Measurements of carbon-14 show that roughly 20% of carbon dioxide emissions in the Los Angeles Basin are likely due to the decay of plants in managed landscapes.

The area’s natural green spaces, like its grasslands and forests, probably aren’t the origin of this unexpectedly large signal. The Los Angeles Basin in California, home to more than 18 million people, is often pegged as a car-centric—and, accordingly, polluted—urban sprawl. But researchers studying carbon dioxide (CO 2 ) emissions in the second-largest metropolitan area in the United States recently found a surprise: Roughly 20% of the CO 2 over Los Angeles derives from biogenic sources such as the decay of plant material.

What’s more, the area’s natural green spaces, like its grasslands and forests, probably aren’t the origin of this unexpectedly large signal, the new research shows. Instead, the biogenic emissions likely derive from the region’s managed landscapes like residents’ lawns and golf courses. Scientists determined this by studying seasonal variations in CO 2 levels over the course of 18 months.

Old Carbon

For this study, John Miller, a carbon cycle scientist at the National Oceanic and Atmospheric Administration in Boulder, Colo., and his colleagues collected samples of air from around the Los Angeles Basin. Working in collaboration with the Megacities Carbon Project, the researchers used accelerator mass spectrometry to analyze the carbon-14 (14C) content of the air samples.

This isotope traces biogenic sources of carbon dioxide like recent plant decay but not the carbon dioxide emitted by factories, power plants, and cars. That’s because the carbon in coal, natural gas, and petroleum is millions of years old, said Miller. “All of the 14C in them has decayed.”

A Green Surprise

“We were expecting to see a more fossil fuel–dominated signal.” Given the amount of 14C they measured, Miller and his collaborators deduced that roughly one fifth of the carbon dioxide in the Los Angeles Basin comes from the biosphere. The remaining 80% comes from the combustion of fossil fuels.

Such a high biogenic fraction was a surprise. “We were expecting to see a more fossil fuel–dominated signal,” said Miller.

Fluctuations in the 14C-containing carbon dioxide furthermore exhibited a perplexing seasonal trend: a spike in the net uptake of 14C around July. This spike is consistent with the timing of the peak growth of the region’s managed urban landscapes, like its golf courses and residents’ lawns, which receive regular watering.

By contrast, the grasslands and forest ecosystems of Southern California, a relatively dry Mediterranean-like climate, exhibit a carbon uptake peak in early spring. These results will be reported tomorrow at AGU’s Fall Meeting 2018 in Washington, D. C.

“This is a novel piece of work and is very relevant to the study of the urban carbon cycle,” said Anna Karion, an atmospheric scientist at the National Institute of Standards and Technology in Gaithersburg, Md., who was not involved in the research. “More of these kinds of measurements need to be made across the country.” “We’re just starting to learn about carbon balance in cities.”

Miller and his colleagues are now starting to make similar measurements in the Washington, D. C./Baltimore area, and they also plan to expand their monitoring nationwide.

There’s a lot more to do, Miller said. “We’re just starting to learn about carbon balance in cities.”

—Katherine Kornei ([email protected]; @katherinekornei), Freelance Science Journalist