Technical Note: Determination of aerosol optical properties by a calibrated sky imager

Cazorla, A.; Shields, Janet E.; Karr, Monette E.; Olmo, F.J.; Burden, Art R.; Alados-Arboledas, L.

doi:10.5194/acp-9-6417-2009

Cited by 44 publications

(33 citation statements)

References 20 publications

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“…In addition, the MABE parameters for these two calibration techniques present values around 5% which indicates a good agreement between measured and modeled UVI data at Granada. This result agrees with other inter-comparison works using the LibRadtran/UVSPEC code and experimental data (e.g., Mayer et al, 1997;Kylling et al, 1998;de Backer et al, 2001). The uncertainty of MBE and MABE parameters given by their standard error is always lower than 0.7%, indicating the high statistical significance of the reported values.…”

Section: Experimental Uv Indexsupporting

confidence: 82%

“…It provides images of the whole sky dome in daytime, and it has been commonly used for atmospheric radiative transfer studies (Cazorla et al, 2008(Cazorla et al, , 2009. The All-Sky Imager is a custom adaptation of a scientific CCD camera, using a digital color video camera mounted with a fish-eye lens (180 • FOV) pointing to the zenith.…”

Section: Ground-based Measurementsmentioning

confidence: 99%

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Influence of the calibration on experimental UV index at a midlatitude site, Granada (Spain)

et al. 2011

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Abstract. The ultraviolet index (UVI) is the most commonly used variable to inform about the level and potential harmful effect of ultraviolet (UV) radiation reaching the Earth's surface. This variable is derived from the output signal of UV radiometers applying conversion factors from calibration methods. This paper focused on the influence of the use of two of these methods (called one-step and two-steps methods) on the experimental UVI measured by a YES UVB-1 radiometer located in a midlatitude station, Granada (Spain) for the period 2006-2009. In addition, it also analyzes the deviation from the UVI values obtained when the manufacturer's calibration factors are applied. For this goal, a detailed characterization of the UVB-1 radiometer from the first Spanish calibration campaign of broadband UV radiometers at the "El Arenosillo" INTA station in 2007 was used. In addition, modeled UVI data derived from the LibRadtran/UVSPEC radiative transfer code are compared with the experimental values recorded at Granada for cloud-free conditions. Absolute mean differences between measured and modeled UVI data at Granada were around 5% using the onestep and two-steps calibration methods, indicating an excellent performance of these two techniques for obtaining UVI data from the UVB-1 radiometer. Conversely, the application of the manufacture's calibration factor produced a large overestimation (∼14%) of the UVI values, generating unreliable alarming high UVI data in summer. Thus, the number of days with an extreme erythemal risk (UVI higher than 10) increased up to 46% between May and September at Granada. This percentage reduced to a more reliable value of 3% when the conversion factors obtained with the two-steps calibration Correspondence to: M. Antón (mananton@unex.es) method are used. These results evidence the need for a sound calibration of the broadband UV instruments in order to obtain reliable measurements.

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Section: Experimental Uv Indexsupporting

confidence: 82%

Section: Ground-based Measurementsmentioning

confidence: 99%

Influence of the calibration on experimental UV index at a midlatitude site, Granada (Spain)

et al. 2011

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“…projecting the polarized sky hemisphere onto a camera chip with a fisheye objective (see e.g. North and Duggin, 1997;Liu and Voss, 1997;Pust and Shaw, 2008;Cazorla et al, 2009;Kreuter et al, 2009). This method will be the subject of this study and we want to explore its feasibility for aerosol characterization.…”

Section: Introductionmentioning

confidence: 99%

Feasibility of polarized all-sky imaging for aerosol characterization

Kreuter

Blumthaler

2013

Atmos. Meas. Tech.

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In this study, we investigate the method of polarized all-sky imaging with respect to aerosol characterization. As a technical frame work for image processing and analysis, we propose Zernike polynomials to decompose the relative Stokes parameter distributions. This defines a suitable and efficient feature vector which is also appealing because it is independent of calibration, circumvents overexposure problems and is robust against pixel noise. We model the polarized radiances of realistic aerosol scenarios and construct the feature vector space of the key aerosol types in terms of the first two principal components describing the maximal variances. We show that, using this representation, aerosol types can be clearly distinguished with respect to fine and coarse mode dominated size distribution and index of refraction. We further investigate the individual influences of varying aerosol properties and solar zenith angle. This suggests that polarized all-sky imaging may improve aerosol characterization in combination with sky scanning radiometers of the existing Aerosol Robotic Network (AERONET) especially at low aerosol optical depths and low solar zenith angles

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“…The semidirect effect describes the warming of the boundary layer, through the absorption of radiation by aerosols, which can lead to evaporation of clouds. This will allow more solar radiation to reach the surface (Ramanathan et al, 2001;Cazorla et al, 2009). The indirect effect concerns the ability of aerosols to act as cloud condensation nuclei which influences the microphysical and optical properties of clouds, thus changing the radiative and precipitation properties and the lifetime of clouds (Ramanathan et al, 2001;Kaufman et al, 2002;Lohmann, 2002;Lohmann and Feichter, 2005;Unger et al, 2009).…”

Section: Introductionmentioning

confidence: 99%

“…To gain a better understanding of the effect of aerosols in the UV, knowledge of the parameters that determine the optical and physical properties of aerosols is essential (Cazorla et al, 2009;Kazadzis et al, 2009). One of these parameters is the Aerosol Optical Depth (AOD), an integral measurement of the combined aerosol scattering and absorption in the atmospheric column (Mulcahy et al, 2009).…”

Section: Introductionmentioning

confidence: 99%

Aerosol Optical Depth measurements at 340 nm with a Brewer spectrophotometer and comparison with Cimel sunphotometer observations at Uccle, Belgium

et al. 2010

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Abstract. The Langley Plot Method (LPM) is adapted for the retrieval of Aerosol Optical Depth (AOD) values at 340 nm from Brewer#178 sun scan measurements between 335 and 345 nm (convoluted with the band pass function of the Cimel sunphotometer filter at 340 nm) performed in Uccle, Belgium. The use of sun scans instead of direct sun measurements simplifies the comparison of the AOD values with quasi-simultaneous Cimel sunphotometer values. Also, the irradiance at 340 nm is larger than the one at 320.1 nm due to lower ozone absorption, thus improving the signal to noise ratio. For the selection of the cloudless days (from now on referred to as calibration quality clear days), a new set of criteria is proposed. With the adapted method, individual clear sky AOD values, for which the selection criteria are also presented in this article, are calculated for a period from September 2006 until the end of August 2010. These values are then compared to quasi-simultaneous Cimel sunphotometer measurements, showing a very good agreement (the correlation coefficient, the slope and the intercept of the regression line are respectively 0.974, 0.968 and 0.011), which proves that good quality observations can be obtained from Brewer sun scan measurements at 340 nm. The analysis of the monthly and seasonal Brewer AODs at Uccle is consistent with studies at other sites reporting on the seasonal variation of AODs in Europe. The highest values can be observed in summer and spring, whereas more than 50% of the winter AODs are lower than 0.3. On a monthly scale, the lowest AOD are observed in December and the highest values occur in June and April. No clear weekly cycle is observed for Uccle. The current cloud-screening algorithm is still an issue, Correspondence to: V. De Bock (veerle.debock@meteo.be) which means that some AOD values can still be influenced by scattered clouds. This effect can be seen when comparing the calculated monthly mean values of the Brewer with the AERONET measurements.

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Technical Note: Determination of aerosol optical properties by a calibrated sky imager

Cited by 44 publications

References 20 publications

Influence of the calibration on experimental UV index at a midlatitude site, Granada (Spain)

Influence of the calibration on experimental UV index at a midlatitude site, Granada (Spain)

Feasibility of polarized all-sky imaging for aerosol characterization

Aerosol Optical Depth measurements at 340 nm with a Brewer spectrophotometer and comparison with Cimel sunphotometer observations at Uccle, Belgium

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