Effect of emission variability on concentration fluctuations in idealised deep urban street canyons

The level of air pollution in a street canyon depends on the non-trivial interplay between vehicular exhaust emissions, atmospheric transport, and physico-chemical transformation of pollutants. In this study, we investigate the temporal dynamics of air pollutant concentration in a deep street canyon orthogonal to the wind direction using a two-box model. Simulations provide insights into both steady-state mean concentrations and the magnitude of concentration fluctuations as pollutant nature (inert or reactive), emission signal stochasticity, and the fundamental rates of turbulent transport and chemical transformations vary. Beyond elucidating the role of different parameters on mean pollution levels, the results reveal that extreme air pollutant concentrations are more likely to occur when the characteristic time scale of vehicular exhaust emissions significantly exceeds the ventilation time scale of the street canyon. The innovative modelling approach paves the way for various applications, particularly in the field of optimal traffic management and the study of citizens' exposure to air pollution.