Pollutant concentrations within deep urban canopies are affected by regional and local sources. Little is known, however, about how the balance between them changes with height. In this study we use roadside measurements of particulates and numerical simulations with a high-resolution mesoscale model (WRF) and a building-resolving large-eddy simulation model (PALM) to determine the relative importance of regional and local sources. It is found that the crossover height  depends sensitively on the building geometry. Implications for urban design and the location of fresh air outlets are discussed.

This work was supported by the Environment and Conservation Fund and the Woo Wheelock Green Fund.