Optimisation of a numerical model to simulate the dispersion and chemical transformations within the oxides of nitrogen/ozone system as traffic pollution enters an urban greenspace

Urban greenspace has many health benefits, including cleaner air than the surrounding streets. In this study, a detailed exercise has been conducted to measure concentrations of NO/NO₂/NO_x and O₃ within an urban greenspace, the University of Birmingham campus, using continuous analysers, as well as transects of NO₂ measured with diffusion tubes. Concentrations have been simulated using the ADMS-Roads model which has been optimised initially using NO_x concentrations for traffic emissions on surrounding roads, background concentrations, and meteorological data considering four candidate sites. Optimisation for prediction of NO₂ shows the critical importance of the NO₂:NO_x ratio in traffic emissions, for which a derivation from atmospheric measurements is consistent with a value derived from optimisation of the model fit to roadside data. After optimisation, the model gives an excellent fit to continuous data measured at roadside. Comparison of model predictions with transects of NO₂ across the greenspace also show generally good model performance. The incorporation of dry deposition processes for the nitrogen oxides into the model leads to a reduction of less than 1% in predicted concentrations, leading to the conclusion that the cleaner air within urban greenspace is primarily the result of dispersion rather than deposition processes.