Characterising urban pollutant ventilation with the tracer age and age spectrum

The mean tracer age and age spectrum are standard tools for the analysis of stratospheric transport and mixing. They characterise the time elapsed between the emission of a chemical constituent and its arrival in the stratosphere. They can also be applied to the ventilation of urban pollutants. By contrast with standard ventilation diagnostics, they require no a priori assumptions. Using large-eddy simulations of street-canyon flows, it is shown that the mean tracer age is significantly smaller than the local mean age obtained by assuming a spatially homogeneous source. The age spectrum (or distribution of tracer ages) is rather broad and shows a complicated dependence on the canyon geometry. Applications to time-dependent inflow forcing will be described.