In this seminar I will discuss the effect that trees have on air flow and pollutant transport. The interaction of trees with their environment is extremely complex: trees provide shading, absorb and emit radiation, evaporate water, cause drag and absorb/deposit pollutants. All these processes are incorporated in to a minimalistic tree model. Analysis of the tree model reveals the parameter space that map the cooling regimes of the tree. The model is then incorporated into the urban large-eddy simulation code uDALES, and the effect of trees on urban air quality is investigated. It is shown that trees can be both beneficial and detrimental to urban air quality, depending on how much they block the interaction between the street and the atmosphere.