Spatiotemporal Trends in Anthropogenic NOx Emissions in the Northern Hemisphere: Insights from Satellite Observations and Atmospheric Modeling

Nitrogen oxides (NO_x = NO + NO₂) are critical atmospheric pollutants with significant implications for human health and are key precursors of ozone and nitrate aerosols. Anthropogenic emissions primarily from sectors such as transportation, industry, and fossil fuel combustion are the main sources of NO_x. The temperate regions of the Northern Hemisphere, which host most industrialized countries and densely populated areas, account for 60~70% of global anthropogenic NO_x emissions. As the harmful effects of air pollution gain global attention, nations have implemented various clean energy policies to address these threats. Effective monitoring of anthropogenic NO_x emissions and control policies relies on accurate, long-term emission inventories. However, existing “bottom-up” inventories suffer from delays in data compilation, making it difficult to timely and accurately monitor the spatiotemporal variations of NO_x emissions.  This study presents an effective top-down inversion framework using TROPOMI satellite NO₂ observations combined with the GEOS-Chem atmospheric chemical transport model to assess recent NO_x emissions. The framework employs a mass balance principle and a two-step inversion approach, extending anthropogenic NO_x emissions in the Northern Hemisphere into 2022 and optimizing emissions from 2019 to 2022 for the temperate regions. Our results show a 1.68% decrease in NO_x emissions in 2020, followed by a 5.72% rebound in 2021. The recovery in China was faster than in other regions, surpassing 2019 levels by July 2020. In 2022, emissions declined across all regions, driven primarily by the Omicron variant, energy shortages, and clean energy policies. By integrating satellite observations, atmospheric modeling, and emission inversion techniques, our study contributes to the growing body of knowledge on how emissions evolve in response to global disruptions.