Articles | Volume 10, issue 9
Model evaluation paper
19 Sep 2017
Model evaluation paper |  | 19 Sep 2017

Evaluation of high-resolution GRAMM–GRAL (v15.12/v14.8) NOx simulations over the city of Zürich, Switzerland

Antoine Berchet, Katrin Zink, Dietmar Oettl, Jürg Brunner, Lukas Emmenegger, and Dominik Brunner

Abstract. Hourly NOx concentrations were simulated for the city of Zürich, Switzerland, at 10 m resolution for the years 2013–2014. The simulations were generated with the nested mesoscale meteorology and micro-scale dispersion model system GRAMM–GRAL (versions v15.12 and v14.8) by applying a catalogue-based approach. This approach was specifically designed to enable long-term city-wide building-resolving simulations with affordable computation costs. It relies on a discrete set of possible weather situations and corresponding steady-state flow and dispersion patterns that are pre-computed and then matched hourly with actual meteorological observations. The modelling system was comprehensively evaluated using eight sites continuously monitoring NOx concentrations and 65 passive samplers measuring NO2 concentrations on a 2-weekly basis all over the city. The system was demonstrated to fulfil the European Commission standards for air pollution modelling at nearly all sites. The average spatial distribution was very well represented, despite a general tendency to overestimate the observed concentrations, possibly due to a crude representation of traffic-induced turbulence and to underestimated dispersion in the vicinity of buildings. The temporal variability of concentrations explained by varying emissions and weather situations was accurately reproduced on different timescales. The seasonal cycle of concentrations, mostly driven by stronger vertical dispersion in summer than in winter, was very well captured in the 2-year simulation period. Short-term events, such as episodes of particularly high and low concentrations, were detected in most cases by the system, although some unrealistic pollution peaks were occasionally generated, pointing at some limitations of the steady-state approximation. The different patterns of the diurnal cycle of concentrations observed in the city were generally well captured as well. The evaluation confirmed the adequacy of the catalogue-based approach in the context of city-scale air pollution modelling. The ability to reproduce not only the spatial gradients but also the hourly temporal variability over multiple years makes the model system particularly suitable for investigating individualized air pollution exposure in the city.

Short summary
We evaluate a new cost-effective method to simulate pollutant dispersion at high resolution on a city-wide domain. The method is based on a catalogue of reference simulations matched to weather observations to produce a sequence of hourly pollution maps. A total of 2 years of simulations are compared with continuous measurements and passive NO2 samplers in the city of Zurich. Spatial and temporal variability proved to be very well reproduced by the method.