Modelling chemistry in aircraft plumes 1: comparison with observations and evaluation of a layered approach

An expanding plume model with chemistry has been used to study the chemical conversion of nitrogen oxides to reservoir species in aircraft plumes. The model represents the plume by several circular or cylindrical layers in order to give a more detailed description of the chemical evolution in the plume. Model simulations of plumes from two B747 aircraft monitored during the POLINAT measurement campaigns in winter 1994 and summer 1995 were performed. The NOx emission indices were 12.5 and 26.1 g kg−1, respectively. The chemical evolution was followed during the phase dominated by the aircraft-induced dispersion (vortex regime), and in the atmospheric-induced dispersion regime up to 15 h. The results were compared to observations of NO, HNO3, HNO2 and CO2 taken within the trailing vortices. The modelled values were in broad agreement with the observations. With a NOx emission index of 26.1 g kg−1, 60% remained as NOx after 15 h at 50°N under summer conditions for emissions at 07 UT. This amount decreased to 40% when a one-layered plume was considered, suggesting that the distribution of NOx leads to different oxidising potentials which again influenced the plume chemistry.