Large-eddy simulation of roadside dispersion and chemical transformation of nitrogen oxides in an urban environment

The objective of this work is to present a high-resolution, urban scale large-eddy simulation (LES) frameworkfor modelling dispersion and chemical transformation of nitrogen oxides (NOx=NO + NO2) from urban traffic. The control-volume based computational fluid dynamics (CFD) simulation platform OpenFOAM has been adopted for LES transport and reactional modelling. A rudimentary chemical kinetic mechanism for NOx and tropospheric ozone (O3) has been introduced as a non-limiting implementation. All urban structures are fully resolved in the solution domain, with dynamic grid refinement allowing local spatial resolutions to reach 10 m and below. A k-way Boolean operation and a reduced convolution Minkowski sum algorithm areused to automate the conversion of available geographical data, such as two-dimensional urban footprints and object heights, into three-dimensional geometrical entities. Said methodology is applied to urban regions in the City of Berlin of up to 2 km × 2km around active roadside air quality measurement stations from the BLUME(Berliner Luftgütemessnetz) network. Particular emphasis is placed on the period in July 2014, where emission observations from measurement campaign and WRF regional model simulations are available.