Atmos. Chem. Phys. Discuss., 12, 20785-20848, 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 1: In-situ optical measurements at GAW and IMPROVE stations
M. Collaud Coen1, E. Andrews2,3, A. Asmi4, U. Baltensperger5, N. Bukowiecki5, D. Day6, M. Fiebig7, A. M. Fjaeraa7, H. Flentje8, A. Hyvärinen11, A. Jefferson2, S. G. Jennings9, G. Kouvarakis10, H. Lihavainen11, C. Lund Myhre7, W. C. Malm6, N. Mihapopoulos10, J. V. Molenar12, C. O'Dowd9, J. A. Ogren3, B. A. Schichtel13, P. Sheridan3, A. Virkkula4, E. Weingartner5, R. Weller14, and P. Laj15
1Federal Office of Meteorology and Climatology, MeteoSwiss, 1530 Payerne, Switzerland
2University of Colorado, CIRES, Boulder, Colorado, 80305, USA
3National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Boulder, Colorado, 80305, USA
4Department of Physics, University of Helsinki, P.O. Box 64, Helsinki, Finland
5Paul Scherrer Institute, Laboratory of Atmospheric Chemistry, 5232 Villigen PSI, Switzerland
6Colorado State University, Cooperative Institute for Research in the Atmosphere, Fort Collins, CO 80523, USA
7NILU – Norwegian Institute for Air Research, Instituttveien 18, 2027 Kjeller, Norway
8German Weather Service, Meteorological Observatory Hohenpeissenberg, Albin-Schwaiger-Weg 10, 82383 Hohenpeißenberg, Germany
9School of Physics, National University of Ireland Galway, Galway, Co. Galway, Ireland
10Environmental Chemistry Processes Laboratory, Dept. of Chemistry, University of Crete, 71003 Heraklion Crete, Greece
11Finnish Meteorological Institute, Erik Palmenin aukio 1, 00560 Helsinki, Finland
12Air Resource Specialists, Inc., 1901 Sharp Point Drive, Suite E, Ft. Collins, CO 80525, USA
13National Park Service, Fort Collins, CO 80523, USA
14Alfred-Wegener-Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany
15UJF-Grenoble 1/CNRS, LGGE UMR5183, 38041 Grenoble, France

Abstract. Currently many ground-based atmospheric stations include in-situ measurements of aerosol physical and optical properties, resulting in more than 20 long-term (>10 yr) aerosol measurement sites in the Northern Hemisphere and Antarctica. Most of these sites are located at remote locations and monitor the aerosol particle number concentration, wavelength-dependent light scattering, backscattering, and absorption coefficients. The existence of these multi-year datasets enables the analysis of long-term trends of these aerosol parameters of the derived light scattering Ångström exponent and backscatter fraction. Since the aerosol variables are not normally distributed, three different methods (the seasonal Mann-Kendall test associated with the Sen's slope, the generalized least squares fit associated with an autoregressive bootstrap algorithm for confidence intervals, and the least-mean square fit applied to logarithms of the data) were applied to detect the long-term trends and their magnitudes for each month. To allow a comparison among measurement sites with varying length of data records, trends on the most recent 10 and 15 yr periods were calculated. No significant trends were found for the three continental European sites. Statistically significant trends were found for the two European marine sites but the signs of the trends varied with aerosol property and location. Statistically significant decreasing trends for both scattering and absorption coefficient were found for most North American stations, although positive trends were found for a few desert and high-altitude sites. No significant trends in scattering coefficient were found for the Arctic or Antarctic stations, whereas the Arctic station had a negative trend in absorption coefficient.

Citation: Collaud Coen, M., Andrews, E., Asmi, A., Baltensperger, U., Bukowiecki, N., Day, D., Fiebig, M., Fjaeraa, A. M., Flentje, H., Hyvärinen, A., Jefferson, A., Jennings, S. G., Kouvarakis, G., Lihavainen, H., Lund Myhre, C., Malm, W. C., Mihapopoulos, N., Molenar, J. V., O'Dowd, C., Ogren, J. A., Schichtel, B. A., Sheridan, P., Virkkula, A., Weingartner, E., Weller, R., and Laj, P.: Aerosol decadal trends – Part 1: In-situ optical measurements at GAW and IMPROVE stations, Atmos. Chem. Phys. Discuss., 12, 20785-20848, doi:10.5194/acpd-12-20785-2012, 2012.
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