Rapidly declining NOx emissions from human activities in China since 2020

Nitrogen oxides (NO_x) form aerosols and ozone in the atmosphere, and are significant contributors to air pollution and climate change. China is the largest emitter, yet accurate and timely information on NO_x emissions in China has been lacking since 2020 due to delays in emissions reporting.

In a study published in Environmental Science and Ecotechnology, researchers developed a satellite-based inversion system for real-time monitoring of regional air pollution emissions. Using data from this system and other independent observations, they show a consistent decline in China’s NO_x emissions from 2020-2022, despite increased fossil fuel consumption.

A reduction in transportation-associated emissions, largely due to the COVID-19 lockdowns, slightly lowered China’s NO_x emissions in 2020. In 2021 and 2022, reductions in NO_x emissions were achieved within the industrial and transportation sectors through stringent air pollution controls. These controls appear to have accounted for more than 70% of the total reduction.

This reduction was corroborated by data from two independent spaceborne instruments — the TROPOspheric Monitoring Instrument (TROPOMI) and the Ozone Monitoring Instrument (OMI). TROPOMI is on board the European Copernicus Sentinel-5 Precursor (S5P), which launched in 2017 and is the most widely used satellite instrument monitoring NO₂ pollution because it offers global daily NO₂ tropospheric vertical column densities (TVCDs) sampled at 13:30 local time with a current resolution of up to 5.5 × 3.5 km. OMI is on the National Aeronautics and Space Administration (NASA)’s EOS-Aura spacecraft, providing global daily NO₂ TVCDs sampled at 13:40 local time with a resolution of 13 × 24 km.

Monthly total NO_x emissions between 2020 and 2022 in China were inferred from satellite retrievals of NO₂ TVCDs using the mass balance method, assuming a localized relation between the changes in NO₂ TVCDs and NO_x emissions.

Despite a decrease in NO_x emissions, the research team did not observe a concurrent decline in China’s CO₂ emissions due to the increased fossil fuel consumption, which suggests a difficulty in achieving coordinated governance of air quality and climate pollutants under the current energy structure.

The authors conclude that a satellite-based inversion system like the one presented in this study could be a crucial part of such a coordinated approach, enabling tracking air pollutant emissions by sector with low latency.