Year of publication
- 2011 (2) (remove)
- Numerical simulation of flow, H2SO4 cycle and new particle formation in the CERN CLOUD chamber (2011)
- To study the effect of galactic cosmic rays on aerosols and clouds, the Cosmic Leaving OUtdoor Droplets (CLOUD) project was established. Experiments are carried out at a 26 m3 tank at CERN (Switzerland). In the experiments, the effect of ionising particle radiation on H2SO4 particle formation and growth is investigated. To evaluate the experimental configuration, the experiment was simulated using a coupled multidimensional CFD – particle model (CLOUD-FPM). In the model the coupled fields of gas/vapour species, temperature, flow velocity and particle properties were computed to investigate the tank's mixing state and mixing times. Simulation results show that the mixing state of the tank's contents largely depends on the characteristics of the mixing fans and a 1-fan configuration, as used in first experiments, may not be sufficient to ensure a homogeneously mixed chamber. To mix the tank properly, 2 fans are necessary. The 1/e response times for instantaneous changes of wall temperature and saturation ratio inside the chamber were found to be in the order of few minutes. Particle nucleation and growth was also simulated and particle number size distribution properties of the freshly nucleated particles (particle number, mean size, standard deviation of the assumed log-normal distribution) were found to be mixed over the tank's volume similar to the gas species.
- Relating hygroscopicity and composition of organic aerosol particulate matter (2011)
- A hygroscopicity tandem differential mobility analyzer (HTDMA) was used to measure the water uptake (hygroscopicity) of secondary organic aerosol (SOA) formed during the chemical and photochemical oxidation of several organic precursors in a smog chamber. Electron ionization mass spectra of the non-refractory submicron aerosol were simultaneously determined with an aerosol mass spectrometer (AMS), and correlations between the two different signals were investigated. SOA hygroscopicity was found to strongly correlate with the relative abundance of the ion signal m/z 44 expressed as a fraction of total organic signal (f44). m/z 44 is due mostly to the ion fragment CO2+ for all types of SOA systems studied, and has been previously shown to strongly correlate with organic O/C for ambient and chamber OA. The analysis was also performed on ambient OA from two field experiments at the remote site Jungfraujoch, and the megacity Mexico City, where similar results were found. A simple empirical linear relation between the hygroscopicity of OA at subsaturated RH, as given by the hygroscopic growth factor (GF) or "ϰorg" parameter, and f44 was determined and is given by ϰorg = 2.2 × f44 − 0.13. This approximation can be further verified and refined as the database for AMS and HTDMA measurements is constantly being expanded around the world. The use of this approximation could introduce an important simplification in the parameterization of hygroscopicity of OA in atmospheric models, since f44 is correlated with the photochemical age of an air mass.