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.

Research highlights
► GRAL-C, a hybrid Lagrangian/Eulerian chemistry, diffusion and transport model was developed.
► Simple O3–NO–NO2 chemistry was simulated with high spatial resolution.
► The model was applied successfully in two urban areas.
► Further research regarding segregation is necessary.