Dramatic Lockdown Fossil Fuel CO₂ Decrease Detected by Citizen Science-Supported Atmospheric Radiocarbon Observations

Abstract

COVID-19 lockdowns resulted in dramatic changes to fossil fuel CO 2 emissions around the world, most prominently in the transportation sector. Yet travel restrictions also hampered observational data collection, making it difficult to evaluate emission changes as they occurred. To overcome this, we used a novel citizen science campaign to detect emission changes during lockdown and engage youth in climate science. Citizen scientists collected grass samples from their garden or local park, from which we analyzed the radiocarbon content to infer the recently added atmospheric fossil fuel CO 2 mole fraction at each sampling location. The local fossil fuel CO 2 mole fractions during lockdown were compared with a “normal” nonlockdown period. Our results from 17 sites in five cities around New Zealand demonstrate dramatic reductions in traffic emissions of 75 ± 3% during the most severe lockdown restriction period. This is consistent with sparse local traffic count information and a much larger decrease in traffic emissions than reported in global aggregate estimates of emission changes. Our results demonstrate that despite nationally consistent rules on travel during lockdown, emission changes varied by location, with inner-city sites typically dominated by bus traffic showing smaller decreases in emissions than elsewhere.

Reductions in California's Urban Fossil Fuel CO2 Emissions During the COVID‐19 Pandemic

“…The spatial sensitivity (“footprint”) of a plant is expected to be very localized (<100 m) but may vary for each sample depending on the local topography and air ventilation conditions. Previous work has shown that plants are predominantly influenced by emissions within 20–40 m (Lichtfouse et al., 2005 ; Turnbull et al., 2022 ). In contrast, atmospheric CO 2 measurements from rooftop/tower sites integrate signals over larger spatial scales (∼10 km) since the inlet is higher above the ground (Kort et al., 2013 ).…”

“…However, the localized spatial sensitivity of plants could be advantageous for studies seeking to investigate emissions at the neighborhood scale or from specific ffCO 2 sources (i.e., individual facilities or roads). Such analyses would require a strategic sampling design, targeting specific emission sources such as major roads (Turnbull et al., 2022 ). Without such targeted sampling, aggregated plant 14 C results in complex urban environments can be difficult to interpret since they represent highly local ffCO 2 emissions that may vary based on individual and immeasurable factors (i.e., human behaviors) within a neighborhood.…”

“…Because plant 14 C reflects the CO 2 assimilated from the atmosphere during photosynthesis, differences in 14 C depletion between plant samples are driven by local differences in ambient 14 CO 2 composition and particularly the amount of fossil fuel influence. Studies around the world have mapped ffCO 2 patterns using a variety of plant species appropriate for their study area including tree rings in LA (e.g., Djuricin et al., 2012 ), evergreen tree leaves in Italy (Alessio et al., 2002 ), corn leaves in the United States (Hsueh et al., 2007 ) and Beijing, China (Xi et al., 2011 ), annual grasses in California (Riley et al., 2008 ; Wang & Pataki, 2010 ), ipê leaves in Rio de Janeiro (Santos et al., 2019 ), turfgrasses in New Zealand (Turnbull et al., 2022 ), and wheat crops in India (Sharma et al., 2023 ). Thus, cities can apply this technique to quantify ffCO 2 patterns by sampling a commonly found plant species that is photosynthetically active during the time integration period of interest.…”

“…This means that more 14 C samples can be analyzed leading to higher spatial resolution urban ffCO 2 datasets than with air samples. During COVID‐19 lockdowns in New Zealand, the 14 C of weekly‐sampled grasses tracked changes in local ffCO 2 emissions that coincided with the stringency of COVID‐related restrictions and detected a 75% ± 3 peak reduction in ffCO 2 emissions (Turnbull et al., 2022 ).…”

Reductions in California's urban fossil fuel CO2 emissions during the COVID-19 pandemic

“…This means that more 14 C samples can be analyzed leading to higher spatial resolution urban ffCO 2 datasets than with air samples. During COVID-19 lockdowns in New Zealand, the 14 C of weekly-sampled grasses tracked changes in local ffCO 2 emissions that coincided with the stringency of COVID-related restrictions and detected a 75% ± 3 peak reduction in ffCO 2 emissions (Turnbull et al, 2022).…”