The U.S. power sector emissions of CO2 and NOx during 2020: Separating the impact of the COVID-19 lockdowns from the weather and decreasing coal in fuel-mix profile

In recent years, the United States power sector emissions of CO2 and NOx have decreased due to declining coal and increasing natural gas and renewables in the fuel-mix. In April 2020, the COVID-19 social restrictions in the United States led to a decline in electricity demand from the commercial and industrial sectors. In this study, we estimate the changes in the emissions of CO2 and NOx from the U.S. power sector due to three factors: 1) weather, 2) the fuel-mix change in the past five years, and 3) the COVID-19 social restrictions. We use a multivariate adaptive regression splines (MARS) model to separate the impacts of outdoor temperature and type-of-day from the COVID-19 on power generation, and the daily operation status of 3013 power units to account for the fuel-mix change. We find that electricity demand changes due to COVID occurred mostly from March to June 2020, with electricity demand generally returning to 2015–2019 levels starting in July 2020. We find the U.S. power sector CO2 emissions, reported by EPA, dropped by 29.8 MTCO2 (−26%) in April 2020, relative to the average April emissions between 2015 and 2019. Of that reduction, we attribute declines of 18.3 ± 4.0 MTCO2 (−18 ± 4%) to the COVID-19 lockdowns, declines of 13.7 ± 4.2 MTCO2 (−12 ± 4%) to a fuel-mix change, and increases of 2.3 ± 1.1 MTCO2 (+2 ± 1%) due to weather variability compared to the five prior years. For the same month, the power sector NOx emissions dropped by 27.6 thousand metric tons (−42%) in April 2020, relative to the past five-year monthly average. Of that reduction, we attribute declines of 10.5 ± 2.4 thousand metric tons (−22 ± 5%) to the COVID-19 lockdowns, declines of 18.5 ± 2.5 thousand metric tons (−28 ± 4%) to a fuel-mix change, and increases of 1.4 ± 0.6 thousand metric tons (+2 ± 1%) due to weather variability. This result highlights the importance of accounting for weather and fuel-mix changes when estimating the impact of COVID-19 on the power sector emissions.