Development and evaluation of GRAL-C dispersion model, a hybrid Eulerian–Lagrangian approach capturing NO–NO₂–O₃ chemistry

Based on two recent publications using Lagrangian dispersion models to simulate NO–NO₂–O₃ chemistry for industrial plumes, a similar modified approach was implemented using GRAL-C (Graz Lagrangian Model with Chemistry) and tested on two urban applications. In the hybrid dispersion model GRAL-C, the transport and turbulent diffusion of primary species such as NO and NO₂ are treated in a Lagrangian framework while those of O₃ are treated in an Eulerian framework. GRAL-C was employed on a one year street canyon simulation in Berlin and on a four-day simulation during a winter season in Graz, the second biggest city in Austria. In contrast to Middleton D.R., Jones A.R., Redington A.L., Thomson D.J., Sokhi R.S., Luhana L., Fisher B.E.A. (2008. Lagrangian modelling of plume chemistry for secondary pollutants in large industrial plumes. Atmospheric Environment 42, 415–427) and Alessandrini S., Ferrero E. (2008. A Lagrangian model with chemical reactions: application in real atmosphere. Proceedings of the 12th Int. Conf. on Harmonization within atmospheric dispersion modelling for regulatory purposes. Croatian Meteorological Journal, 43, ISSN: 1330-0083, 235–239) the treatment of ozone was modified in order to facilitate urban scale simulations encompassing dense road networks. For the street canyon application, modelled daily mean NOₓ/NO₂ concentrations deviated by +0.4%/−15% from observations, while the correlations for NOₓ and NO₂ were 0.67 and 0.76 respectively. NO₂ concentrations were underestimated in summer, but were captured well for other seasons. In Graz a fair agreement for NOₓ and NO₂ was obtained between observed and modelled values for NOₓ and NO₂. Simulated diurnal cycles of NO₂ and O₃ matched observations reasonably well, although O₃ was underestimated during the day. A possible explanation here might lie in the non-consideration of volatile organic compounds (VOCs) chemistry.