REPORT SERIES IN AEROSOL SCIENCE N:o 137 (2012) DYNAMICS OF NEUTRAL AND CHARGED AEROSOL PARTICLES

Atmospheric aerosol particles have two major effects on everyday life: firstly, they have various climate effects and, secondly, they have adverse health effects. The particles affect climate directly by scattering and absorbing solar radiation and indirectly by acting as cloud condensation nuclei, CCN, with a net effect assumed to be a cooling one. Increase in CCN number concentration may result in brighter and longer lasting clouds, which in turn would increase the amount of radiation scattered back to space. Recent studies suggest that majority of the atmospheric aerosol particles originate from the particle formation occurring via gas-toparticle conversion. However, the freshly-formed particles are not large enough to impact neither health nor climate and they are most susceptible to removal by collisions with larger pre-existing particles. Consequently, the knowledge of both the formation and the growth rate of particles are crucially important when assessing the health and climate effects of atmospheric new particle formation. The purpose of this thesis is to increase our knowledge of the dynamics of neutral and charged aerosol particles with a specific interest towards the particle growth rate and processes affecting the aerosol charging state. A new model, Ion-UHMA, which simulates the dynamics of neutral and charged particles, was developed for this purpose and it was shown to be capable of reproducing new particle formation and growth events as observed in the measurements. Simple analytical formulae that can be used to estimate the growth rate due to various processes were derived, tested against the simulated data and used to study the effect of charged particles on the growth rate of a particle population. It was found that the average growth rate of freshly-formed particles could be significantly increased due to enhanced condensation onto charged particles. Furthermore, the charged particles were found to increase the growth rate due to selfcoagulation and coagulation scavenging by a factor of 1.5–2. Finally, recent data-analysis methods that have been applied to the aerosol charging states obtained from the measurements were modified for a charge asymmetric framework. The methods were then used on the data measured in Helsinki, Finland, and tested on the data obtained from aerosol dynamics simulations. The methods were found to be able to provide reasonable estimates on the growth rate and proportion of particles formed via ion-induced nucleation, provided that the growth rate is high enough and that the charged particles do not grow much more rapidly than the neutral ones. A simple procedure for estimating whether the methods are suitable for analysing data obtained in specific conditions was provided. In this thesis, the dynamics of neutral and charged aerosol particles were studied in detail. Combined with the current field and chamber measurements of neutral and charged particles, the analytical tools provided here can be used to study the atmospheric new particle formation and growth more comprehensively and, eventually, to decrease the uncertainty related to climate effects of aerosol particles.