Carbon dioxide emissions in Los Angeles fell 33% in April of 2020 compared with previous years, as roads emptied and economic activity slowed due to the COVID-19 pandemic, according to a new study in Geophysical Research Letters. In the Washington, D.C./Baltimore region, emissions of carbon dioxide, or CO2, dropped by 34% during the same period.

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The study was led by scientists at NASA’s Jet Propulsion Laboratory (JPL), the National Institute of Standards and Technology (NIST) and the University of Notre Dame.

While the emissions reductions are significant, the method that scientists used to measure them may have the greater long-term impact.

In both locations, scientists had previously installed networks of sensors on rooftops and towers to monitor the concentration of CO2 in the air. They used the data from those sensor networks to estimate the drop in emissions.

This might seem an obvious way to estimate emissions, but this is not how it’s usually done. Most cities estimate their emissions by tallying up the effects of activities that cause emissions, such as the number of vehicle miles traveled or the square footage of buildings heated and cooled. These are called “bottom up” methods because they are mostly based on activities on the ground.

This new study demonstrates that “top-down” methods, based on measuring the concentration of CO2 in the air, can produce reliable emissions estimates. Scientists were able to test those methods when emissions suddenly dropped due to COVID-19.

“This was a completely unanticipated experiment, and one we don’t ever want to do again,” said lead author and JPL data scientist Vineet Yadav. “But our results show that we were able to detect the onset of emissions reductions to within a few days.”

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