Modelling chemistry in aircraft plumes 2: the chemical conversion of NOx to reservoir species under different conditions

An expanding plume model with chemistry has been used to study the chemical conversion of nitrogen oxides to reservoir species in aircraft plumes under various conditions. A plume from a B747 was studied, where the NOx emission index was 26.1 g kg−1. Several sensitivity tests were performed for emissions at 07 UT at 50°N under summer conditions. The amount of NOx remaining in the plume after 15 h was between 58 and 69%, assuming background concentrations of ozone and NOx ranging from 50 to 200 ppb and 50 to 200 ppt, respectively. A somewhat higher range in the chemical conversion of NOx was calculated when changing the temperature from 200 to 240 K. The remaining NOx in the plume increased to 90% in a study where the aircraft emissions were released in air masses already exposed to aircraft emissions some hours before. The rate of chemical conversion of NOx to reservoir species was highly dependent on the number of sunlit hours during the 15 h period. The diurnal, seasonal and latitudinal variations in the chemical conversion of NOx to reservoir species were studied. As a first approximation, the conversion rates can be used to modify the aircraft NOx emissions at cruise altitudes used in mesoscale models.