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

Citation

Yañez, Cindy et al. (2022), Reductions in California's urban fossil fuel CO2 emissions during the COVID-19 pandemic, Dryad, Dataset, https://doi.org/10.7280/D1F98G

Abstract

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. 

Methods

On-road carbon dioxide (CO2) data was collected using a cavity ringdown spectrometer (Picarro, Inc) installed inside a mobile laboratory. Freeways in the Los Angeles metropolitan area in California were surveyed using the mobile laboratory on July weekdays in 2019, 2020, and 2021. Alongside the measurements of ambient CO2 mixing ratios on the freeways, we also measured meteorological variables using a compact weather sensor (METSENS500 Campbell Scientific, Inc) and position data using a global satellite positioning device (GPS 16X, Garmin, Ltd.). The CO2 data was calibrated using a two-point linear correction based on measurements of known mixing ratios before and after each day of data collection. Data was synchronized into five-second intervals and gridded into 100m road intervals. Enhancements of CO2 ("CO2xs") were calculated by subtracting a background from all measurements, which we characterized using flask sample data from NOAA's Global Monitoring Division's site in Cape Kumukahi, Hawaii (Dlugokencky et al., 2021). We filtered the data to only include measurements collected on freeways that spatially overlapped with the 2020 dataset. We also only include data collected during daytime hours (11AM - 4PM, local time).

Plants for radiocarbon analysis were sampled by community scientists across the state of California in 2020 and 2021. Community scientists mailed annual grasses in paper envelopes with the sample location and collection date. To prepare the samples for radiocarbon analysis, we washed them and weighed out approximately 4 mg of the latest growing biomass. Then we sealed the samples into quartz tubes with cuprous oxide. The samples were then evacuated and combusted for three hours at 900°C. The resulting CO2 was purified cryogenically and converted to graphite using a sealed tube zinc reduction method (Xu et al., 2007). The graphite was then analyzed for radiocarbon at the W. M. Keck Carbon Cycle Accelerator Mass Spectrometer facility at the University of California, Irvine. 

Funding

National Science Foundation, Award: DGE-1839285