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. This paper describes the CityChem extension of the Eulerian urban dispersion model EPISODE.The development of the CityChem extension was driven by the need to apply the model inlargely populated urban areas with highly complex pollution sources of particulate matterand various gaseous pollutants.The CityChem extension offers a more advanced treatment of the photochemistry in urban areas andentails specific developments within the sub-grid components for a more accuraterepresentation of dispersion in proximity to urban emission sources.Photochemistry on the Eulerian grid is computed using a numerical chemistry solver. Photochemistry in the sub-grid components is solved with a compact reaction scheme,replacing the photo-stationary-state assumption.The simplified street canyonmodel (SSCM) is used in the line source sub-grid model to calculate pollutant dispersionin street canyons.The WMPP (WORM Meteorological Pre-Processor) is used in the point source sub-grid modelto calculate the wind speed at plume height.The EPISODE–CityChem model integrates the CityChem extension in EPISODE, with thecapability of simulating the photochemistry and dispersion of multiple reactive pollutantswithin urban areas.The main focus of the model is the simulation of the complex atmosphericchemistry involved in the photochemical production of ozone in urban areas.The ability of EPISODE–CityChem to reproduce the temporal variation of majorregulated pollutants at air quality monitoring stations in Hamburg, Germany, wascompared to that of the standard EPISODE model and the TAPM (The Air Pollution Model) air quality model usingidentical meteorological fields and emissions. EPISODE–CityChem performs betterthan EPISODE and TAPM for the prediction of hourly NO2 concentrations at the trafficstations, which is attributable to the street canyon model.Observed levels of annual mean ozone at the five urban background stations in Hamburgare captured by the model within ±15  %.A performance analysis withthe FAIRMODE DELTA tool for air quality in Hamburg showed that EPISODE–CityChem fulfils themodel performance objectives for NO2 (hourly), O3 (daily max. of the 8 h running mean) and PM 10 (daily mean) set forth inthe Air Quality Directive, qualifying the model for use in policy applications.Envisaged applications of theEPISODE–CityChem model are urban air quality studies, emission control scenarios inrelation to traffic restrictions and the source attribution of sector-specific emissionsto observed levels of air pollutants at urban monitoring stations.

geoscientific model development

The Eulerian urban dispersion model EPISODE – Part 2: Extensions to the source dispersion and photochemistry for EPISODE–CityChem v1.2 and its application to the city of Hamburg