Ambient sesquiterpene concentration and its link to air ion measurements B. Bonn1,*, A. Hirsikko1, H. Hakola2, T. Kurtén1, L. Laakso1, M. Boy3, M. Dal Maso1, J. M. Mäkelä4, and M. Kulmala1 1Department of Physical Sciences, University of Helsinki, Helsinki, Finland 2Finnish Meteorological Institute, Helsinki, Finland 3National Center of Atmospheric Research, Boulder, CO, USA 4Tampere University of Technology, Institute of Physics, Tampere, Finland *now at: Estonian University of Life Sciences, Department of Plant Physiology, Tartu, Estonia
Abstract. Ambient air ion size distributions have been measured continuously at the
Finnish boreal forest site in Hyytiälä since spring 2003. In general,
these measurements show a maximum of air ions below 1.0 nm in diameter. But
this physical characterization does not provide any information about the
ion's chemical composition, which is one key question regarding the
explanation of nucleation events observed. In this study we propose a link of
the observed maximum of negative air ions between 0.56 and 0.75 nm to the
so-called stabilised Criegee biradical, formed in the reaction of biogenic
sesquiterpenes with ozone and predominantly destroyed by its reaction with
ambient water vapour. Calculations of the electron and proton affinities of
120 kJ mol−1 (1.24 eV) and of 960 kJ mol−1 support
this link. Other possible candidates such as sulphuric acid derived clusters
are unable to explain the observations made. By using this approach, we are
able to calculate the ambient concentration of sesquiterpenes at the air ion
instrument inlet with a high time resolution on the daily and seasonal scale.
The estimated concentration is found to reveal the same seasonal pattern as
emission measurements conducted at shoot level. As expected for biogenic
VOCs, the concentration is obtained highest during summer (maximum values of
about 100 pptv) and smallest during winter (minimum less than 1 pptv).
Because of the sesquiterpenes high reactivity and its low ambient
concentrations, this approach can be a first step in understanding their
emission and their impact on atmospheric chemistry in more detail. The
findings presented are highly relevant for emission budgets too, since boreal
forests are extended over large areas of the globe.
Citation: Bonn, B., Hirsikko, A., Hakola, H., Kurtén, T., Laakso, L., Boy, M., Dal Maso, M., Mäkelä, J. M., and Kulmala, M.: Ambient sesquiterpene concentration and its link to air ion measurements, Atmos. Chem. Phys., 7, 2893-2916, doi:10.5194/acp-7-2893-2007, 2007.