Carbon dioxide emissions in Los Angeles fell 33% in April 2020 from previous years, as roads emptied and economic activity slowed due to the COVID-19 pandemic, new study finds in Geophysical research letters. In the Washington, DC / Baltimore area, emissions of carbon dioxide, or CO2, fell 34% over the same period.
The study was conducted by scientists from NASA’s Jet Propulsion Laboratory (JPL), the National Institute of Standards and Technology (NIST) and the University of Notre Dame.
While emission reductions are significant, the method scientists use to measure them may have the greatest long-term impact.
In both locations, scientists had already installed sensor networks on rooftops and towers to monitor the concentration of CO2 in the air. They used data from these sensor networks to estimate the drop in emissions.
This may seem like an obvious way to estimate emissions, but it’s usually not the way to do it. Most cities estimate their emissions by counting the effects of activities that cause emissions, such as the number of kilometers traveled by vehicles or the square footage of buildings heated and cooled. These are called “bottom up” methods because they are mainly based on activities in the field.
This new study shows that “top-down” methods, based on measuring the concentration of CO2 in air, can produce reliable emission estimates. Scientists were able to test these methods when emissions suddenly dropped due to COVID-19.
“It was a completely unplanned experiment, and one we never want to do again,” said JPL lead and data scientist Vineet Yadav. “But our results show that we were able to detect the onset of emission reductions within days.”
Scientists have developed top-down methods to measure CO2 emissions for several years. “This study shows that the technology has matured enough to produce reliable results and can be put into use,” said Kimberly Mueller, scientist and co-author of NIST. This would give cities an important new tool in their efforts to reduce emissions.
Top-down estimates are difficult to make because most CO emissions2 in the air over cities does not come from local emissions. Most of it is there naturally, and some of it is emitted outside the city boundaries and comes on the wind. The trick is to determine the amount of CO2 in the air above the city was generated locally.
“My doctoral advisor used to describe the atmosphere like a big cup of coffee,” Mueller said. “You added the cream and you try to stir the coffee to see where and when you put the cream.”
To stir the atmospheric coffee, the scientists used data on the speed, direction and other factors of the wind. This allowed them to estimate where emissions were coming from in a city and their magnitude.
Although difficult to achieve, top-down measurements have several advantages. First, they can provide relatively quick feedback on the effectiveness of emission reduction efforts. If a city changes traffic patterns or increases transit, for example, top-down estimates can provide data on whether those efforts are actually leading to reduced emissions.
Additionally, a recent study, also co-authored by Mueller, found that U.S. cities often underestimate their emissions when using only bottom-up methods. Another recent study has shown that the combination of bottom-up and top-down methods increases accuracy. (Both studies were partially funded by NIST.)
“Accurate measurements are essential to any strategy for managing greenhouse gas emissions,” said James Whetstone, head of NIST’s greenhouse gas measurement group and co-author of the study. “This is the only way to know if you are progressing towards your goals.”
NIST, NASA and other research partners are using the Los Angeles and Washington, DC / Baltimore area sensor networks to develop and test top-down methods to obtain more accurate emission estimates. This research project focuses on cities in part because cities represent a large and growing share of global CO2 emissions.
The 33% and 34% emission reductions in Los Angeles and the DC / Baltimore region represent reductions from April’s average emissions of the previous two years. The researchers used three different methods to detect the change in emissions based on atmospheric measurements, all of which detected the drop occurring at the same time.
“These independent statistical tests on different pieces of the puzzle gave consistent results,” said Subhomoy Ghosh, computer scientist and co-author of Notre Dame. “It gives us confidence in the results.”
Additionally, the methods appeared to work well in both locations, despite very different environmental conditions. In Los Angeles, relatively clean air comes in from the Pacific. DC and Baltimore, on the other hand, regularly receive emissions from cities and power plants in the west. Also in DC and Baltimore, emission models must take into account the effects of spring, when plants turn green again and start to draw CO.2 air. Los Angeles experiences less seasonal variation in uptake by plants.
“These methods were robust enough to work in very different contexts,” Mueller said. “The fact that these methods work in both places means the results were no fluke.”