Declined Decrease of Tropospheric NO2 in the Past Decade

Ground and satellite observations show that air pollution regulations in the United States (US) and Europe have resulted in substantial reductions in emissions and corresponding improvements in air quality over the last several decades. However, large uncertainties remain in evaluating how recent regulations and natural sources affect the observed pollutant trends. Here we investigate the trends of tropospheric nitrogen dioxide (NO₂) over US and Europe in the period of 2005-2018 to explore the possible sources controlling tropospheric NO₂. We found that emission inventories overestimated reductions of anthropogenic nitrogen oxides (NO_x) emissions over both continents. The natural sources have important influences on the observed trends. This work reveals the large uncertainties in NO_x emission estimates, and highlights the importance to understand the natural sources affecting the distribution of this critical air pollution component.