Incorporating 15N into the Multi-Resolution Emission Inventory to Simulate the Spatiotemporal Variations of δ15N in Emitted NOx over the Pearl River Delta Region, China

Nitrogen oxides (NOx), comprising nitric oxide (NO) and nitrogen dioxide (NO2), are key precursors of atmospheric nitrate, a major component of fine particulate matter (PM2.5) that critically affects air quality, human health, and ecosystems. Emission inventories provide detailed spatial and temporal information on NOx sources, while stable isotope techniques offer an additional constraint for source apportionment. Here, we incorporated stable nitrogen isotopes (14N, 15N) into the widely used Multi-resolution Emission Inventory for China (MEIC) over South China, with a focus on the Pearl River Delta (PRD) region, one of the most highly urbanized and industrialized regions in China, using an isotopic mass–balance model. The 2008 MEIC inventory indicated that NOx emissions across South China were spatially heterogeneous, dominated by transportation sources, and concentrated mainly in the PRD and other urban clusters. We then compared the simulated isotopic composition of emitted NOx with atmospheric measurements to assess the role of emission sources in controlling atmospheric nitrate (NO3−). The simulated δ15N(NOx) values were found to generally underestimate the observed δ15N(NO3−) values. This discrepancy highlights the need for future 15N-enabled air quality modeling to better represent both source contributions and atmospheric processing, thereby improving source apportionment, emission inventory evaluation, and our understanding of reactive nitrogen cycling.