U.S. Urban Testbed Projects: Methods, Uncertainties, and Goals for Future Efforts

Urban greenhouse gas (GHG) emissions account for a large portion of overall global GHG emissions; thus, city and regional governments are essential implementers of effective GHG mitigation.  Understanding the significance of their GHG emissions, city governments worldwide have committed to targets that may eventually rival or exceed those of their respective national governments, as defined through Intended Nationally Determined Contributions (INDCs) under the Paris Climate Agreement.   However, effective implementation of policy mechanisms to reduce urban GHG emissions requires policy performance assessment. Performance metrics for GHG reduction policies will be substantially enhanced by scientifically credible emissions quantifications based on a variety of approaches.

 To support the effort of improving measurement of GHG emissions in urban environments, several federal agencies including the National Institute for Standards and Technology (NIST), NOAA, NASA, DOE, NSF, etc. along with academic, research, and private entities have been working within a handful of domestic urban areas to improve both (1) the assessment of  GHG emissions accuracy by a variety of measurement technologies, and (2) the tools that can better assess GHG inventory data at urban GHG mitigation scales based upon these measurements. These research efforts have so far been focused domestically due to the quantity and quality of emission source/sink activity information and the logistical ability to instrument urban areas.  This enables these cities to be suitable test-bed environments.  Urban studies have been conducted in six U.S. metropolitan areas: Indianapolis (INFLUX experiment), Los Angeles (the Megacities project), Salt Lake City, Oakland, Boston, and the Washington/Baltimore area (the Northeast Corridor GHG Measurements project).  These cities represent a diversity in meteorological, terrain, demographic, and emissions characteristics and generally involve multiple measurement systems (e.g. meteorological, LIDAR, GHG concentration, etc.) and integrated observing approaches (e.g. aircraft mass balance, inventory-based emissions modeling, statistical models based on atmospheric GHG measurements and transport-dispersion models).  It is expected that the lessons that are learned can be exported to other cities both domestically and internationally.

In April, 2016, NIST brought together many of the U.S. research teams working to improve urban GHG measurement capabilities. These researchers are also focused on further resolving spatial and sectoral GHG attribution, understanding temporal variability of emissions as well as their annual trends.  These research teams employ both methods using atmospheric observations of GHG concentrations coupled with an atmospheric transport-dispersion model (i.e. top down) and emission process-based approaches (bottom-up).  Much has been learned from this work including:

1. Overall consensus that top-down methods utilizing atmospheric GHG observations coupled with transport models are generally successful in quantifying total CO₂ and methane (CH₄) emissions from cities (using either aircraft-based measurements or tower-based continuous GHG concentration observations mainly in winter months);
2. Bottom-up products such as those produced from emissions modeling, e.g., the Vulcan/Hestia system, are able capture the spatial and temporal distribution of CO₂ within urban areas especially where emissions activity data is readily available; and,
3. Better understanding of urban CH₄ emissions and a general agreement that atmospheric methods can better attribute CH₄ emissions to their various sectoral sources (e.g. natural gas, landfills, dairy farms, etc.) and that coupling top-down and bottom-up approaches and results is essential to broadening the science base supporting inventory data.

Regardless of these accomplishments, many research challenges remain, including how best to use the bevy of information being collected in these areas (including use of night-time data and the need to significantly improve the fidelity of lower atmospheric transport simulation), to isolate biospheric fluxes from the anthropogenic signals.

This presentation will give an overview of urban greenhouse gas research efforts in the U.S. and the results of the April NIST workshop, sharing lessons learned and identifying community needs and future goals.  We highlight the need for:

• Strengthening cooperation and information sharing between the research groups involved and the general scientific and emissions data compilation communities;
• Further development of method standardization, or best practices, for both top-down and bottom-up estimation methods; and,
• Improved validation and evaluation, along with improvement, of high-resolution transport and dispersion models needed for top-down approaches.

Overall, we stress that investment in urban greenhouse gas research and high-quality measurements (including those of meteorology, trace gases, imagery, etc.) across the United States provides a strong platform for support and expansion of advancing measurement method capabilities with the excellent potential to effectively inform mitigation policy in the areas where it is will most likely to be applied: cities.