30The use of sky cameras for nocturnal aerosol characterization is discussed in this study. 31Two sky cameras are configured to take High Dynamic Range (HDR) images at and thermal radiation (direct effect), and acting as cloud droplet nuclei that leads to 60 changes in the cloud properties and lifetime (indirect effect). 61At night there is no solar radiation, but aerosol radiative forcing in the longwave 62 range can be significant for large particles like desert dust and sea salt [Stier et al., 2007; 63 Sicard et al., 2014]. In addition, at night the aerosol indirect effect still works and could 64 provide changes on nocturnal cloud properties [Ramanathan et al., 2001; Kaufman et al. 65 2005; Rosenfeld et al. 2006]. These changes could contribute to global warming since 66 clouds at night absorb part of the longwave radiation emitted by Earth, and then, they 67 re-emit radiation back to the Earth surface [Ramanathan et al., 1989; NASA Facts, 68 1999; Wild, 2012]. Moreover, the knowledge of aerosol properties at night is important 69 for the aerosol characterization in polar areas and in winter seasons, which present low 70 sunshine duration values [Stone et al., 2010; Tomasi et al., 2015]. 71Some instruments and techniques are used for the characterization of aerosol 72 properties at night: lidar systems provide backscatter and extinction profiles using Fernald-Sasano retrievals [Klett, 1981[Klett, , 1985 Fernald, 1984;Sasano, 1984] [Ansmann et al., 2001;Pérez-Ramírez et al., 2008Berkoff et al., 2011; Barreto et 79 al., 2013 Barreto et 79 al., , 2016 Barreto et 79 al., , 2017. The AOD measurements at night provide information to 80 discriminate between the extinction of fine and coarse mode [O'Neill et al., 2003] brighter. This information could be combined with the spectral AOD, also obtained 91 from the Moon (lunar photometry) to retrieve aerosol characteristics. 92A sky camera can be used to obtain relative sky radiance near the Moon since it 93 records the full hemispherical sky radiance measuring different wavelength intervals 94 and it can operate at night with an appropriate exposure time (ET). It should be noted 95 also that sky cameras usually present a low signal to noise ratio. Cloud detection is the 96 most spread use of sky cameras [Long et al., 2006; Calbó and Sabburg, 2008; Cazorla et 97 al., 2008a; Ghonima et al., 2012;Kazantzidis et al., 2012; Mandat et al., 2014, Alonso 98 et al., 2014 though they have been used with other purposes [Horváth et al., 2002; 99 Cazorla et al., 2008b;Kreuter et al., 2009; Sigernes et al., 2014], including the 100 retrieval of sky radiances in daytime [Voss and Zibordi, 1989; López-Alvarez et al., 101 2008; Román et al., 2012; Toshing et al., 2013; Chauvin et al., 2015]. The CCD spectral response, given by the manufacturer, is shown in Fig. 1a. This 183 response is only available up to 700 nm, but red channel seems to have certain 184 sensitivity to longer wavelengths. As mentioned above, the camera also co...
Abstract. The aim of this paper is to analyze the suitability of the high-mountain stations Mauna Loa and Izaña for Langley plot calibration of Sun photometers. Thus the aerosol optical depth (AOD) characteristics and seasonality, as well as the cloudiness, have been investigated in order to provide a robust estimation of the calibration uncertainty as well as the number of days that are suitable for Langley calibrations. The data used for the investigations belong to the AERONET and GAW-PFR networks, which maintain reference Sun photometers at these stations with long measurement records: 22 years at Mauna Loa and 15 years at Izaña. In terms of clear-sky and stable aerosol conditions, Mauna Loa (3397 m a.s.l.) exhibits on average 377 Langley plots (243 morning and 134 afternoon) per year suitable for Langley plot calibration, whereas Izaña (2373 m a.s.l.) shows 343 Langley plots (187 morning and 155 afternoon) per year. The background AOD (500 nm) values, on days that are favorable for Langley calibrations, are in the range 0.01–0.02 throughout the year, with well-defined seasonality that exhibits a spring maximum at both stations plus a slight summer increase at Izaña. The statistical analysis of the long-term determination of extraterrestrial signals yields to a calibration uncertainty of ∼ 0.25–0.5 %, this uncertainty being smaller in the visible and near-infrared wavelengths and larger in the ultraviolet wavelengths. This is due to atmospheric variability produced by changes in several factors, mainly the AOD. The uncertainty cannot be reduced based only on quality criteria of individual Langley plots and averaging over several days is shown to reduce the uncertainty to the needed levels for reference Sun photometers.
Abstract. Arid regions are a major source of mineral dust aerosol. Transport from these sources can have a great impact on aerosol climatology in distant regions. In order to assess the impact of dust on climate we must understand how dust properties change after long distance transport from sources. This study addresses the changes in columnar aerosol properties when mineral dust outbreaks from western Africa arrive over the eastern Caribbean after transport across the Atlantic Ocean, a transit of 5–7 days. We use data from the NASA Aerosol Robotic Network (AERONET) located at five Caribbean and two western Africa sites to characterize changes in columnar aerosol properties: aerosol optical depth (AOD), size distribution, single scattering albedo, and refractive indexes. We first characterized the local aerosol climatology at each site and then using air mass back trajectories we identified those days when trajectories over Caribbean sites back-tracked to western Africa. Over the period 1996–2014 we identify 3174 days, an average of 167 days per year, when the air mass over the Caribbean sites could be linked to at least one of the two western Africa sites. For 1162 of these days, AOD data are available for the Caribbean sites as well as for the corresponding western Africa sites about 5–7 days earlier, when the air mass passed over these sites. We identified dust outbreaks as those air masses yielding AOD ≥ 0.2 and an Ångström exponent below 0.6. On this basis of the total 1162 days, 484 meet the criteria for mineral dust outbreaks. We observe that the AOD at 440 nm decreases by about 0.16 or 30 % during transport. The volume particle size distribution shows a similar decrease in the volume concentration, mainly in the coarse mode. The single scattering albedo, refractive indexes, and asymmetry factor remain unchanged. The difference in the effective radius over western Africa sites with respect to Caribbean sites ranges between 0 and −0.3 µm. Finally we conclude that in about half of the cases only non-spherical dust particles are present in the atmosphere over the western Africa and Caribbean sites, while in the other cases dust particles were mixed with other types of aerosol particles.
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