Wild Grasses Give California Scientists Neighborhood-By-Neighborhood Emissions Readings

In Southern California, automobile emissions are the biggest source of carbon dioxide in the air. But during the pandemic, when there were fewer cars on the road, those levels decreased.

Now, scientists from the University of California have found a unique way to show which neighborhoods’ air pollution returned to pre-pandemic levels after the lifting of COVID-19 restrictions. By analyzing wild grass samples, the researchers were able to figure out the levels of carbon dioxide emissions in the areas where the samples were taken.

During their study, the researchers found that the reduction in carbon dioxide levels on regional highways in 2020 was about 60 percent.

The detailed findings demonstrated that by 2021, the emissions in some areas of California had gone up again, while in other parts of the state — mostly the richer areas — they had not, according to a press release from the University of California (UC), Riverside.

“We think that these differences in rebound reflect how people returned to in-person work. As we all experienced, in more affluent areas, people were able to continue to work from home for longer,” Claudia Czimczik, professor in the Department of Earth System Science at UC Irvine, told EcoWatch in an email.

The study, “Reductions in California’s Urban Fossil Fuel CO2 Emissions During the COVID-19 Pandemic,” was published in the journal AGU Advances.

To obtain the wild grasses used in the study, the researchers accepted the help of citizen scientists.

“Community scientists sent us hundreds of wild grass samples. We analyzed them for their radiocarbon content, which is a proxy for fossil fuel emissions,” said assistant professor of climate change & sustainability at UC Riverside Francesca Hopkins, who was a co-author of the study, in the press release.

The study’s lead author Cindy Yañez, formerly of UC Riverside and now an Earth System Sciences doctoral student at UC Irvine, said plants keep a record of carbon emissions.

“Plants absorb CO2 during photosynthesis and incorporate it into their tissues, recording a snapshot of local fossil fuel inputs in the process,” Yañez said in the press release.

Radiocarbon is in every living thing but not in fossil fuels, which is why it works as a proxy for fossil fuel emissions.

“Radiocarbon is made naturally in the atmosphere, oxidized to CO2, and taken up by plants. All living things (that fix carbon via photosynthesis or eat plants or eat plant eaters) are naturally labeled with a known amount of radiocarbon,” Czimczik told EcoWatch. “We are all a tiny bit radioactive (this is not a health concern). However, radiocarbon has a relatively short half-life of about 5.5 thousand years. Fossil fuels are derived from plants that lived millions of years ago. All the radiocarbon these ancient plants contained when they were alive has decayed away by radioactive decay.”

The grass samples the researchers collected showed that emissions levels in locations along the coast in Orange County had kept to their pandemic lows, while levels in Pasadena and Los Angeles were higher, exceeding those of the state overall. The samples also revealed that post-pandemic emissions levels in San Francisco were lower than those in Los Angeles.

“We believe many of the differences we saw can be attributed to how many people are able to continue working remotely,” Hopkins said in the press release. “Other likely factors include the number of low-emission or electric vehicles in an area, and distance from industrial warehouses, with heavy big-rig traffic.”

When the study focused on particular San Francisco streets, the data revealed that the emissions on the Great Highway had remained low into 2021 because it hadn’t reopened to cars until the end of that year. In contrast, the amount of radiocarbon in the grass revealed that the carbon dioxide levels on 19th Avenue had gone way up because traffic had been redirected there.

For the researchers, wild grasses provided an easier way of measuring carbon levels than other methods.

“People traditionally measure radiocarbon in the air by collecting large, 2-liter flasks of air, which you then have to take to a lab and extract. It’s time consuming,” Hopkins said in the press release. “Grass already got the CO2 out of the air and did a lot of the work for us. It’s less expensive and lets us measure in new locations that weren’t previously possible.”

Urban tower networks or satellites can provide a low-cost way of measuring carbon in cities, but they don’t provide surveys on a neighborhood level. These detailed studies are necessary for developing policies to lower greenhouse gas emissions in the fight against climate change.

Cities could limit traffic on a specific road or shut off another emissions source and use analyses of the area’s grass to calculate its effect.

To measure the radiocarbon levels in the grass was a complex process that can be used for other solid materials, like charcoal.

“The grasses are washed, dried, and a subsample is combusted with an oxidizer (copper oxide) at 1000C in a sealed glass cuvette; all carbon in the sample becomes CO2, all nitrogen N2 etc, and minerals remain as ash. Then, we release the gasses into a vacuum and freeze out the CO2 to purify it. We can reduce the CO2 to solid graphite powder. The powder is pressed into a target and put into an accelerator mass spectrometer. This machine separates carbon isotopes (12C, 14C) in the sample and counts them,” Czimczik told EcoWatch. “The radiocarbon content is the ratio of 14C to 12C relative to a known standard. Samples that have small ratios contain more fossil fuel-derived carbon than samples that have large ratios. This method works for any organic solid, fresh grass, charcoal from the Bronze age etc.”

Any annual plant can be used to detect carbon levels in the atmosphere.

“Technically, any plant with an annual life cycle (plants without storage organs) can be used. The leaves of annual plants take up CO2 from the air and use this carbon to build new leaves, flowers etc.,” Czimczik said. “The grasses we targeted are invasive, easily identifiable, and found across the entire state of California. We did not want people to travel far from where they lived or accidentally collect native (and potentially threatened) plants. Plants are also easy/cheap to mail. We can process 10-20 plant samples in the lab per day.”

Czimczik said the researchers hope their method of quantifying an area’s CO2 levels will be expanded to other areas of California as well as other parts of the country.

“We think the method is powerful. We just secured funding to purchase another 14C-measuring machine that can handle more of these kinds of samples. We like to combine this data with other data streams, e.g.., continuous observations of CO2 from urban tower networks and satellites. We also think that this method is particularly interesting for cities that do not have this kind of infrastructure. We are hoping to start working in one of these cities in the coming year,” Czimczik told EcoWatch.

Czimczik said their research can help people know that what they’re doing to help fight global warming is having a positive effect.

“Mitigating climate change requires effort. We can help verify that the effort is working. We think people need to ‘see’ that they can make a difference to stay motivated,” Czimczik said.

Having the numbers can also let researchers know if the efforts being made to improve the air quality in certain communities are proving successful.

“As the world gets more serious about mitigating climate-warming CO2, we need metrics to let us know if we’re doing a good job of reducing it,” said Hopkins in the press release. “We also want to make sure communities most affected by emissions are getting the air quality benefits of reducing CO2.”