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

Fossil fuel carbon dioxide emissions (ffCO2) constitute the majority of
greenhouse gas emissions and are the main determinent of global climate
change. The COVID-19 pandemic caused wide-scale disruption to human
activity and provided an opportunity to evaluate our capability to detect
ffCO2 emission reductions. Quantifying changes in ffCO2 levels is
especially challenging in cities, where climate mitigation policies are
being implemented but local emissions lead to spatially and temporally
complex atmospheric mixing ratios. Here, we used direct observations of
on-road CO2 mixing ratios with analyses of the radiocarbon (14C) content
of annual grasses collected by community scientists in Los Angeles and
California, USA to assess reductions in ffCO2 emissions during the first
two years of the COVID-19 pandemic. With COVID-19 mobility restrictions in
place in 2020, we observed a significant reduction in ffCO2 levels across
California, especially in urban centers. In Los Angeles, CO2 enhancements
on freeways were 60 ± 16% lower and ffCO2 levels were 38-52% lower than in
pre-pandemic years. By 2021, California's ffCO2 levels rebounded to
pre-pandemic levels, albeit with substantial spatial heterogeneity related
to local and regional pandemic measures. Taken together, our results
indicate that a reduction in traffic emissions by ~60% (or 10-24% of Los
Angeles' total ffCO2 emissions) can be robustly detected by plant 14C
analysis, and pave the way for mobile- and plant-based monitoring of ffCO2
emissions in cities without CO2 monitoring infrastructure such as those in
the Global South.