Atmospheric Gaseous Sulfuric Acid Measurements:Implications for Aerosol Formation by Homogeneous Nucleation

This work is focused on the presentation and interpretation of measurements of gaseous sul- furic acid, which we have made at Hyytiälä (Finland) in the framework of the European pro- jects BACCI (Research Unit on Biosphere-Aerosol-Cloud-Climate-Interactions) and QUEST (Quantification of Aerosol Nucleation in the European Boundary Layer). These measurements were conducted using Chemical Ionization Mass Spectrometry (CIMS) and represent one of the few long-term gaseous sulfuric acid measurements. Sulfuric Acid is probably the key trigger of new particle formation by nucleation. On the basis of our sulfuric acid measu- rements the nucleation process was examined considering the aerosol size distributions which were simultaneously measured. The H2SO4 data were compared with particle and meteorological parameters in order to characterize conditions for nucleation. In the next step it was examined to what extent the number concentration of small particles depends on the sulfuric acid concentration. Previous theoretical and experimental investigations showed that new particle concentrations obey a linear or a quadratic dependence on the gaseous sulfuric acid concentration. Both dependencies were observed in this work which indicates that activation (linear) as well as kinetic nucleation theory (quadratic) can explain the formation of new particles in this region