Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder

NOAA Observations Help EPA Track Emissions of a Family of Greenhouse Gases

For the first time, the U.S. Environmental Protection Agency (EPA) is using NOAA atmospheric measurements to help support a national inventory of emissions from an important family of greenhouse gases. 

The gases, called hydrofluorocarbons or HFCs, were developed to replace industrial chemicals responsible for destroying ozone in the stratosphere and creating the annual ozone hole over Antarctica. While less damaging to the ozone layer, human-made HFCs—used primarily in stationary air conditioning, refrigeration and foam-blowing applications—are super-potent greenhouse gases in their own right.

As ozone-destroying predecessors like chlorofluorocarbons—also potent greenhouse gases—have been phased out, HFC emissions and their concentrations in the atmosphere have increased dramatically. 

A graph depicting the rapid rise in the greenhouse gas hydrofluorocarbon-134.This graph depicts the rapid rise in the super-potent greenhouse gas hydrofluorocarbon-134, one of a family of gases developed developed to replace industrial chemicals responsible for destroying ozone in the stratosphere and creating the annual ozone hole over Antarctica. Measurements are in parts per trillion. Credit: Global Monitoring Laboratory

Because of HFCs’ growing climate impact, the Kigali Amendment of the Montreal Protocol has established a schedule for the international phase-down of their future production and consumption. In the United States, the American Innovation and Manufacturing Act of 2020 calls for an 85 percent reduction in HFC production and consumption by 2036.

Estimating emissions: Top down and bottom up

Greenhouse gas emissions estimates are generally calculated by one of two primary types of methods. “Bottom up” inventories are developed through an accounting of all activities that generate emissions, the application of measurement-based or model-based emissions factors to those activities and use of facility-specific data, where available. This is the approach used by the EPA and the basis for reporting national emissions and sinks annually to the United Nations Framework Convention on Climate Change.   

Greenhouse gas emissions can also be estimated by careful measurements of atmospheric samples collected downwind of a source region. This method is known as “atmosphere-based” or “top down,” and is a standard research approach used by NOAA’s Global Monitoring Laboratory.

A  graph showing regional emissions trends for HFC-134 in parts per trillionThe Global Monitoring Laboratory's new Emission Tracker provides emission magnitudes for certain potent greenhouse gases, such as replacements for ozone-destroying synthetic chemicals, derived from atmospheric measurements at sites across the U.S. This graph shows regional emissions trends for HFC-134 in parts per trillion. Credit: Global Monitoring Laboratory

This year, EPA has included a comparison of NOAA’s atmospheric emissions estimates of four HFCs to its own inventory-based estimates in the just-released U.S. Inventory of Greenhouse Gas Emissions and Sinks, based on results first reported in a 2017 Geophysical Research Letters study by a team that included scientists from NOAA, CIRES, EPA, and Lawrence Berkeley National Laboratories.

“The comparisons show reasonable consistencies between the two independently derived estimates, suggesting a robust understanding of HFC emissions,” said Lei Hu, a CIRES scientist at the Global Monitoring Laboratory who leads a team supplying the atmosphere-based estimates. “Where we see differences, further analysis will be beneficial.” 

HFCs well-suited for comparing inventory-based and atmosphere-based estimates

Accurate estimates of human-caused greenhouse gas emissions are important for understanding contributions to climate change, and for evaluating the effectiveness of emissions reduction activities. While “top down” and “bottom up” approaches each have limitations, they can complement each other, and comparisons can help target measurement or data collection improvements. Improved understanding of GHG emissions helps provide policymakers guidance when developing and assessing emissions reduction strategies. 

HFCs are some of the most suitable greenhouse gases for comparisons between the bottom-up inventory and atmosphere-based estimates because they are engineered chemicals with no natural sources, and they persist in the atmosphere, which reduces the complexity of estimating emissions.  

“Interagency collaboration strengthens our collective abilities to quantify HFC emissions in the U.S.,” said Ariel Stein, the director of the Global Monitoring Laboratory. “We look forward to more opportunities to work together in understanding emissive processes and guiding emission reductions.”

The HFC emissions estimates have recently been incorporated into the Global Monitoring Laboratory's new Emission Tracker web page, which provides atmosphere-based estimates at national and regional levels for 2008-2014 for HFCs and several other synthetic gases. The  laboratory plans to continue to provide national- and global-scale observations of the atmosphere and build tools that guide informed decisions for a Climate Ready Nation.

This story was written by NOAA Communications

CIRES at CU Boulder has partnered with NOAA since 1967.


Lei Hu
CIRES and NOAA scientist
Theo Stein
NOAA Communications
Karin Vergoth
CIRES Communications

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