Spatiotemporal variations of atmospheric nitrogen deposition in China during 2008-2020

In recent decades, China has been a hotspot for reactive nitrogen (N) deposition due to intensive fossil fuel burning and increased agricultural activities. The Chinese government has implemented active measures to protect air quality and reduce N deposition. In this study, we combined monthly measurements of monitoring sites and satellite data to construct random forest (RF) models for the estimation of monthly N deposition in China during 2008-2020. This provides N deposition estimation in latest years and has a finer temporal resolution. The RF models can address more complicated relationships between variables and performed well in estimating N deposition compared to the measurements with average correlation coefficients of 0.79 and 0.77 for dry and wet N deposition, respectively. We found that mean annual total N deposition was 22.0 +/- 0.8 kg N ha(-1) yr(-1) during 2008-2020, which accounted for 64% (+/- 4%) of reactive N emissions in China. Although the total N deposition was high, it has stabilized and decreased slightly over the past decade, with dry and wet N deposition contributing equally to the total N deposition. NHx deposition (F-NHx) still slowly increased (0.09 kg N ha(-1) yr(-2)) during 2015-2020, when N fertilizer application and livestock started to decrease. This is due to the rapid decline in NOx and SO2 emissions during the same period. Our results provide a new perspective on the spatiotemporal variations in the total N deposition in China. It is necessary to optimize other emission reduction strategies to mitigate air pollution and N deposition.