Atmos. Chem. Phys., 11, 12369-12386, 2011
© Author(s) 2011. This work is distributed
under the Creative Commons Attribution 3.0 License.
Aerosol hygroscopicity and CCN activation kinetics in a boreal forest environment during the 2007 EUCAARI campaign
K. M. Cerully1, T. Raatikainen2,3, S. Lance4,5, D. Tkacik6, P. Tiitta7,8, T. Petäjä9, M. Ehn9,*, M. Kulmala9, D. R. Worsnop3,7,9,10, A. Laaksonen3,7, J. N. Smith7,11, and A. Nenes1,2,12
1School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, USA
2School of Earth and Atmospheric Science, Georgia Institute of Technology, Atlanta, GA, USA
3Finnish Meteorological Institute, Helsinki, Finland
4Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
5Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA
6Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, USA
7Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
8Atmospheric Chemistry Research Group, North-West University, Potchefstroom, South Africa
9Department of Physics, University of Helsinki, Helsinki, Finland
10Aerodyne Research Incorporated, Billerica, MA, USA
11National Center for Atmospheric Research, Boulder, CO, USA
12Institute of Chemical Engineering and High-Temperature Chemical Processes, Foundation for Research and Technology Hellas, Patras, Greece
*currently at: Institute for Energy and Climate Research, Forschungszentrum Jülich, Jülich, Germany

Abstract. Measurements of size-resolved cloud condensation nuclei (CCN) concentrations, subsaturated hygroscopic growth, size distribution, and chemical composition were collected from March through May, 2007, in the remote Boreal forests of Hyytiälä, Finland, as part of the European Integrated project on Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) campaign. Hygroscopicity parameter, κ, distributions were derived independently from Continuous Flow-Streamwise Thermal Gradient CCN Chamber (CFSTGC) and Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) measurements. CFSTGC-derived κ values for 40, 60, and 80 nm particles range mostly between 0.10 and 0.40 with an average characteristic of highly oxidized organics of 0.20 ± 0.10, indicating that organics play a dominant role for this environment. HTDMA-derived κ were generally 30% lower. Diurnal trends of κ show a minimum at sunrise and a maximum in the late afternoon; this trend covaries with inorganic mass fraction and the m/z 44 organic mass fraction given by a quadrupole aerosol mass spectrometer, further illustrating the importance of organics in aerosol hygroscopicity. The chemical dispersion inferred from the observed κ distributions indicates that while 60 and 80 nm dispersion increases around midday, 40 nm dispersion remains constant. Additionally, 80 nm particles show a markedly higher level of chemical dispersion than both 40 and 60 nm particles. An analysis of droplet activation kinetics for the sizes considered indicates that most of the CCN activate as rapidly as (NH4)2SO4 calibration aerosol.

Citation: Cerully, K. M., Raatikainen, T., Lance, S., Tkacik, D., Tiitta, P., Petäjä, T., Ehn, M., Kulmala, M., Worsnop, D. R., Laaksonen, A., Smith, J. N., and Nenes, A.: Aerosol hygroscopicity and CCN activation kinetics in a boreal forest environment during the 2007 EUCAARI campaign, Atmos. Chem. Phys., 11, 12369-12386, doi:10.5194/acp-11-12369-2011, 2011.
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