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São José dos Campos, Brazil

Silva A.A.,Institute Estudos Avancados
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2011

Clouds are the main tropospheric agent in the modulation of solar ultraviolet (UV) radiation in the Earth's surface. Clouds produce both attenuation and enhancement of radiation. This work shows how the local cloud cover (LCC) can partially explain mismatches between overpass satellite and ground-based instruments in the erythemal dose rate (EDR) for an urban, tropical site in Southern Hemisphere. Best agreement occurs for the scarcer cloud cover of winter. The Ozone Monitoring Instrument (OMI) EDR is on average 15% higher than ground-based measurements for cloudless skies. This value is due to both OMI algorithm misconception about aerosols and instrumental uncertainty. © 2011 Elsevier Ltd.

Garcia R.D.M.,Institute Estudos Avancados
Journal of Physics: Conference Series | Year: 2012

A couple of issues raised in a recent work on Fresnel boundary and interface conditions for polarized radiative transfer are discussed. The first issue concerns a normalization that has to be performed on the transmission matrix relating the transmitted and incident Stokes vectors at a smooth interface between different materials. It was found that the normalization step is not performed in a correct way in many existing works and so the effects of the two most frequent discrepancies encountered in these works are evaluated for a simple model of an atmosphere-water system. The second issue has to do with a choice of the sign of the parallel component of the amplitude coefficient of reflection R l when constructing the reflection matrix that relates the reflected and incident Stokes vectors at a smooth interface. A computational approach based on the limit to the case with constant index of refraction is used to determine the appropriate sign of R l for polarized radiative transfer calculations in multilayers with different indices of refraction.

Silva A.A.,Institute Estudos Avancados
Journal of Geophysical Research: Atmospheres | Year: 2013

Three Brewer spectrophotometers were set up in three tropical sites of South America (in the Bolivian Altiplano and seashore and biomass burning areas of Brazil) to measure the total ozone column (TOC). Only TOC measurements with uncertainties ≤1% (1σ) were considered. Typically, the standard deviation for the diurnal sets of measurements was predominantly ≤1% for two of these sites. The average variability in TOC ranged from 6.3 Dobson units (DU) to 16.8 DU, and the largest variability reached 54.3 DU. Comparisons between ground-based and satellite (Total Ozone Mapping Spectrometer (TOMS)) data showed good agreement with coefficients of determination ≤0.83. However, the quality of the ground-based measurements was affected by the weather condition, especially for one of the sites. Visual observation of the sky from the ground during the measurements with one of the Brewers added to the satellite data of reflectivity and aerosol index supports that statement. Key Points Three Brewers operating simultaneously in three South American sites Diurnal variability in the total ozone column (TOC) reaches 54.3 DU The analysis of TOC measurements is supported by the observation of the weather ©2013. American Geophysical Union. All Rights Reserved.

Silva A.A.,Institute Estudos Avancados
Photochemical and Photobiological Sciences | Year: 2013

The incidence of ultraviolet radiation (UVR) under noon overcast skies was investigated in an urban, tropical site. Overcast skies stimulate people to be outdoors more freely and carelessly than under a cloudless scenario, thereby representing a situation with UVR. 153 occurrences of noon overcast skies were reported for erythemal dose rates (EDR) of up to 0.395 W m-2 (Extreme UV-Index) and average of 0.121 W m-2 (Moderate UVI). Erythemal doses varied from 0.4 to 21.6 Standard Erythema Dose (SED) units and averaged 7.7 SED. 34% of the occurrences of Extreme UVI were events of radiation enhancement up to 19% above the summer peak value of 0.332 W m-2. The elapsed time for such events ranged from 2 to 7 minutes and averaged 4.5 minutes. Cloud genera referring to EDR were predominantly Cu followed by As in summer, and Cu plus Sc and As in the other seasons. For events of radiation enhancement, cloud genera featured a reduction in the occurrence of Cu and an increase in the occurrence of As and Ac. © 2013 The Royal Society of Chemistry and Owner Societies.

Silva A.A.,Institute Estudos Avancados
Photochemical and Photobiological Sciences | Year: 2015

The diffuse (Dif) component of ultraviolet radiation (UVR) plays an important role in the daily exposure of humans to solar radiation. This study proposes a semi-empirical method to obtain the Dif component of the erythemal dose rate, or the erythemally weighted irradiance, (EDRDif) calculated from synchronized measurements of the Dif component of UVR (UVDif) and the global (G) irradiances of both UVR (UVG) and the erythemal dose rate (EDRG). Since the study was conducted in the tropics, results involve a wide range of solar zenith angles to which EDRDif is seasonally dependent. Clouds are the main atmospheric agent affecting Dif radiation. The ratio between Dif and G (Dif/G) showed a quadratic dependence on cloud cover with a coefficient of determination r2 = 0.79. The maxima of EDRDif were mainly above the moderate range (>137.5 mW m-2) of the UV-Index and reached the extreme range (>262.5 mW m-2) for the spring-summer period. The fraction of the global daily erythemal dose (daily EDG) corresponding to Dif radiation (daily EDDif) ranged from 936 J m-2 to 5053 J m-2 and averaged 2673 J m-2. Daily EDDif corresponded to at least 48% of daily EDG for a practically cloudless sky. Therefore, Dif radiation is a real threat. Lighter skin people (types I and II) can get sunburnt in a couple of minutes under such an incidence of radiation. Moreover, accumulative harm can affect all skin types. © 2015 The Royal Society of Chemistry and Owner Societies.

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