Decadal Variabilities in Tropospheric Nitrogen Oxides Over United States, Europe, and China

Global trends in tropospheric nitrogen dioxide (NO₂) have changed dramatically in the past decade. Here, we investigate tropospheric NO₂ variabilities over United States, Europe, and E. China in 2005–2018 to explore the mechanisms governing the variation of this critical pollutant. We found large uncertainties in the trends of anthropogenic nitrogen oxides (NO_x) emissions, for example, the reductions of NO_x emissions, derived with different approaches and data sets, are in the range of 35%–50% over the United States and 15%–45% over Europe in 2005–2018. By contrast, the analysis in this work indicates declines of anthropogenic NO_x emissions by about 40% and 25% over the United States and Europe, respectively, in 2005–2018, and about 20% over E. China in 2012–2018. However, the shift of major NO_x sources from power generation to industrial and transportation sectors has led to noticeable diminishing effects in emission controls. Furthermore, satellite measurements exhibit the influence of NO₂ background levels over the United States and Europe, which offset the impacts of anthropogenic emission declines, resulting in flatter trends of tropospheric NO₂ over the United States and Europe. Our analysis further reveals underestimation of background NO₂ by chemical transport models, which can lead to inaccurate interpretations of satellite measurements. We use surface in-situ NO₂ observations to diagnose the satellite-observed NO₂ trends and find top-down NO_x emissions over urban grids represent the changes in anthropogenic NO_x emissions better. This work highlights the importance of comprehensive applications of different analysis approaches to better characterizing atmospheric composition evolution.