Building on the laboratory experiments with H2SO4 cluster ions and the atmospheric gaseous H2SO4 measurements one expects large HSO-Aa Ww and H+Aa Ww cluster ions to be at least occasionally present in the cold upper troposphere. If so, such ions would induce new aerosol particle formation and therefore would represent fingerprints of INU. Using our aircraft-based large ion-mass spectrometer LIOMAS we have made a search for such large cluster ions in the upper troposphere (Eichkorn et al., 2002; Arnold et al., 2006).
Figure 7 shows as an example time-series of ion composition data obtained by LIOMAS in cloud-free upper troposphere air during one aircraft flight (Arnold et al., 2006). Given are flight altitude and fractional abundances of positive ions with mass numbers larger than 200, 300, and 600 amu. While the aircraft was cruising mostly around 8000 m altitude it occasionally intercepted air masses containing large negative and positive cluster ions with mass numbers larger than 600 amu. Negative and positive ions behave rather similar which suggests that one or several trace gases X are present which cluster to negative and positive ions. From the measured ions the concentrations of X were inferred. These range between 1-4 x 106 cm-3 which is very similar to the gaseous H2SO4 concentrations previously measured by our group in the upper troposphere around 8000 m altitude (see Figure 6). This
suggests at least that X is not more abundant than gaseous H2SO4 which implies that most likely X can be identified as H2SO4.
Building on the inferred X and assuming X to be gaseous H2SO4 the rate J of ion-induced nucleation was calculated (Arnold et al., 2006). Around 8000m altitude Ji ranges mostly from 3-25 cm-3 s-1. The latter value is equal to the maximum possible Ji = Q where Q is the ionization rate. When Ji = Q, all ions become stable aerosol particles. Also calculated was the rate Jh of homogeneous bi-molecular nucleation (which does not require ions, Arnold et al., 2006). For the flight sections with low concentration of X, Ji tends to exceed Jh whereas for the section with high x concentrations Jh exceeds Ji. Even the highest Jh of about 100 cm-3 s-1 are much smaller than the highest possible Jh dictated by the rate at which an H2SO4 molecule collides with another H2SO4 molecule. Note that a nucleation rate of 3 cm-3s-1 is already relatively high allowing the formation of more than 10,000 aerosol particles cm-3hr-1.
The above example of the atmospheric situation encountered during a single upper troposphere flight demonstrates that in the cold upper troposphere CR mediated INU can be very efficient. However this example also demonstrates that HONU is also efficient and may become even more efficient than INU.
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