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Formation of aerosols in the lower Stratosphere and upper Troposphere

Final Activity Report Summary - FAST (Formation of aerosols in the lower stratosphere and upper troposphere)

Owing to their potential influence on climate atmospheric aerosol particles have received considerable attention during recent years. Their formation processes are thus of importance. Several air borne experiments have identified the upper troposphere and lower stratosphere as one of the major formation regions. The sulfate aerosol particles are formed via gas to particle conversion and as the primary mechanism is regarded the binary homogeneous nucleation of sulfuric acid (H2SO4) and water (H2O). However, predicted nucleation rates are usually much lower than the nucleation rates inferred from particle measurements showing that our understanding of particle formation processes in the atmosphere is far from complete. Ternary homogeneous nucleation (H2O, H2SO4 and ammonia) and ion-mediated nucleation were suggested as nucleation mechanism that may explain the gap between measurements and simulations. However, the understanding of these processes is still evolving and quantitatively very uncertain. Further, measurements show that binary homogeneous nucleation rates may be enhanced by atmospheric variability and mixing and may occur under conditions which are otherwise unfavourable for nucleation.

The aim of our project was to investigate nucleation and growth processes of sulfate aerosol in the atmosphere theoretically using model simulations but in close connection to available observations. Especially, we focused on when, where and under which conditions enhanced nucleation rates are occurring. The main objectives of the project were:
- nucleation processes of atmospheric aerosols in general, growth of aerosol particles to climate relevant sizes;
- development of parameterisation for mixing;
- case studies with experimental data;
- binary and ternary homogeneous nucleation and dynamic processes which could lead to aerosol formation.

To fulfil the objectives several case studies with a microphysical box model were performed. A case study was performed on how binary homogeneous nucleation rates are influenced by the usage of different spatial and temporal resolutions of meteorological data sets. The simulations with the model showed that due to differences in the temperature history significant differences in the nucleation rate can occur. In another case study, it was investigated how atmospheric variability can influence the nucleation rate. Therefore, fluctuations were superimposed on the synoptic trajectories. The model simulations showed that atmospheric variability has a negligible influence on the binary homogeneous nucleation rate in the upper troposphere and lower stratosphere. To investigate how aerosol particles grow to climate relevant sizes we simulated the formation of a polar stratospheric cloud. These clouds play a major role in the formation of the Arctic and Antarctic ozone hole. Case studies with experimental data were performed for measurements made in the free Arctic troposphere. Thereby, it was investigated how sub-grid scale velocity influences the nucleation rate challenging also the last objective of the project. It could be shown that sub-grid scale vertical velocity has an influence on aerosol formation processes of sulfate aerosols. The case studies performed help to improve our understanding of atmospheric nucleation processes and thus to improve the agreement between models and measurements.