Retrieval of aerosol single-scattering albedo and polarized phase function from polarized sun-photometer measurements for Zanjan's atmosphere

Open Access Atmospheric Measurement Techniques Atmos. Meas. Tech., 6, 2659–2669, 2013 www.atmos-meas-tech.net/6/2659/2013/ doi:10.5194/amt-6-2659-2013 © Author(s) 2013. CC Attribution 3.0 License. Retrieval of aerosol single-scattering albedo and polarized phase function from polarized sun-photometer measurements for Zanjan’s atmosphere A. Bayat1 , H. R. Khalesifard1,2 , and A. Masoumi3 1 Department of Physics, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran 2 Center for Climate Change and Global Warming, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran 3 Department of Physics, Faculty of Sciences, University of Zanjan, Zanjan 45371-38791, Iran Correspondence to: A. Bayat () Received: 14 March 2013 – Published in Atmos. Meas. Tech. Discuss.: 4 April 2013 Revised: 2 September 2013 – Accepted: 4 September 2013 – Published: 15 October 2013 Abstract. The polarized phase function of atmospheric aerosols has been investigated for the atmosphere of Zanjan, a city in northwest Iran. To do this, aerosol optical depth, Ångström exponent, single-scattering albedo, and polarized phase function have been retrieved from the measurements of a Cimel CE 318-2 polarized sun-photometer from February 2010 to December 2012. The results show that the maximum value of aerosol polarized phase function as well as the polarized phase function retrieved for a specific scattering angle (i.e., 60◦ ) are strongly correlated (R = 0.95 and 0.95, respectively) with the Ångström exponent. The latter has a meaningful variation with respect to the changes in the complex refractive index of the atmospheric aerosols. Furthermore the polarized phase function shows a moderate negative correlation with respect to the atmospheric aerosol optical depth and single-scattering albedo (R = −0.76 and −0.33, respectively). Therefore the polarized phase function can be regarded as a key parameter to characterize the atmospheric particles of the region – a populated city in the semi-arid area and surrounded by some dust sources of the Earth’s dust belt. 1 Introduction Physical and optical properties of the atmospheric aerosols are from the major uncertainties in the global climate changes (IPCC, 2007). In order to reduce the lack of extensive and reliable information about aerosols and their im- pacts on atmosphere, they have been widely investigated by ground-based measurements and satellite remote sensing suites (Heintzenberg et al., 1997; Kaufman et al., 2002). Ground-based measurements are ideal for reliable and continuous derivation of local aerosol optical and physical properties due to negligible effects of surface background on the measurements, and satellite measurements provide less accurate information about aerosols but in a global coverage (Holben et al., 1998; Dubovik et al., 2002). Satellite remote sensing and ground-based measurements are complementary methods to study aerosols properties and their effects on climate. They have been investigated by using active (e.g., spaceborne and ground-based lidars) (Bösenberg et al., 2003; Winker et al., 2007) and passive (e.g., spaceborne spectrometers and ground-based sun-photometers) instruments (Holben et al., 1998; Prospero et al., 2002; Ginoux et al., 2012). Iran is located within Earth’s so-called dust belt. Many cities in the western, eastern, southern, and central parts of this country have been subjected to dust events of different strengths, especially during the recent years. Previous observations show that the Tigris–Euphrates basin in the west, the Arabian Peninsula in the south and southwest, and the arid region between the Caspian and Aral seas in the north are the main external sources for the observed dust activities in this region (Prospero et al., 2002; Leon and Legrand, 2003; Goudie et al., 2006; Bayat et al., 2011; Abdi et al., 2011, 2012; Sabetghadam et al., 2012; Masoumi et al., 2013). There are also some minor active dust sources inside the Published by Copernicus Publications on behalf of the European Geosciences Union. 2660 Iranian Plateau (Abdi et al., 2011, 2012; Masoumi et al., 2013). Zanjan, a city in northwest Iran, is located in a mountainous region at 36.70◦ N, 48.51◦ E, and 1800 m above the mean sea level (a.m.s.l). Based on the recordings of Zanjan’s Meteorological office, the average of sunlight hours for this city is more than 7 h per day (Samimi et al., 1997). Considering the geographical location as well as the climatological conditions and lack of measured data for the region, groundbased measurements in this city provide valuable information on the dust activities as well as aerosol types and their optical and physical properties. Aerosol classification using ground-based remote sensing techniques can help to improve the estimation of aerosol radiative impact on climate and the accuracy of satellite retrievals (Dubovik et al., 2002; Cattrall et al., 2005; Giles et al., 2012). Various methods based on aerosol optical and physical properties have been used to classify different types of aerosols from ground-based sun-photometer (SPM) measurements. The extinction of sunlight by aerosols when it passes vertically through the atmosphere from the top of the atmosphere to the surface is called the aerosol optical depth (τa ). This parameter and its spectral dependence with respect to wavelength (i.e., the Ångström exponent, α) are the commonly used parameters to distinguish the dominant aerosol types (e.g., Toledano et al., 2007; Kalapureddy et al., 2009; Bayat et al., 2011; Boselli et al., 2012; Masoumi et al., 2013). Studies (Gobbi et al., 2007; Basart et al., 2009) have shown that the derivative of α or the spectral difference of α-wavelength pairs together with τa and the particle’s effective radius can be used to infer different aerosol types. In other works, variations of aerosol single-scattering albedo (the ratio of scattering to extinction coefficients of particles, ω0 ) with respect to their sizes have been used to distinguish aerosol types (Omar et al., 2005; Mielonen et al., 2009; Lee et al., 2010; Russell et al., 2010; Giles et al., 2012). As a result of the mentioned methods, τa , α, ω0 , and size distributions retrieved from non-polarized measurements of SPM are commonly used to categorize different types of aerosols, but the polarization ones are mainly neglected. Polarized sky radiance resulting from interaction between sunlight and atmospheric particles strongly depends on the presence of aerosols in the atmosphere, and can be monitored by looking at the aerosol polarized phase function, qa (2) (Vermeulen et al., 2000; Li et al., 2004, 2006). The qa (2) is indicative of the linear polarization of the scattered light that has been generated by the atmospheric aerosols (Li et al., 2004). Based on the relationship between the polarized sky radiance measured by SPM and its theoretical estimation from applying the Mie scattering theory, (...truncated)