The potential role of air ions in new particle formation at the Waldstein site
2 Leibniz-Institute for Tropospheric Research
P 2.2 in Research in its Prime: First Results of Ongoing Research
Mechanisms governing the process of secondary new particle formation are widely discussed, but our understanding of these mechanisms is still limited. In particular, there is no general consensus on the role of air ions in new particle formation.
Over the last decade a set of new instruments has become available, capable of measuring the number size distribution of neutral and charged clusters in size ranges relevant for particle nucleation. One of these instruments is the neutral cluster and air ion spectrometer (NAIS) which measures neutral clusters down to 2 nm diameter and ions down to diameters of 0.8 nm. During an eight-week measurement campaign we deployed a NAIS to study new particle formation events at the Waldstein ecosystem research site in the Fichtelgebirge mountain range, NE Bavaria, Germany.
The Fichtelgebirge mountains are well known for their elevated background radiation levels. The radiation originates from the bedrock, containing an increased concentration of uranium and thorium. Additionally, the fissured granitic bedrock promotes the molecular diffusion of gaseous radon. As tropospheric air ions mainly form due to radioactive decay, their concentrations may be elevated at the Waldstein and consequently play a key role in particle formation at that specific location.
Indeed the ion concentrations at the Waldstein showed to be slightly elevated in compare to other continental measurement sites. Further, an increase in ion concentrations was observed prior to the onset of particle formation events. The period of time between the appearance of elevated ion concentrations and the formation of numerous small particles of the same diameter was in the range of 10 – 60 minutes. Once formed, the particles and ions grow by condensation of precursor gases and coagulation. During this growth process the particles tend to grow faster than the ions, to eventually recover the temporal advance of the ions. This observations are giving a strong hint towards a significant role of ions in the formation and growth of secondary aerosol particles.