Retrieval of aerosol microphysical properties by means of sun and star photometry at Granada, Spain

Abstract

Knowledge of aerosol microphysical properties is essential to understand the role of atmospheric aerosols in radiative forcing. Passive remote-sensing techniques allow the retrieval of columnar aerosol size distribution from spectral measurements of aerosol optical depth by applying numerical methods. These methods are widely applied to sun photometers. For night-time retrievals of aerosol optical depth, the GFAT (Atmospheric Physics Group of the University of Granada), operates a star photometer with channels at 380, 436, 500, 670, 880, and 1020 nm in the city of Granada (southeastern Spain, 36.83° N, 3.58° W, 680 m a.s.l.). The GFAT also operates a Cimel CE-318 sun photometer with filters at the same wavelengths. To our knowledge, the variations between daytime and night-time columnar aerosol microphysical properties have not been studied until now. In this work, the applicability of an inversion code based only on direct irradiance measurements is presented to retrieve aerosol size distributions, for both daytime and night-time. From March 2007 to February 2009, statistical analyses and time evolutions of the effective radius and the properties of the fine mode such as mean radius, width, and particle volume concentration are analysed. Daytime effective radius values are large in summer and small in winter, whereas no clear seasonal pattern emerges at night-time. During daytime, the fine mode radius does not show any remarkable pattern, whereas at night-time, this parameter shows a clear seasonal pattern with large values in summer and small values in winter. On the other hand, fine mode widths do not show any pattern, neither daytime nor night-time, although large values of this parameter are found during daytime in all seasons. The fine-particle volume concentration presents no seasonal pattern during the daytime, whereas at night-time it shows larger values in summer than in other seasons.

Acknowledgements

This work was supported by Spanish Ministry of Science and Technology projects CGL2008-01330-E/CLI (Spanish Lidar Network), CGL2010-18782, and CSD2007-00067; by Andalusian Regional Government projects P10-RNM-6299 and P08-RNM-3568; and by the Aerosols, Clouds, and Trace Gases Research Infrastructure (ACTRIS) project (EU INFRA-2010-1.1.16-262254). The authors would like express their gratitude to the NASA/Goddard Space Flight Center, National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory (ARL), Naval Research Laboratory for use of the HYSPLIT transport and dispersion model. We thank Dr Kowalski for revising the English of the manuscript and also the anonymous referees for their suggestions to improve this work.