Opinion: Coordinated Development of Emission Inventories for Climate
Forcers and Air Pollutants
Steven Smith1, Erin McDuffie2, Molly Charles1
1Joint Global Change Research Institute, Pacific Northwest National Laboratory, College Park, MD
2AAAS Science and Technology Policy Fellowships, Washington, D.C

Presentation Topics
1. Possible implications of a new SLCF methodology report
2. Historical context to understand key differences in Air Pollutant and GHG inventories
3. Benefits & challenges of various coordination approaches
4. Recommendations & final thoughts

IPCC SLCF Expert Meeting #4
April 11, 2022

Possible implications of a new Short-Lived Climate Forcer (SLCF)
Methodology Report“Additional SLCF-focused guidelines (i.e., from the IPCC) may help tocoordinate air pollutant and GHG inventory methodological approaches andcould enhance the ability to consider synergies between both air pollution
and GHG mitigation strategies.”
Smith, S. J., McDuffie, E. E., and Charles, M.: Opinion: Coordinated Development of Emission Inventories for Climate Forcers and Air
Pollutants, Atmos. Chem. Phys. Discuss. [preprint], https://doi.org/10.5194/acp-2021-1059, in review, 2022.

Possible implications of a new Short-Lived Climate Forcer (SLCF)
Methodology Report“Additional SLCF-focused guidelines (i.e., from the IPCC) may help tocoordinate air pollutant and GHG inventory methodological approaches andcould enhance the ability to consider synergies between both air pollution
and GHG mitigation strategies.”
Smith, S. J., McDuffie, E. E., and Charles, M.: Opinion: Coordinated Development of Emission Inventories for Climate Forcers and Air
Pollutants, Atmos. Chem. Phys. Discuss. [preprint], https://doi.org/10.5194/acp-2021-1059, in review, 2022.

What do we mean by ‘coordination’? nd what factors need to beconsidered when developing SLCF methodological guidance?

Possible implications of a new Short-Lived Climate Forcer (SLCF)
Methodology Report“Additional SLCF-focused guidelines (i.e., from the IPCC) may help tocoordinate air pollutant and GHG inventory methodological approaches andcould enhance the ability to consider synergies between both air pollution
and GHG mitigation strategies.”
Smith, S. J., McDuffie, E. E., and Charles, M.: Opinion: Coordinated Development of Emission Inventories for Climate Forcers and Air
Pollutants, Atmos. Chem. Phys. Discuss. [preprint], https://doi.org/10.5194/acp-2021-1059, in review, 2022.

What do we mean by ‘coordination’ and what factors need to beconsidered when developing SLCF methodological guidance?

Inventories of GHGs vs Short-Lived ‘Air Pollutant’ Climate Forcers

All air pollutants(APs) listed here
are direct/indirect
climate forcers.

There are additional
categories of APs suchas metals and other
HAPs that are included
in many national
inventories.

Historical Context (#1): Development of GHG and AP inventories
have been distinct – driven by differences in their end uses andreporting requirements
Greenhouse Gas Inventories

Air Pollutant Inventories•

Starting ~1993 – inventories developed by national
governments (Annex I) for the purpose of tracking
emission trends and mitigation progress•

Starting ~1970s – inventories developed by individual
governments (national, regional, local) as needed (and as
capacity allowed), for the purpose of developing
regulations to reduce air pollution levels•

Currently available international reporting framework andguidance for national GHG inventories (anthropogenic
sources only) (IPCC guidance)•

No global international reporting framework for national
air pollutant inventories (regional reporting exists, e.g,
EMEP)

•

Required UNFCCC emissions reporting (Annex I countries)
following IPCC 2006 ‘good practice’ guidance (soon to be
implemented by all signatories, under the Enhanced
Transparency Framework of the Paris Agreement)•

Currently, established AP inventory programs in mostOECD countries and many research institutions

Historical Context (#1): Development of GHG and AP inventories
have been distinct – driven by differences in their end uses andreporting requirements
Greenhouse Gas Inventories

Air Pollutant Inventories•

Starting ~1993 – inventories developed by national
governments (Annex I) for the purpose of tracking
emission trends and mitigation progress•

Starting ~1970s – inventories developed by individual
governments (national, regional, local) as needed (and as
capacity allowed), for the purpose of developing
regulations to reduce air pollution levels•

Currently available international guidance for national
GHG inventories (anthropogenic sources only) (IPCC
guidance)•

No global international reporting framework for national
air pollutant inventories (regional reporting exists, e.g,
EMEP)

•

Required UNFCCC emissions reporting (Annex I countries)
following IPCC 2006 ‘good practice’ guidance (soon to be
implemented by all signatories, under the Enhanced
Transparency Framework of the Paris Agreement)•

Currently, established AP inventory programs in mostOECD countries and many research institutions

Historical Context (#1): Development of GHG and AP inventories
have been distinct – driven by differences in their end uses andreporting requirements
Greenhouse Gas Inventories

Air Pollutant Inventories•

Starting ~1993 – inventories developed by national
governments (Annex I) for the purpose of tracking
emission trends and mitigation progress•

Starting ~1970s – inventories developed by individual
governments (national, regional, local) as needed (and as
capacity allowed), for the purpose of developing
regulations to reduce air pollution levels•

Currently available international guidance for national
GHG inventories (anthropogenic sources only) (IPCC
guidance)•

No global international reporting framework for national
air pollutant inventories (regional reporting exists, e.g,
EMEP)

•

Required UNFCCC emissions reporting (Annex I countries),soon to be implemented by all signatories, under the
Enhanced Transparency Framework of the Paris
Agreement•

Currently, established AP inventory programs in mostOECD countries and many research institutions, but noglobal reporting mechanism

For GHGs, Emissions are the policy target

For APs, Concentrations are the end regulatory target

Inventory Context {Equivalent Particulate Matter (PM) Emissions}
One major factor shaping the use & development of inventories is socio-economic status.
For each country group, theevolution from 1990-2019 isshown by the dotted trail.

● Socio-economic
development is anoverriding priority for low
SDI countries● AP will be large concern,
but inven. not as critical● CO2 emissions are
relatively lower on the RHS
and AP uncertainty
generally higher● AP emission inventories
become more importantand, arguably useful,toward the left and lowerleft

SDI = Socio-Demographic Index

Historical Context (#2): Due to different end uses, GHG and AP
inventories have similar but distinct data needs
Greenhouse Gas Inventories

Air Pollutant Inventories•

Large volumes of activity and emission factor data (less sofor combustion CO2), w/ country and facility-specificinformation required for higher IPCC Tier methods.

•

Large volumes of activity and emission factor data, often
required to be at the facility (or sub-national) level anddependent on operational conditions, or
performance/presence of emission control devices•

Limited country and facility-specific data available indeveloping countries, but IPCC guidance is provided for Tier
1 estimates with default emission factors.

•

Limited availability of data at the necessary level of detail inmany countries, where a Tier 1 approach (that does notconsider facility-specific information) may not be appropriate•

Reported data to the UN are at the national level (sufficientfor mitigation tracking, but not scientific impacts)•

Reported data for regulatory and impact analyses need tohave spatial and temporal detail•

National inventory activities are often conducted byenvironmental ministries (and reported by UNFCCC national
focal point) – not typically the same groups that develop
national air pollution inventories (e.g., USA)•

Inventory activities (if present) are often supported byregulatory processes in developed countries

Historical Context (#2): Due to different end uses, GHG and AP
inventories have similar but distinct data needs
Greenhouse Gas Inventories

Air Pollutant Inventories•

Large volumes of activity and emission factor data (less sofor combustion CO2), w/ country and facility-specificinformation required for higher IPCC Tier methods.

• Large volumes of activity and emission factor data, often
required to be at the facility (or sub-national) level anddependent on operational conditions, or
performance/presence of emission control devices•

Limited country and facility-specific data available indeveloping countries, but IPCC guidance is provided for Tier
1 estimates with default emission factors.

• Limited availability of data at the necessary level of detail inmany countries, where a Tier 1 approach (that does notconsider facility-specific information) may not be appropriate•

Reported data to the UN are at the national level (sufficientfor mitigation tracking, and most scientific impacts)• Reported data for regulatory and impact analyses need tohave higher spatial and temporal detail•

National inventory activities are often conducted byenvironmental ministries (and reported by UNFCCC national
focal point) – not typically the same groups that develop
national air pollution inventories (e.g., USA)

• Inventory activities (if present) are often supported byregulatory processes in developed countries

Historical Context (#2): Due to different end uses, GHG and AP
inventories have similar but distinct data needs
Greenhouse Gas Inventories

Air Pollutant Inventories•

Large volumes of activity and emission factor data (less sofor combustion CO2), w/ country and facility-specificinformation required for higher IPCC Tier methods.

• Large volumes of activity and emission factor data, often
required to be at the facility (or sub-national) level anddependent on operational conditions, or
performance/presence of emission control devices•

Limited country and facility-specific data available indeveloping countries, but IPCC guidance is provided for Tier
1 estimates with default emission factors.

• Limited availability of data at the necessary level of detail inmany countries, where a Tier 1 approach (that does notconsider facility-specific information) may not be appropriate•

Reported data to the UN are at the national level (sufficientfor mitigation tracking, and most scientific impacts)• Reported data for regulatory and impact analyses need tohave higher spatial and temporal detail•

National inventory activities are often conducted byenvironmental ministries (and reported by UNFCCC national
focal point) – not always the same groups that develop
national air pollution inventories (e.g., USA)

• Inventory activities (if present) are often supported by longstanding regulatory processes in developed countries

Historical Context (#3): AP and GHG inventories typically differ in
uncertainty reporting
Greenhouse Gas Inventories

Air Pollutant Inventories••

Limited focus on estimating AP uncertainties byregulatory agencies due to wide variety of data sources
and methodologies across sectors.

•

Uncertainties often higher in developing countries due
to data gaps and information on local technologies andusage patterns•

Uncertainties will become more important for policy
analyses and mitigation actions as emissions are
reduced; BUT lack of uncertainty estimates should not
be an impediment for mitigation action

Standard IPCC methodologies to report uncertainties(Approach 1 and 2)

Inventories of GHGs vs Short-Lived ‘Air Pollutant’ Climate Forcers

There are both benefits and challenges with coordinating AP and
GHG inventory activities… and a range of possible approaches
Possible benefits
Full coordination/harmonization:•

Could help countries without current programs to establish
sustainable systems to develop GHG and AP inventories

Challenges•

GHG and AP inventories often have very different end uses
(e.g., AP regulatory analyses vs. GHG mitigation tracking)•

Underlying driver data at different scales may be available atdifferent times, since national fuel consumption statistics areoften updated more quickly than sub-national data•

Air pollutant methodologies can be more data intensive and,
therefore, can have longer lag times (e.g., MOVES)•Key emission categories can also differ substantially between
air pollutant and GHG inventories, suggesting potentiallydifferent development priorities between the two

Partial Coordination:•

Increased engagement across AP and GHG communities could:• Help to better align emission source and chemical specie
definitions, methodological approaches, and facilitate
additional data collection• facilitate productive engagement between research groups
and GHG and AP inventory compilers• Better align and inform gridding (temporal and spatial)
methodologies – required to assess environmental andhealth impacts and tools to compare top-down(observation-based) and bottom-up emission estimates (per
IPCC 2019 Refinements).

With limited resources available for inventory (AP & GHG)
development, any coordination needs to ensure that acountry’s regulatory needs are met and reflect the unique
circumstances

There are both benefits and challenges with coordinating AP and
GHG inventory activities… and a range of possible approaches
Possible benefits
Full coordination/harmonization:•

Could help countries without current programs to establish
sustainable systems to develop GHG and AP inventories

Challenges•

GHG and AP inventories often have very different end uses
(e.g., AP regulatory analyses vs. GHG mitigation tracking)•

Underlying driver data at different scales may be available atdifferent times, since national fuel consumption statistics areoften updated more quickly than sub-national data•

Air pollutant methodologies can be more data intensive and,
therefore, can have longer lag times (e.g., MOVES)•Key emission categories can also differ substantially between
air pollutant and GHG inventories, suggesting potentiallydifferent development priorities between the two

Partial Coordination:•

Increased engagement across AP and GHG communities could:• Help to better align emission source and chemical specie
definitions, methodological approaches, and facilitate
additional data collection• facilitate productive engagement between research groups
and GHG and AP inventory compilers• Better align and inform gridding (temporal and spatial)
methodologies – required to assess environmental andhealth impacts and tools to compare top-down(observation-based) and bottom-up emission estimates (per
IPCC 2019 Refinements).

With limited resources available for inventory (AP & GHG)
development, any coordination needs to ensure that acountry’s regulatory needs are met and reflect the unique
circumstances

There are both benefits and challenges with coordinating AP and
GHG inventory activities… and a range of possible approaches
Possible benefits
Full coordination/harmonization:•

Could help countries without current programs to establish
sustainable systems to develop GHG and AP inventories

Partial Coordination:•

Increased engagement across AP and GHG communities could:• Help to better align emission source and chemical specie
definitions, methodological approaches, and facilitate
additional data collection• facilitate productive engagement between research groups
and GHG and AP inventory compilers• Better align and inform gridding (temporal and spatial)
methodologies – required to assess environmental andhealth impacts and tools to compare top-down(observation-based) and bottom-up emission estimates (per
IPCC 2019 Refinements).

Challenges•

GHG and AP inventories often have very different end uses
(e.g., AP regulatory analyses vs. GHG mitigation tracking)• Air pollutant methodologies can be more data intensive and,
therefore, can have longer lag times (e.g., MOVES)•

Underlying driver data at different scales may be available atdifferent times, since national fuel consumption statistics areoften updated more quickly than sub-national data•

Key emission categories can also differ substantially between
air pollutant and GHG inventories, suggesting potentiallydifferent development priorities between the twoWith limited resources available for inventory (AP & GHG)
development, any coordination needs to ensure that acountry’s regulatory needs are met and reflect the unique
circumstances

There are both benefits and challenges with coordinating AP and
GHG inventory activities… and a range of possible approaches
Possible benefits
Full coordination/harmonization:•

Could help countries without current programs to establish
sustainable systems to develop GHG and AP inventories

Partial Coordination:•

Increased engagement across AP and GHG communities could:• Help to better align emission source and chemical specie
definitions, methodological approaches, and facilitate
additional data collection• facilitate productive engagement between research groups
and GHG and AP inventory compilers• Better align and inform gridding (temporal and spatial)
methodologies – required to assess environmental andhealth impacts and tools to compare top-down(observation-based) and bottom-up emission estimates (per
IPCC 2019 Refinements).

Challenges•

GHG and AP inventories often have very different end uses
(e.g., AP regulatory analyses vs. GHG mitigation tracking)• Air pollutant methodologies can be more data intensive and,
therefore, can have longer lag times (e.g., MOVES)•

Underlying driver data at different scales may be available atdifferent times, since national fuel consumption statistics areoften updated more quickly than sub-national data•

Key emission categories can also differ substantially between
air pollutant and GHG inventories, suggesting potentiallydifferent development priorities between the twoWith limited resources available for inventory (AP &GHG) development, any coordination needs to ensurethat a country’s regulatory needs are met and reflect
their unique circumstances

Final Thoughts/Recommendations

Please reach out to discuss additional comments and/or ideas!
(ssmith@ppnl.gov and erin.mcduffie@wustl.edu)

General thoughts on coordination• IPCC SLCF methodology could help fill a gap for global AP methodologies and/or increase coordination between AP & GHG communities• Keep end uses in mind during methodology development (e.g., mitigation tracking vs regulatory development vs. scientific assessment)• Full GHG & AP coordination – may not always useful (or possible), but could benefit countries that without current inventory systems• Partial GHG & AP coordination – align sector and chemical definitions (or documenting where different), facilitating compiler-researcher engagement, etc.

Recommendations relevant to this expert meeting:••••Chemical compounds - consider indirect & direct forcers; establish flexibility to allow for new information on NMVOCs and PM speciation
Sectoral detail - define the appropriate level of detail for each sector and establish flexible reporting to reflect regionally key sources
Tiered Methodology Approach - assess where Tier 1 is and is not appropriate (e.g., where Tier 1 would produce unreliable estimates)
Reporting flexibility - assess what level of flexibility in reporting requirements would be helpful (e.g., sectoral detail by emission type)

Caveats for national AP reporting (as would be done to UNFCCC)• National reporting does not provide the spatial and temporal detail needed for AP impact analysis• When done well can provide a useful indication of trends• Unlike combustion CO2, when done poorly, can result in grossly incorrect trends (need more than just a quality flag)• Note that some AP inventory systems do not produce true time series (need to flag this)Related topics for continued discussion••••

How to balance resource limitations for inventory (GHG or AP) development/provide guidance on identifying key sources?
Can the inventory community leverage existing work from the research community (both bottom-up and top-down)?
What recommendations or methodologies are appropriate/useful for spatial and temporal emissions allocation?
Can general IPCC uncertainty guidance be applied to APs?