Improved Modelling Of Soil No (X) Emissions In A High Temperature Agricultural Region: Role Of Background Emissions On No2 Trend Over The Us

EPA reports a steady decline of US anthropogenic NO (x) emissions in 2005-2019 summers, while NO2 vertical column densities (VCDs) from the OMI satellite over large spatial domains have flattened since 2009. To better understand the contributing factors to a flattening of the OMI NO2 trends, we investigate the role of soil and lightning NOx emissions on this apparent disagreement. We improve soil NO (x) emissions estimates using a new observation-based temperature response, which increases the linear correlation coefficient between GEOS-Chem simulated and OMI NO2 VCDs by 0.05-0.2 over the Central US. Multivariate trend analysis reveals that soil and lightning NO (x) combined emissions trends change from -3.95% a(-1) during 2005-2009 to 0.60% a(-1) from 2009 to 2019, thereby rendering the abrupt slowdown of total NO (x) emissions reduction. Non-linear inter-annual variations explain 6.6% of the variance of total NO (x) emissions. As background emissions become relatively larger with uncertain inter-annual variations, the NO2 VCDs alone at the national scale, especially in the regions with vast rural areas, will be insufficient to discern the trend of anthropogenic emissions.