Atmos. Chem. Phys. Discuss., 12, 20849-20899, 2012
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Aerosol decadal trends – Part 2: In-situ aerosol particle number concentrations at GAW and ACTRIS stations
A. Asmi1,15, M. Collaud Coen2, J. A. Ogren3, E. Andrews3,4, P. Sheridan3, A. Jefferson3,4, E. Weingartner5, U. Baltensperger5, N. Bukowiecki5, H. Lihavainen6, N. Kivekäs6, E. Asmi6, P. P. Aalto1, M. Kulmala1, A. Wiedensohler7, W. Birmili7, A. Hamed7,8, C. O'Dowd9, S. G. Jennings9, R. Weller10, H. Flentje11, A. Mari Fjaeraa12, M. Fiebig12, C. Lund Myhre12, A. G. Hallar13, and P. Laj14
1Department of Physics, University of Helsinki, P.O. Box 64, 00014, Finland
2Federal Office of Meteorology and Climatology, MeteoSwiss, 1530 Payerne, Switzerland
3NOAA Earth System Research Laboratory, Boulder, USA
4Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, USA
5Paul Scherrer Institute, Laboratory of Atmospheric Chemistry, Villigen PSI, 5232, Switzerland
6Finnish Meteorological Institute, Erik Palmenin aukio 1, 00560 Helsinki, Finland
7Leibniz Institute for Tropospheric Research, Germany
8Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
9School of Physics, National University of Ireland Galway, Galway, Co. Galway, Ireland
10Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany
11Deutcher Wetterdienst, Meteorological Observatory Hohenpeissenberg, Albin-Schwaiger-Weg 10, 82383 Hohenpeißenberg, Germany
12NILU – Norwegian Institute for Air Research, Instituttveien 18, 2027 Kjeller, Norway
13Desert Research Institute, Storm Peak Laboratory Division of Atmospheric Sciences, Steamboat Springs, USA
14UJF-Grenoble 1/CNRS, LGGE UMR 5183, Grenoble, 38041, France
15Laboratoire de Météorologie Physique, CNRS-Université Blaise Pascal, UMR6016, Clermont Ferrand, France

Abstract. We have analysed the trends of total aerosol particle number concentrations (N) measured at long-term measurement stations involved either in the Global Atmosphere Watch (GAW) and/or EU infrastructure project ACTRIS. The sites are located in Europe, North America, Antarctica, and on Pacific Ocean islands. The majority of the sites showed clear decreasing trends both in the full-length time-series, and in the intra-site comparison period of 2001–2010, especially during the winter months. Several potential driving processes for the observed trends were studied, and even though there are some similarities between N trends and air temperature changes, the most likely cause of many Northern Hemisphere trends was found to be decreases in the anthropogenic emissions of primary particles, SO2 or some co-emitted species. We could not find a consistent agreement between the trends of N and particle optical properties in the few stations with long timeseries of all of these properties. The trends of N and the proxies for cloud condensation nuclei (CCN) were generally consistent in the few European stations where the measurements were available. This work provides a useful comparison analysis for modelling studies of trends in aerosol number concentrations.

Citation: Asmi, A., Collaud Coen, M., Ogren, J. A., Andrews, E., Sheridan, P., Jefferson, A., Weingartner, E., Baltensperger, U., Bukowiecki, N., Lihavainen, H., Kivekäs, N., Asmi, E., Aalto, P. P., Kulmala, M., Wiedensohler, A., Birmili, W., Hamed, A., O'Dowd, C., Jennings, S. G., Weller, R., Flentje, H., Mari Fjaeraa, A., Fiebig, M., Lund Myhre, C., Hallar, A. G., and Laj, P.: Aerosol decadal trends – Part 2: In-situ aerosol particle number concentrations at GAW and ACTRIS stations, Atmos. Chem. Phys. Discuss., 12, 20849-20899, doi:10.5194/acpd-12-20849-2012, 2012.
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