Fig. 1. Substantial anthropogenic and soil NO_x emissions lead to high NO₂ levels over the North China Plain (NCP).

Panels a and b show the anthropogenic NO_x emissions in July 2017 from the Multi-resolution Emission Inventory for China (MEIC) and the soil NO_x emissions calculated from the Berkeley-Dalhousie Soil NO_x Parameterization (BDSNP) implemented GEOS-Chem, respectively. The thick blue lines outline the NCP region. The total emissions in the NCP are shown in the inset. Panel c shows the fraction of soil NO_x emissions from fertilizer application to the total soil NO_x emissions. Panel d compares the BDSNP soil NO_x emissions to nine field measurements across China (locations given in the Panel b and Supplementary Table 2, with the correlation coefficient and mean bias shown in the inset. Panel e compares the GEOS-Chem simulated tropospheric NO₂ columns over the NCP with averaging kernel applied to the POMINO, DOMINO, and QA4ECV satellite products (Methods). The colored box-and-whisker plots (5th, 25th, 50th, 75th, and 95th percentiles, and mean values denoted as dots) represent NO₂ columns from the observation, GEOS-Chem BASE simulation, and a sensitivity model simulation with soil NO_x emissions excluded (NoSoil). The comparisons are grouped for the high anthropogenic NO_x emission model grids (defined as grids with the 20% anthropogenic/soil NO_x emission ratio greater than 2, accounting for 20% of the NCP grids), and high soil NO_x emission model grids (defined as grids with the 20% anthropogenic/soil NO_x emission ratio smaller than 0.5, accounting for 30% of the NCP grids) (Supplementary Fig. 1c). We use the emission ratio of 20% as the criteria here as the July soil NO_x emissions in the NCP are about 20% of the anthropogenic NO_x emissions (a, b). Supplementary Figure 3 compares the spatial distributions.