Urban GHG Emissions Measurement and Monitoring System (Urban-GEMMS)

A Prototype of the Integrated Urban U.S. Greenhouse Gas Measurement, Monitoring, and Information System

NOAA’s Air Resources Laboratory (ARL) is developing Urban-GEMMS, an operational capability to measure and model U.S. emissions of greenhouse gases (GHG), in collaboration with other OAR labs (Global Monitoring Laboratory and Chemical Sciences Laboratory) as well as the National Institute for Standards and Technology (NIST). Initially, this capability integrates existing, mature capabilities into an urban-scale operational GHG monitoring system covering the Washington DC-Baltimore region. Implementation of this operational system to other urban areas, and to larger scales (e.g., regional, national) is planned.

The primary goal of the urban-scale prototype is to reduce the lag between data collection and the availability of GHG emissions estimates. Current estimates are generally available on an annual basis. Creating a product on a more frequent basis enables and allows for more effective collaboration between regional and local city planners and federal agencies to monitor and evaluate their GHG mitigation policies. This prototype system is based on contemporaneous observations of GHG concentrations in the atmosphere; it complements and provides an independent quality control check on traditional emissions estimating methods.

Monitoring greenhouse gas sources

This GHG Measurement, Monitoring, and Information System constitutes a portion of the US government’s initiative to address greenhouse gas emissions. The initial version of the prototype system being developed will target carbon dioxide (CO₂₎, as it is the most significant GHG contributing to climate change, and approximately 70% of anthropogenic CO₂ emissions are generated in urban areas. The next phase of the system will add methane (CH₄), as it is also a large contributor to climate change and has significant urban emissions. This program will foster a better understanding of GHG emissions, which is essential to designing and evaluating efforts to reduce emissions. This system aims to not only improve the accuracy of emissions estimates, but also to provide more frequent insights into the results of efforts to reduce emissions. As GHG emissions mitigation policies are adopted, stakeholders will be able to monitor the results of their efforts.

Urban-GEMMS starts with atmospheric measurements of CO2 in the DC-Baltimore region and provides estimates of what emissions must have been to create the current CO2 concentrations. It is classified as a top-down emissions-estimating system, as it starts from observable concentrations of CO2 in the air. The process of using downwind observations to estimate emissions is used in a number of air pollution applications and is commonly called an inversion. The system is under active, collaborative development, and all aspects of the methodology described below are currently being developed, tested, and optimized.

The methodology

CO₂ Observations

NIST’s Northeast Corridor GHG observation network provides fundamental input data to the prototype system. This network of tower-based carbon dioxide and methane observation stations was established in 2015 with the goal of quantifying GHG emissions in urban areas in the northeastern United States. A focus of the network is on Baltimore, MD, and Washington, DC, with a high density of stations in these two urban areas. Additional observing stations are available in the northeastern US to provide supplementary data on emissions throughout this complex region with high population density and multiple metropolitan areas.

Meteorological modeling

A fundamental component of Urban-GEMMS is a high-resolution meteorological simulation in the region, assimilating a variety of meteorological measurements, A key source of the meteorological measurements assimilated into the modeling are provided by ARL’s UrbanNet. This simulation is the result of a data reanalysis rather than a forecast. It is carried out after the fact, and uses meteorological measurements during the period of the simulation to create the best possible characterization of the atmosphere throughout that time period. This high-resolution simulation ties into the transport and dispersion model and other system components, as described below.

HYSPLIT footprints

The NOAA HYSPLIT atmospheric transport and dispersion model creates a series of “footprints,” to estimate the sensitivity of measured CO2 concentrations to upwind surface fluxes. The footprint fields generated by HYSPLIT are one of the key inputs into the inversion model to estimate CO2 emissions.

Biogenic emissions

Photosynthesis and other biogenic processes affect CO2 concentrations in the atmosphere and must be accounted for to estimate anthropogenic emissions. The measurements of CO2 in the region used for emissions estimates reflect both anthropogenic and biogenic influences. In Urban-GEMMS, the Vegetation Photosynthesis and Respiration Model (VPRM) is used to estimate biogenic sources and sinks within the modeling domain so they are accounted for in CO2 emissions estimates.

Background concentrations / boundary fluxes

CO2, influenced by upwind sources and sinks, enters into the modeling domain which must be accounted for in emission estimates. These boundary fluxes create a background concentration that is subtracted from the urban- area CO2 measurements to isolate the urban-area emissions. Urban-GEMMS utilizes CO2 measurements at upwind stations to estimate these background fluxes.

HYSPLIT generated footprints driven by the HRRR NWP model output for the NIST tower in Arlington, VA, covering measurements taken during the afternoon hours (12-4pm EST) in January 2019. Higher footprint values reveal regions that are more susceptible to surface fluxes that impact the measurements at this location and time window. HYSPLIT simulations suggest that for those hours, Arlington, VA was more sensitive to CO2 fluxes from the south-southwest and northwest. The diamonds show the locations of all NIST towers.

Anthropogenic emissions prior estimates

Urban-GEMMS starts with a first guess (or a prior estimate) of the emissions in the region, and then uses the CO2 measurements to estimate how these emissions should be adjusted so that they better match the CO2 measurements. The first guess emissions are based on the Greenhouse Gas and Air Pollutants Emissions System (GRA2PES) emissions inventory system developed by NOAA’s Chemical Sciences Laboratory (CSL) in collaboration with NIST.

Inversion using CarbonTracker-Lagrange

Once the above system components are assembled for a given month, they are used as inputs to the CarbonTracker-Lagrange (CT-L) inversion model developed by NOAA’s Global Monitoring Laboratory (GML). Conceptually, CT-L uses the time series of measured atmospheric concentrations of CO2 attributed to anthropogenic sources in the DC Baltimore region to estimate how much CO2 would have had to be emitted upwind of the measurement sites to produce the observed signals. The HYSPLIT footprints driven with the meteorological model fields provide the linkage between the emissions and observed concentrations required by the inversion technique.

Uncertainties

A fundamental component of Urban-GEMMS is estimates of uncertainties in the emissions estimates being generated. Stakeholders need to be aware of these uncertainties in order to better interpret the results of the system. Analysis of uncertainties in each of the system elements, and the entire system, will be continuously carried out in the operational system and included in the results provided by the system. For example, the meteorological modeling results will be compared against observations in the region, e.g., from ARL’s UrbanNet. The system’s overall emissions estimates will be evaluated by comparison against mobile- and aircraft-based measurements periodically carried out in the region. To this end, NOAA carries out mobile GHG measurements with a specially outfitted vehicle, known as the Air Resources Car, throughout the Washington DC-Baltimore region. Flight-based GHG measurement campaigns are carried out in the region in collaborations between NOAA, NIST, the University of Maryland, and other institutions.

Post-processing, publishing and distribution of results

Urban-GEMMS is currently configured to generate monthly emissions estimates, estimates for a given month are anticipated to be available within two months. In addition to overall regional emission estimates for a given month, spatially and temporally resolved estimates for each month will also be provided for the region. It is understood that uncertainties will generally increase with estimates made on smaller and smaller temporal or spatial scales. The ultimate resolution(s) provided by the system will be determined based on a consideration of stakeholder needs as well as an analysis of relative uncertainties. Once the estimates are complete for a given month, they will be provided on a public-facing website, including trends and graphical, tabular, and other summaries of the results. More detailed data products will also be available for download. The goal is to make the outputs as useful and relevant to stakeholders as possible. Focus groups and other mechanisms will be used during the development of the system — and after the system becomes operational — to ensure that this goal is met.