Links between two different types of spectra of charged nanometer aerosol particles


We have, since 2007, continuously measured the electrical mobility distribution of small (<1.5 nm in diameter) corona-generated one-second-aged air ions, using our Small Air Ion Spectrometer (KAIS), in urban area, in the center of the town of Tartu, Estonia. We have simultaneously measured the mobility distributions of natural air ions (0.42-7.4 nm in diameter) with the Balanced Scanning Mobility Analyzer (BSMA). In this work we employ these data to establish certain links between the concurrent spectra of two types, especially for days with new particle formation events. We elaborated and tested an automatic classification method, which selected the spectra according their shape. In the case of the BSMA negative ions, we obtained four classes of the spectra which are associated with the intermediate ion (1.6-7.4 nm in diameter) nucleation burst events and rain-type events. The spectra within these classes are characterized by special shapes and they are called ``event-like spectra″. The first class demonstrates the strongest increase in the concentration of all ions with the mobilities below 0.8 cm(2)V(-1) s(-1) (above 1.25 nm in diameter), it mainly contains spring and early summer spectra, recorded around midday. In the second class, the events are weaker; it contains a large number of late spring morning time spectra, when relative humidity (RH) tends to decrease. The third and the fourth classes contain many spectra, which resemble to short isolated events. These spectra are excluded from the further analysis. In the first class, the precipitation spectra make about 34% of all the event-like spectra, in the other classes about 10%. In the case of the BSMA positive ions, only the first and second classes were present. The precipitation spectra make about 20% of all event-like classes. About 45% of the event-like spectra are without precipitation, but with RH>90%, and these spectra are excluded from the further analysis. The spectra with RH<50% formed 12% of the respective classes. The established links with the concurrent KAIS classes imply that especially the BSMA positive ion burst events are often connected with some extra trace gases, which can override the dependence on the meteorological conditions. All the ions within the BSMA ion burst spectra are definitely older than about 1 s. (C) 2010 Elsevier B.V. All rights reserved.