DOI: 10.7280/d1f98g
title: Reductions in California's urban fossil fuel CO2 emissions during the COVID-19 pandemic

description: 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. 
description: On-road carbon dioxide (CO<sub>2</sub>) 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
 CO<sub>2</sub> 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 CO<sub>2</sub>
 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 CO<sub>2</sub>
 ("CO<sub>2xs</sub>") 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
 CO<sub>2</sub> 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.