Geosci. Model Dev., 12, 3357–3399, 2019 https://doi.org/10.5194/gmd-12-3357-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. 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 Matthias Karl1, Sam-Erik Walker2, Sverre Solberg2, and Martin O. P. Ramacher1 1Chemistry Transport Modelling, Helmholtz-Zentrum Geesthacht, Geesthacht, Germany 2Norwegian Institute for Air Research (NILU), Kjeller, Norway Correspondence: Matthias Karl ([email protected]) Received: 14 December 2018 – Discussion started: 11 January 2019 Revised: 17 May 2019 – Accepted: 23 June 2019 – Published: 1 August 2019 Abstract. This paper describes the CityChem extension of teorological fields and emissions. EPISODE–CityChem per- the Eulerian urban dispersion model EPISODE. The devel- forms better than EPISODE and TAPM for the prediction of opment of the CityChem extension was driven by the need hourly NO2 concentrations at the traffic stations, which is to apply the model in largely populated urban areas with attributable to the street canyon model. Observed levels of highly complex pollution sources of particulate matter and annual mean ozone at the five urban background stations in various gaseous pollutants. The CityChem extension offers Hamburg are captured by the model within ±15 %. A per- a more advanced treatment of the photochemistry in urban formance analysis with the FAIRMODE DELTA tool for air areas and entails specific developments within the sub-grid quality in Hamburg showed that EPISODE–CityChem ful- components for a more accurate representation of disper- fils the model performance objectives for NO2 (hourly), O3 sion in proximity to urban emission sources. Photochemistry (daily max. of the 8 h running mean) and PM10 (daily mean) on the Eulerian grid is computed using a numerical chem- set forth in the Air Quality Directive, qualifying the model istry solver. Photochemistry in the sub-grid components is for use in policy applications. Envisaged applications of the solved with a compact reaction scheme, replacing the photo- EPISODE–CityChem model are urban air quality studies, stationary-state assumption. The simplified street canyon emission control scenarios in relation to traffic restrictions model (SSCM) is used in the line source sub-grid model to and the source attribution of sector-specific emissions to ob- calculate pollutant dispersion in street canyons. The WMPP served levels of air pollutants at urban monitoring stations. (WORM Meteorological Pre-Processor) is used in the point source sub-grid model to calculate the wind speed at plume height. The EPISODE–CityChem model integrates the City- Chem extension in EPISODE, with the capability of simu- 1 Introduction lating the photochemistry and dispersion of multiple reactive pollutants within urban areas. The main focus of the model Air quality (AQ) modelling plays an important role by as- is the simulation of the complex atmospheric chemistry in- sessing the air pollution situation in urban areas and by sup- volved in the photochemical production of ozone in urban porting the development of guidelines for efficient air qual- areas. The ability of EPISODE–CityChem to reproduce the ity planning, as highlighted in the current Air Quality Direc- temporal variation of major regulated pollutants at air qual- tive (AQD) of the European Commission (EC, 2008). The ity monitoring stations in Hamburg, Germany, was compared main air pollution issues in European cities are the human to that of the standard EPISODE model and the TAPM (The health impacts of exposure to particulate matter (PM), ni- Air Pollution Model) air quality model using identical me- trogen dioxide (NO2) and ozone (O3), while the effects of Published by Copernicus Publications on behalf of the European Geosciences Union. 3358 M. Karl et al.: EPISODE – Part 2: The CityChem extension air pollution due to sulfur dioxide (SO2), carbon monox- dispersion and local photochemistry. In particular, the model ide (CO), lead (Pb) and benzene have been reduced dur- allows the user to retrieve concentrations at the sub-grid ing the last 2 decades due to emission abatement measures. scale in specified locations of the urban area. Moreover, the Tropospheric (ground-level) ozone is a secondary pollutant EPISODE model is an integral part of the operational Air generated in in photochemical reaction cycles involving two Quality Information System AirQUIS 2006 (Slørdal et al., classes of precursor compounds, i.e. nitrogen oxides and 2008). volatile organic compounds (VOCs), initiated by the reac- Part one (Hamer et al., 2019) of this two-part article series tion of the hydroxyl (OH) radical with organic molecules. provides a detailed description of the EPISODE model sys- For health protection, a maximum daily 8 h mean threshold tem, including the physical processes for atmospheric pollu- for ozone (120 µgm−3) is specified as a target value in the tant transport, the photo-stationary-state (PSS) approxima- European Union, which should not be exceeded at any AQ tion, the involvement of nitric oxide (NO), NO2 and O3, monitoring station on more than 25 d yr−1. However, about sub-grid components, and the interaction between the Eule- 15 % of the population living in urban areas is exposed to rian grid and the sub-grid processing of pollutant concentra- ozone concentrations above the European Union (EU) target tions. Part one examines the application of EPISODE to air value (EEA, 2015). Traffic is a major source of nitrogen ox- quality scenarios in the Nordic winter setting. During win- ides (NOx D NO2 C NO) and highly contributes to the pop- tertime in northern Europe, the PSS assumption is a rather ulation exposure to ambient NO2 concentrations in urban ar- good approximation of the photochemical conversion occur- eas because these emissions occur close to the ground and are ring close to the emission sources. However, when the so- distributed across densely populated areas. Urban emissions lar ultraviolet (UV) radiation is stronger, in particular dur- of ozone precursors are transported by local and/or regional ing summer months or at more southerly locations, net ozone air mass flows towards suburban and rural areas, which can formation may take place in urban areas at a certain distance be impacted by O3 pollution episodes (Querol et al., 2016). from the main local emission sources (Baklanov et al., 2007). Eulerian chemistry-transport model (CTM) systems using EPISODE in its routine application does not allow for the numerical methods for solving photochemistry (including treatment of photochemistry involving VOCs and other reac- chemical reaction schemes with varying degrees of detail) tive gases leading to the photochemical formation of ozone. have mainly been used for regional-scale air quality studies. In this part, the features of the CityChem extension for Recent nested model approaches using regional CTM sys- treating the complex atmospheric chemistry in urban areas tems have been applied to capture pollution processes from and specific developments within the sub-grid components the continental scale to the local scale using between 1 and for a more accurate representation of near-field dispersion in 5 km resolution and a temporal resolution of 1 h for the inner- proximity to urban emission sources are described. Atmo- most domain (e.g. Borge et al., 2014; Karl et al., 2015; Pe- spheric chemistry on an urban scale is complex due to the tetin et al., 2015; Valverde et al., 2016). Regional AQ models large spatial variations of input from anthropogenic emis- can give a reliable representation of O3 concentrations in the sions. VOCs related to emissions from traffic are involved in urban background, but due to their limitation in resolving the chemical conversion in urban areas. Therefore, it has become near-field dispersion of emission sources and photochemistry necessary to simulate a large number of chemical interactions at the sub-kilometre scale, i.e. in street canyons, around in- involving NOx,O3, VOCs, SO2 and secondary pollutants. In dustrial stacks and on the neighbourhood level, they cannot order to use comprehensive photochemical schemes in urban provide the information needed by urban policymakers for AQ models involving VOC interactions, the highest priority population exposure mapping, city planning and the assess- for the initial development was to reduce the number of com- ment of abatement measures. pounds and reactions to a minimum, while maintaining the Urban-scale AQ models overcome the limitation inherent essential and most important aspects of chemical reactions in regional-scale models by taking into account details of the taking place in the urban atmosphere on the relevant space urban topography, wind flow field characteristics, land use scales and timescales. information and the geometry of local pollution sources. The CityChem offers a more advanced treatment for the pho- urban AQ model EPISODE developed at the Norwegian In- tochemistry of multiple gaseous pollutants on the Eulerian stitute for Air Research (NILU) is a 3-D Eulerian grid model grid, as well as for dispersion close to point emission sources that operates as a CTM, offline coupled with a numerical (e.g. industrial stacks) and line emission sources (open roads weather prediction (NWP) model. EPISODE is typically ap- and streets). plied with a horizontal resolution of 1 × 1 km2 over an en- tire city with domains of up to 2500 km2 in size. The Eule- 1. Photochemistry on the Eulerian grid uses a numerical rian grid component of EPISODE simulates advection, verti- chemistry solver. The available chemistry schemes in- cal and/or horizontal diffusion, background transport across clude (1) EMEP45 (Walker et al., 2003), which resulted the model domain boundaries, and photochemistry. Several from an appropriate reduction of the former EMEP (Eu- sub-grid-scale modules are embedded in EPISODE to rep- ropean Monitoring and Evaluation Programme) chem- resent emissions (line source and point sources), Gaussian istry scheme (Simpson, 1995); (2) EmChem03-mod, Geosci.