)

Overview• Quick overview of EPA SLCF emission inventory approach• Thoughts on uncertainty• Differences with GHG methods• Applicability to other countries• Known gaps and questions

Short-lived climate forcer emissions data
• National Emission Inventory• Criteria pollutants: NOx, SO2, CO, PM2.5, PM10, Pb• Precursors: NH3, Volatile organic compounds (VOC)• Many hazardous air pollutants• Annual, county level emissions, developed over a 3-year cycle• Modeling inventories• Temporal allocation: monthly, day of week, hourly• Spatial surrogates: population density, highways• Profiles for chemical speciation of VOCs and PM (including organic carbon / elemental carbon)• Future year projections• Emissions from vegetation and geological processes are calculated within the chemicaltransport modeling system: sea spray aerosol, lightning NO, biogenic VOC, etc.

What are the data used for?
• Input for use in regulatory and research air quality models• Assess health and environmental risks of ozone, PM2.5, and air toxics• Quantify risks and benefits of U.S. air quality regulatory options• Prioritize air quality management based on sources and trends• Global assessments of long-range transport of air pollutants andshort-lived climate forcers• Requires detailed representation of• sectors and control technologies• chemical speciation• location and timing of emissions

What are the data sources?
• Data provided by State, Local, and Tribal air agencies• Supplemented by data developed by the US EPA• Quality assurance programs• Emission estimation methods described in technical support andguidance documents
Point sources
Power plants, industrial
facilities
Facility-level data
Largest sources requirecontinuous emission
monitors
Some large sources
require stack tests

Non-point sources
Residential heating,agriculture, oil and gas
production
County totals
Sector-specificdatabases of emission
factors and functions

Mobile sources
Vehicles, trains, planes,
ships
Location-specificactivity data
Emissions from fuelcombustion as well as
evaporation and tirewear

Event sources
Prescribed & wildfires
Day and locationspecific activity data
Combines data fromsatellite-based
detections and reports
on acreage burned

Chemical speciation of PM and VOC• Used to disaggregate total VOC and PM emissions compiled in the NEI into
their component species for air quality models• SPECIATE database of profiles on a percent mass basis• What are the data sources?
• Peer-reviewed literature• Emission source tests by government or industry• New data rigorously reviewed with quality scoring criteria• Bray et al. (2019) reviewed profiles and recommended priority updates• Pros and Cons of this approach• Can fill in the gaps for sources where no speciated emission tests are available• Profiles require review when emission estimates are updated to ensure that the profiles
are representative of the sources they are assigned to• Potential issue: profile and emission factor may be measured under different conditions

How are the data organized?
• Source Classification Code (SCC)• Four-tiers starting with broad
sectors, down to resolving specificequipment and control
technologies• More than 9,000• WebFIRE emission factor databaselists existing emission factors,
including AP-42 emission factors,
for some SCC codes• Cross-reference tables map
SPECIATE profiles to SCC codes• Example for Residential Wood
Heating Appliances• 19 different SCCs
• Tier 1: Fuel Combustion Other• Tier 2: Residential Wood
• Tier 3: Woodstoves• Tier 4: Woodstove: Non-catalytic
Woodstoves: Pellet Fired[2104008053]

U.S. National Emissions (Gg per year)

90,000Note that wildfire emissions are not included in these totals

80,00070,000CO

60,00050,00040,00030,00020,000NOx

10,000SO2
20192018

2017

2016

2015

2014

2013

2012

2011

2010

2009

2008

2007

2006

2005

2004

2003

2002

-Source: U.S. Emission Report to the Convention on Long-range Transboundary Air Pollution (2021)

US 2019 EMISSIONS BY SECTOR AND POLLUTANTCOElectric utilities

Industrial and other fuel combustion

Chemical and manufacturing

Metals processing

Petroleum and related industries

Other industrial processes

Solvent Use

Storage and transport of fuels

Waste

Highway vehicles

Off-road vehicles

Agriculture

NOX

VOC

SO2

PM10

PM2.5

NH3

Source: U.S. Emission Report to the Convention on Long-range Transboundary Air Pollution (2021)

U.S. Black carbon emissions (Gg per year)
350
300
250

On-roadvehicles

200
150

Non-roadequipment

100

MarineTrains50
Industry and Commercial

02011

2017

2025

Agriculture

Other

Industry and Commercial

Electricity

Residential wood

AircraftTrainsMarineNon-road equipment

On-road vehicles

Waste disposal

Source: U.S. National Report to Arctic Council’s Expert Group on Black Carbon and Methane (2021)

Uncertainty• Challenges• Many large datasets• Difficult to independently assess with observations• Quantifying uncertainty requires resources• Types of uncertainty• Unknown emissions• Unresolved variability• Incomplete activity data
• Errors (can be reduced through quality assurance techniques)• Recent assessment by Day et al. (2019)• Trend: models or direct measurements are replacing emission factors• Opportunities to compare across inventories for outliers• Continuous assessment and frequent updates are essential

Uncertainty example: Stationary sources
ContinuousEmission
Monitor
Source Testing
Material Balance

Cost

Category emission model
Local / Sector Emission Factors

AP-42
Engineering Judgement

Reliability of emission estimate• Emissions from stationary industrial
facilities can vary depending on
operating procedures, equipment type,and emission control technologies• Default emission factors may not match
specific practices at a facility• Material balance may only be accurate
in processes where nearly all thematerial is converted to emissions• Recommend large sources are directlymeasured, and continuously measured
if there is considerable temporalvariabilityDifferences with GHG emission methods• The time and place of emissions is important, not just annual total• This requires more activity data
• May require time- and spatially-varying emission factors• For some sectors where GHG methods use fuel-based emission
factors, some SLCF emission factors use a different basis• Ship emissions are tracked using distance travelled between satellite-based
automatic identification system checks• NEI includes aircraft take-offs and landings, but not cruise emissions• Sector aggregations can differ, for example industrial combustion andprocess emissions

Applicability to other countries• Methods and guidance can be applicable in other contexts• Emission System Structure• Data management and quality assurance• Many globally important emission sources are highly variable and U.S.
emission test conditions may not be applicable• Open burning: types of materials and burning conditions• Residential cooking: large range of conditions• Rare in U.S.: for example, brick kilns, three-wheeled motor vehicles• Very challenging to develop representative Tier I methods using fuelbased emission factors for many sources, given diversity of emissions
processes

Gaps and questions• EPA methods are a hybrid of state-provided emissions data andemissions developed using nationally consistent methods – what isthe right mix for the international community?
• Do existing IPCC emission categories have enough specificity tofacilitate evaluation of reported inventories?
• To what extent do emission factors resolve the differences between
particulate matter, condensable PM, intermediate volatility
compounds, and volatile organic matter?