Your lawn can identify the traces of vehicle pollution

University of California scientists have a new way to demonstrate which neighborhoods returned to pre-pandemic levels of air pollution after COVID restrictions ended. It relies on plants absorbing carbon dioxide, and not all carbon dioxide being the same.

Carbon-14 is created in the atmosphere when nitrogen is struck by cosmic rays. It’s an unstable isotope, so over time it decays. The carbon in oil has had so much time all the carbon-14 has disappeared, so carbon dioxide created from burning oil will be unusually low in carbon-14 compared to natural carbon dioxide.

“Plants absorb CO2 during photosynthesis and incorporate it into their tissues, recording a snapshot of local fossil fuel inputs in the process,” said study lead Cindy Yañez, formerly of UCR, now an Earth System Sciences doctoral student at UC Irvine. 

The study published in AGU Advances using a mobile laboratory shows the CO2 drop was roughly 60%. 

By analyzing grass samples from across the state, the same study also showed in fine detail that some parts of California were back to high levels of emissions by 2021, while others — generally in more affluent areas — were not. 

“Community scientists sent us hundreds of wild grass samples. We analyzed them for radiocarbon content, which is a proxy for fossil fuel emissions,” said Francesca Hopkins, UC Riverside assistant professor of climate change and study co-author.

The team’s grass samples revealed that coastal Orange County retained their pandemic-related reduction in emissions, and that San Francisco has fared better than Los Angeles. Both the Los Angeles metro area and Pasadena had bigger pollution rebounds than the coast, or than the state as a whole. 

“We believe many of the differences we saw can be attributed to how many people are able to continue working remotely,” Hopkins said. “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.”

Our complementary approaches captured the heterogeneous reality of mandated and voluntary movement restrictions in California during the pandemic. Our study focused on a region rich in high quality datasets (i.e., previous ¹⁴C records, a neighborhood scale bottom-up inventory, and an in-situ tower network) which allowed us to assess ffCO₂ emission reductions in the context of long-term trends. Mobile surveys can detect year-to-year differences in ffCO₂ trends from the on-road sector with high confidence, but further work is needed to relate on-road CO₂ enhancements to vehicle emissions and their drivers.

Yañez et al. 2022.

📰 Full press release at Eurekalert.
🔬 Reductions in California’s Urban Fossil Fuel CO₂ Emissions During the COVID-19 Pandemic at AGU Advances.