An Integrative Framework for Quantifying, Modelling, and Scenario-Testing Reactive Nitrogen Emissions for Air Quality and Climate Co-Benefits: A Case Study from Ondo State, South-West Nigeria

Reactive nitrogen (Nr) emissions from agriculture and fossil fuel combustion drive air pollution (PM₂.₅) and climate change (N₂O), yet no integrated framework exists for managing these interconnected impacts in Nigeria. This study developed and validated an integrative mathematical framework to quantify Nr sources, simulate atmospheric chemistry and climate impacts, and evaluate intervention scenarios for co-benefits in Ondo State, South-West Nigeria. A spatially explicit emissions inventory was compiled using primary surveys (187 farmers, 300 households), traffic counts, and online databases (EDGAR, FAO CountrySTAT, OpenAQ). A dynamic compartmental model (Python, SciPy) simulated NH₃, NOₓ, and N₂O fluxes, secondary PM₂.₅ formation (ISORROPIA II), and N₂O radiative forcing (IPCC GWP100). Model validation against six months of PM₂.₅ sensor data yielded Nash-Sutcliffe efficiency values of 0.68–0.79. Synthetic fertilizers dominated Nr emissions (52.7%), followed by livestock (21.0%), road transport (20.9%), and residential generators (15.1%). The model revealed that 37% of urban PM₂.₅ originates from agricultural NH₃ transported from rural areas. Calibrated N₂O emission factor (0.0087) was 13% lower than IPCC default. Scenario analysis showed that combined agricultural interventions (enhanced-efficiency fertilizers + precision application) reduce PM₂.₅ by 32.7% and N₂O by 44.0%; adding combustion reductions achieves 58.1% PM₂.₅ reduction but no additional climate benefit. No trade-offs were identified. Integrated nitrogen management in Nigeria presents a "win-win" opportunity for air quality and climate action. A decision-support dashboard and STEM education toolkit were developed to translate findings for policymakers and communities.