Mass budget simulation of NOx and CO for the evaluation of calculated emissions for the city of Augsburg (Germany)

The Augsburg field experiment (EVA) campaigns had been carried out in 1998. The aim of this experiment was the evaluation of the urban emissions of NOx and CO that had been derived from the emission model of Institut fuer Energiewirtschaft und Rationelle Energieanwendung (IER), University of Stuttgart. Using a mass balance technique, the real emissions had been estimated from advective fluxes of NOx and CO. These fluxes had been calculated from ground-based and aircraft measurements of wind and concentrations of NOx and CO. Applying the mass balance technique, processes like deposition, turbulent transports, and chemical transformations had been neglected. Simulations with the KAMM/DRAIS model system accompanied and completed the experimental analyses. The results of these simulations are presented in this paper. This model system possesses a mass budget module that allows to calculate the contributions of all relevant processes to the budget of chemical species like CO and NOx. One objective of the model study was to estimate as to how much of the NOx and CO emissions could be described solely by advective fluxes. It could be deduced that about 85% of the NOx and about 95% of the CO emissions can be determined from the horizontal and vertical advective fluxes. The remaining fractions of 15% and 5% indicate the uncertainty of the estimation of the emissions due to the neglect of other processes except the advective fluxes. These uncertainties are remarkably smaller than the errors in the experimental estimation of the emissions. The second aim of the model study was the assessment of the uncertainty of the modelled urban emissions. For this purpose an estimation of the urban emissions has been carried out based only on the calculations of the horizontal fluxes through the lateral, vertical planes along the six flight tracks of the aircraft and the vertical advective fluxes through the ceiling of the surrounded area. The results of three evaluations have been compared: (I) the first used the KAMM/DRAIS model results that were based on the emissions from the IER emission model; (II) the second combined the KAMM/DRAIS wind field and the NOx and CO concentrations being measured along the flight tracks on board the aircraft; and (III) the third only used the aircraft measurements of wind and concentrations. Summarizing the results of the evaluation cases I and II it can be concluded that the modelled CO emissions are about a factor of 2 lower than the real ones and the modelled NOx emissions correspond to the real ones within an uncertainty of about 50%. The results of cases II and III indicate the sensitivity of the calculated net fluxes to the flow field.