AEROSOL OPTICAL PROPERTIES FROM TROPOSPHERIC LIDAR AND SUN PHOTOMETER DURING THE GOA AEROSOL ARCTIC CAMPAIGNS 2005 AND 2006 AT ALOMAR Álvaro Bastidas 1 Edith Rodríguez 2 , Max Frioud 3 , Michael Gausa 3 , Kerstin Stebel 4 , Natalia Prats 2 , Sandra Mogo 2,5 , Benjamín Torres 2 , Carlos Toledano 2 , Alberto Berjón 2 , Victoria Cachorro 2 , Ángel M. de Frutos 2 1 Laser and Optics Spectroscopy Group, Physics School, Universidad Nacional / Medellín – Colombia 2 GOA-Group of Atmospheric Optics, University of Valladolid / Valladolid – Spain 3 Artic Lidar Observatory for Middle Atmosphere Research (ALOMAR) / Andenes – Norway 4 Norwegian Institute for Air Research / Tromsø – Norway 5 Department Physics, University of Beira Interior/covilha- Portugal 1. INTRODUCTION Tropospheric aerosols have an important role in our climate for their contribution to cloud formations and sunlight attenuation affecting critically the global radiative balance. Depending on various generating sources, tropospheric aerosols may vary greatly in both time and space. Several models for the study of the optical properties of the tropospheric aerosols have been developed. However, the optical properties of aerosols, governed by physical parameters such as particle density and size distribution, have not yet been well characterized in the Artic zone. • In the Arctic, long term observations, especially in-situ and a few aerosol lidar measurements, are carried out at Ny- Ålesund/Svalbard (79°N) [Ritter (2004)]. The location of ALOMAR, north of the Arctic Circle and on an island, a few hundred meters from seashore and about 30 km off the continent, makes it ideal for investigations related to Arctic phenomena such as noctilucent clouds, polar stratospheric clouds and Arctic haze. For air masses from the north and north-west ALOMAR can represent a reference station for almost unpolluted, clear air. Air masses passing above ALOMAR with back-trajectories from the north-east transport continental air from the urban centres in northern Russia, whereas western and south- western back-trajectories have a strong maritime influence. ALOMAR’s year-round capability is essential for long term studies to also include intra-annual variations. Previous studies report synergetic use of Lidar and sun- photometer [Landulfo (2003), Müller (2003), Veselovskii (2006)]. The Lidar and sun–photometer can provide information regarding microphysical properties of the • Alvaro Bastidas Laser and Optics Spectroscopy Group, Physics School, Universidad Nacional de Colombia Sede Medellín – Colombia +57 44309887 e-mail: aebastid@unalmed.edu.co aerosols by combining observations at different optical wavelengths. We present the results of Artic campaigns during summertime in 2005 and 2006 with the simultaneous observations by Cimel sun photometer and Lidar, extending the former 2002 and 2003 campaigns conducted by the Group of Atmospheric Optics of Valladolid [Toledano (2006)]. 2. INSTRUMENTS 2.1 The Sun-photometer The Cimel Electronique CE-318 sun photometer is an automatic sun and sky radiometer, with spectral interference filters centered at selected wavelengths: 340, 380, 440, 500, 670, 870, 1020 and 1640 nm for aerosol measurements. The filter band pass has 2 nm FWHM in the UV and 10 nm FWHM for the visible and infrared regions. The Cimel sun photometer is the standard instrument of AERONET network [Holben (1998)]. Direct sun measurements are performed at these wavelengths (1.2º field of view) to determine aerosol optical depth (AOD) and another channel at 940 nm is used for water vapor content retrieval. Sky radiance measurements are acquired at 6 wavelengths (from 440 to 1640 nm) in the solar almucantar and principal plane (for details, see [Holben (1998), Holben (2001)], http://aeronet.gsfc .asa.gov ; and http://www-loa.univ-lille1.fr/photons/ ). Inversion algorithms can then be applied to the sky and direct sun measurements to retrieve aerosol size distribution, single scattering albedo, phase function and complex refractive index [Dubovik (2000), Holben (1998)]. The Cimel sun photometer was calibrated at El Arenosillo within the Spanish Network for Aerosol Measurements (RIMA), in close collaboration with AERONET and according to the AERONET protocols. The direct sun channels were calibrated by inter-comparison with a master instrument. For the sky channels calibration an integrating sphere was utilized. This procedure yields to an estimated accuracy of 0.01-0.02 for the absolute AOD error (wavelength dependent) and 5% relative error for the radiance in the sky channels.