Surface UV Irradiance Research Articles

Application of satellite and ground-based data to investigate the UV radiative effects of Australian aerosols

An understanding of the effect of aerosols on biologically- and photochemically-active UV radiation reaching the Earth's surface is important for many ongoing climate, biophysical, and air pollution studies. In particular, estimates of the UV characteristics of the most common Australian aerosols will be valuable inputs to UV Index forecasts, air quality studies, and assessments of the impact of regional environmental changes. By analyzing climatological distributions of Australian aerosols we have identified sites where co-located ground-based UV-B and ozone measurements were available during episodes of relatively high aerosol activity. Since at least June 2003, surface UV global irradiance spectra (285–450 nm) have been measured routinely at Darwin and Alice Springs in Australia by the Australian Bureau of Meteorology (BoM). Using co-located sunphotometer measurements at Darwin and Alice Springs, we identified several episodes of relatively high aerosol activity. Aerosol air mass types were analyzed from sunphotometer-derived angstrom parameter, MODIS fire maps and MISR aerosol property retrievals. To assess aerosol effects we compared the measured UV irradiances for aerosol-loaded and clear-sky conditions with each other and with irradiances simulated using the libRadtran radiative transfer model for aerosol-free conditions. We found that for otherwise similar atmospheric conditions, smoke aerosols over Darwin reduced the surface UV irradiance by as much as 40–50% at 290–300 nm and 20–25% at 320–400 nm near active fires (aerosol optical depth, AOD, at 500 nm ∼ 0.6). Downwind of fires, the smoke aerosols over Darwin reduced the surface irradiance by 15–25% at 290–300 nm and ∼ 10% at 320–350 nm (AOD at 500 nm ∼ 0.2). The effect of smoke increased with decrease of wavelength and is strongest in the UV-B. The aerosol attenuation factors calculated for the selected cases suggest smoke over Darwin has an effect on surface 340–380 nm irradiances that is comparable to that produced by smoke over Sub-Saharan Africa. Dust activity was very low at Alice Springs during 2004, therefore we were not able to identify strong dust events to fully assess the UV effect of dust. For the cases studied, smoke aerosols seem to produce a stronger reduction in surface UV irradiances than dust aerosols.

Investigation of aerosol components influencing atmospheric transfer of UV radiation in Baltic Sea region

Linking of atmospheric aerosol size distributions and optical properties via predefined aerosol components was investigated. The measured aerosol volume distributions were decomposed to Optical Properties of Aerosols and Clouds (OPAC) components, and aerosol optical properties were calculated for a mixture of those components. The obtained aerosol optical properties were then used for modeling the surface UV irradiances with the libRadtran radiative transfer code. The results were verified with the columnar aerosol characteristics obtained from Aerosol Robotic Network (AERONET) station Tõravere (58.26°N, 26.46°E) and clear‐sky surface UV measurements in Pärnu, Estonia (58.38°N, 24.51°E). The best decomposition results were obtained with four OPAC components, when their lookup characteristics varied within ±10%. Variation of aerosol optical properties in 17 days was influenced by the following aerosol components: soot, 1.2 ± 1.4%; insoluble, 23.1 ± 8.3%; water‐soluble, 44.0 ± 10.8%; accumulation mode sea salt, 31.6 ± 6.2% of total aerosol volume. The average refractive index (for λ = 440 nm) of the component mixture was of 1.42 − 0.013i. Interpretation of the soot component was disputable, since similarly high soot concentrations corresponded to the secondary particles in polluted atmosphere and the nucleation bursts in clean atmosphere. The sea‐salt component showed a correlation with the aerosol residence time over sea. The water‐soluble component and the additional “biomass haze” component represented partly the same aerosol volume in the diameter range of 0.18–1.8 μm. The surface UV irradiances modeled with the AERONET data and the fitted aerosol components were highly correlated with each other, but both model results underestimated the UV extinction by aerosol.

Deriving surface ultraviolet radiation from CERES surface and atmospheric radiation budget: Methodology

We describe an algorithm that retrieves the surface UVB (280–315 nm) and UVA (315–400 nm) irradiances from the Surface and Atmosphere Radiation Budget (SARB) product of Clouds and the Earth's Radiant Energy System (CERES). The SARB product we use here routinely calculates the vertical profiles of shortwave, longwave, and window channel irradiances with inputs of retrievals from imagers collocated with CERES. The top of the atmosphere broadband irradiance from SARB is constrained by CERES broadband irradiance. The shortwave spectrum in the SARB calculation is divided into 15 bands, and the two ultraviolet spectral bands, band 5 (298.5–322.5 nm) and band 6 (322.5–357.5 nm), are used to generate surface UVB and UVA irradiances. In this study, we develop a set of ratio lookup tables to derive surface UVB and UVA irradiances from SARB band 5 and band 6 outputs. We show that the ratio of band 5 to UVB irradiance is sensitive to total column ozone, solar zenith angle, surface albedo, and the atmospheric profile in cloud‐free conditions; in cloudy conditions, the ratio of band 5 to UVB irradiance is also sensitive to cloud optical depth and height. Additionally, we show that the ratio of band 6 to UVA irradiance is sensitive to solar zenith angle, surface albedo, and cloud optical depth. We also derive a UV index from the UVB irradiance. Our algorithm may be applied at any surface elevation or surface type, including snow and ice. Surface UV irradiances derived from the lookup table that we created agree well with those computed by the high‐resolution, multistream radiative transfer code, with differences ranging from −10% to +4% for UVB and UVA irradiances. The relative differences for the UV index are higher, ranging from −26% to +16%.

Climatology of surface ultraviolet-radiation in Valparaiso, Chile

Abstract Despite the lack of long-term records, it is possible to describe many of the short term characteristics, dependencies and climatology of surface UV irradiance. This paper describes the climatology of on ground UV irradiance at Valparaiso (33.05°S, 71.63°W, sea level), Chile. The dependence of UV-B irradiance on ozone and on other climate variables is discussed with reference to our observations conducted during the last four years. Special attention was paid to detect “ozone events” by surface UV irradiance measurements. By analyzing time series of the UV-B/UV-A ratio, we suppressed the cloud variability effect and detected events that implied ozone column changes of about 15%. According to our measurements, during the last four years, the ozone column over Valparaiso was not affected negatively by the Antarctic ozone hole phenomenon.

Fast simulation tool for ultraviolet radiation at the earth’s surface

FastRT is a fast, yet accurate, UV simulation tool that computes downward surface UV doses, UV indices, and irradiances in the spectral range 290 to 400 nm with a resolution as small as 0.05 nm. It computes a full UV spectrum within a few milliseconds on a standard PC, and enables the user to convolve the spectrum with user-defined and built-in spectral response functions including the International Commission on Illumination (CIE) erythemal response function used for UV index calculations. The program accounts for the main radiative input parameters, i.e., instrumental characteristics, solar zenith angle, ozone column, aerosol loading, clouds, surface albedo, and surface altitude. FastRT is based on look-up tables of carefully selected entries of atmospheric transmittances and spherical albedos, and exploits the smoothness of these quantities with respect to atmospheric, surface, geometrical, and spectral parameters. An interactive site, http://nadir.nilu.no/~olaeng/fastrt/fastrt.html, enables the public to run the FastRT program with most input options. This page also contains updated information about FastRT and links to freely downloadable source codes and binaries.

Aerosol ultraviolet absorption experiment (2002 to 2004), part 1: ultraviolet multifilter rotating shadowband radiometer calibration and intercomparison with CIMEL sunphotometers

Radiative transfer calculations of UV irradiance from total ozone mapping spectrometer (TOMS) satellite data are frequently over- estimated compared to ground-based measurements because of the presence of undetected absorbing aerosols in the planetary boundary layer. To reduce these uncertainties, an aerosol UV absorption closure experiment has been conducted at the National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC) site in Greenbelt, Maryland, using 17 months of data from a shadowband radi- ometer (UV-multifilter rotating shadowband radiometer (UV-MFRSR), U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Network) colocated with a group of three sun-sky CIMEL radiometers (rotating reference instruments of the NASA Aerosol Robotic Network (AERONET)). We describe an improved UV-MFRSR on-site calibration method augmented by AERONET-CIMEL measurements of aerosol ex- tinction optical thickness (t a) interpolated or extrapolated to the UV- MFRSR wavelengths and measurement intervals. The estimated t a is used as input to a UV-MFRSR spectral-band model, along with indepen- dent column ozone and surface pressure measurements, to estimate zero air mass voltages V0 in three longer wavelength UV-MFRSR chan- nels (325, 332, 368 nm). Daily mean ^V0&, estimates and standard de- viations are obtained for cloud-free conditions and compared with the on-site UV-MFRSR Langley plot calibration method. By repeating the calibrations on clear days, relatively good stability (62% in ^V0& )i s found in summer, with larger relative changes in fall-winter seasons. The changes include systematic day-to-day ^V0& decline for extended peri- ods along with step jump changes after major precipitation periods (rain or snow) that affected the diffuser transmission. When daily ^V0& values are used to calculate t a for individual 3-min UV-MFRSR measurements on the same days, the results compare well with interpolated AERONET t a measurements (at 368 nm most daily 1s root mean square (rms) differences were within 0.01). When intercalibrated against an AERO- NET sunphotometer, the UV-MFRSR is proven reliable to retrieve t a , and hence can be used to retrieve aerosol column absorption in the UV. The advantage of the shadowband technique is that the calibration ob- tained for direct-sun voltage can then be applied to diffuse-radiance volt- age to obtain total and diffuse atmospheric transmittances. These trans- mittances, in combination with accurate t a data, provide the basis for estimating aerosol column absorption at many locations of the USDA UV-B Monitoring and Research network and for correction of satellite estimations of surface UV irradiance. © 2005 Society of Photo-Optical Instru-

Deriving an effective aerosol single scattering albedo from spectral surface UV irradiance measurements

Abstract The modification of clear-sky global and diffuse irradiances and the direct-to-diffuse irradiance ratio by the aerosol single scattering albedo, ω , as a function of aerosol optical depth, τ a , and solar zenith angle (SZA) is investigated using radiative-transfer model calculations. The model-derived relations are combined with UV irradiances at the surface and the aerosol optical depth measured with a Brewer MkIII spectroradiometer, with the aim to develop and test an indirect method of estimating an effective single scattering albedo. The uncertainties introduced from different sources are quantitatively discussed in order to determine the overall uncertainty of the method. The overall accuracy in determining ω indirectly depends strongly on the radiation quantity used and the amount of aerosols, increasing the uncertainty of the method in situations with τa at 340 nm below 0.2. From the three radiation quantities, the direct-to-diffuse irradiance ratio provides the highest accuracy in the estimation of the effective ω . As an example, the effective ω is determined for 2 days with different aerosol loadings and composition. Finally, 5 years of measurements of global irradiance and τ a under cloud-free conditions are analysed in order to estimate the range of variation of the effective ω at Thessaloniki, Greece. For τ a at 340 nm larger than 0.8, ω ranges between 0.85 and 0.99, while for lower-aerosol optical depths the derived effective ω covers the entire range from 0.64 to 0.99.

Improved sky imaging for studies of enhanced UV irradiance

Abstract. A recent World Meteorological Organisation report discussed the importance of continued study of the effect of clouds on the solar UV radiation reaching the earth's surface. The report mentions that the use of all-sky imagery offers the potential to understand and quantify cloud effects more accurately. There are an increasing number of studies investigating the enhancement of surface solar, UV irradiance, and UV actinic flux, using automated CCD and sky imagers. This paper describes new algorithms applicable to a commercially available all-sky imager (TSI-440), for research investigating cloud enhanced spectral UV irradiance. Specifically, these include three new algorithms relating to cloud amount at different spatial positions from 1) zenith and 2) from the solar position and 3) the visible brightness of clouds surrounding the sun. A possible relationship between UV enhancement and the occurrence of near-sun cloud brightness is reported based on this preliminary data. It is found that a range of wavelength dependent intensities, from 306 to 400 nm, can occur in one day for UV enhancements. Evidence for a possible decreasing variation of intensity with longer wavelengths is also presented.

Modelling the ultraviolet environment at the surface of Mars and design of the Beagle 2 UV sensor

This thesis describes a multi-layer radiative transfer UV model which was used to aid in the design of the UV sensor on Beagle 2, which will soon provide the first ever in situ measurement of UV flux at the martian surface. The model uses the delta-Eddington approximation for diffuse flux and new low temperature gas absorption cross-sections and aerosol optical properties. Dust, H2O clouds and morning fogs are found to modify the martian surface UV spectrum. Dust storms have been shown to attenuate the surface UV flux by more than an order of magnitude, though some UV persists even at extremely high optical depths. The seasonal variation of surface UV irradiance was found to produce maximum exposure areas highly dependent upon dust activity over the martian year. Dust activity is also shown to distort the annual latitudinal dose, with high dust loading in the southern hemisphere resulting in a higher annual dose than in the north. The introduction of O3 abundances of 1.64 x 1017cm-2 into the model resulted in only partial protection for micro-organisms, since wavelengths shorter than 230 nm still penetrate to the surface. DNA-weighting of a martian UV spectrum shows the surface UV environment to be 103 times more damaging than on Earth. UV detection signatures of atmospheric phenomena and anticipated events for the case of Beagle 2 are presented: a dust devil encounter creates a double minimum profile in the UV flux, and solar eclipses by Phobos produce a single minimum. Clouds increase the diffuse/direct UV ratio, and fogs create a distinct dip in the morning profile when normalised to clear days.

The variability of Mercury's exosphere by particle and radiation induced surface release processes

Mercury's close orbit around the Sun, its weak intrinsic magnetic field and the absence of an atmosphere ( P surface<1×10 −8 Pa) results in a strong direct exposure of the surface to energetic ions, electrons and UV radiation. Thermal processes and particle-surface-collisions dominate the surface interaction processes leading to surface chemistry and physics, including the formation of an exosphere ( N⩽10 14 cm −2) in which gravity is the dominant force affecting the trajectories of exospheric atoms. NASA's Mariner 10 spacecraft observed the existence of H, He, and O in Mercury's exosphere. In addition, the volatile components Na, K, and Ca have been observed by ground based instrumentation in the exosphere. We study the efficiency of several particle surface release processes by calculating stopping cross-sections, sputter yields and exospheric source rates. Our study indicates surface sputter yields for Na between values of about 0.27 and 0.35 in an energy range from 500 eV up to 2 keV if Na + ions are the sputter agents, and about 0.037 and 0.082 at an energy range between 500 eV up to 2 keV when H + are the sputter agents and a surface binding energy of about 2 eV to 2.65 eV. The sputter yields for Ca are about 0.032 to 0.06 and for K atoms between 0.054 to 0.1 in the same energy range. We found a sputter yield for O atoms between 0.025 and 0.04 for a particle energy range between 500 eV up to 2 keV protons. By taking the average solar wind proton surface flux at the open magnetic field line area of about 4×10 8 cm −2 s −1 calculated by Massetti et al. (2003, Icarus, in press) the resulting average sputtering flux for O is about 0.8–1.0×10 7 cm −2 s −1 and for Na approximately 1.3–1.6×10 5 cm −2 s −1 depending on the assumed Na binding energies, regolith content, sputtering agents and solar activity. By using lunar regolith values for K we obtain a sputtering flux of about 1.0–1.4×10 4 cm −2 s −1. By taking an average open magnetic field line area of about 2.8×10 16 cm 2 modelled by Massetti et al. (2003, Icarus, in press) we derive an average surface sputter rate for Na of about 4.2×10 21 s −1 and for O of about 2.5×10 23 s −1. The particle sputter rate for K atoms is about 3.0×10 20 s −1 assuming lunar regolith composition for K. The sputter rates depend on the particle content in the regolith and the open magnetic field line area on Mercury's surface. Further, the surface layer could be depleted in alkali. A UV model has been developed to yield the surface UV irradiance at any time and latitude over a Mercury year. Seasonal and diurnal variations are calculated, and Photon Stimulated Desorption (PSD) fluxes along Mercury's orbit are evaluated. A solar UV hotspot is created towards perihelion, with significant average PSD particle release rates and Na fluxes of about 3.0×10 6 cm −2 s −1. The average source rates for Na particles released by PSD are about 1×10 24 s −1. By using the laboratory obtained data of Madey et al. (1998, J. Geophys. Res. 103, 5873–5887) for the calculation of the PSD flux of K atoms we get fluxes in the order of about 10 4 cm −2 s −1 along Mercury's orbit. However, these values may be to high since they are based on idealized smooth surface conditions in the laboratory and do not include the roughness and porosity of Mercury's regolith. Further, the lack of an ionosphere and Mercury's small, temporally and spatially highly variable magnetosphere can result in a large and rapid increase of exospheric particles, especially Na in Mercury's exosphere. Our study suggests that the average total source rates for the exosphere from solar particle and radiation induced surface processes during quiet solar conditions may be of the same order as particles produced by micrometeoroid vaporization. We also discuss the capability of in situ measurements of Mercury's highly variable particle environment by the proposed NPA-SERENA instrument package on board ESA's BepiColombo Mercury Planetary Orbiter (MPO).

A new approach to estimating the albedo for snow‐covered surfaces in the satellite UV method

This paper describes a new method for estimating snow albedo for satellite retrieval of surface UV irradiance and daily UV doses over snow‐covered terrains. The method is based on combining satellite and meteorological analysis data. The satellite data exploited in this work are the measured reflectivities of the Total Ozone Mapping Spectrometer/Nimbus 7 instrument that coincides with the European Centre for Medium‐Range Weather Forecasts ERA‐15 reanalyzed meteorological data. We compared satellite‐retrieved UV daily doses to the ground‐based measurements of two Finnish and five Canadian sites. The comparison clearly showed that the new snow albedo approach improves the accuracy of the satellite‐retrieved UV doses.

Actinic flux determination from measurements of irradiance

The actinic flux describes the number of photons incident at a point, while the irradiance describes the radiant energy crossing a surface. An algorithm for conversion of irradiance to downwelling actinic flux has been developed and tested. The algorithm uses a simple method to distinguish between a cloudy and a cloudless sky. It is necessary to separate cloudy and cloudless situations, as the irradiance to actinic flux conversion depends on the radiance field which is rather different for cloudy and cloudless skies. Surface UV irradiance and downwelling actinic flux spectra were measured at five European locations which were representative of different climates. A total of 9015 synchronized actinic flux and irradiance spectra were available to test the proposed algorithm. The measured irradiance spectra were used to estimate downwelling actinic flux spectra. The estimated actinic flux spectra were compared with the measured actinic flux spectra for all cloud and aerosol situations encountered, a wide range of solar zenith angles, and surface conditions. The average ratio of the reproduced to measured downwelling actinic flux is 1.021 ± 0.085 in the UV‐B and 1.015 ± 0.105 in the UV‐A. In general, the performance of the algorithm is better at smaller solar zenith angles.

Aerosol single scattering albedo retrieved from measurements of surface UV irradiance and a radiative transfer model

Aerosol single scattering albedo (ω), the ratio of aerosol scattering coefficient to total aerosol extinction coefficient, at UV wavelengths is an important aerosol radiative parameter in determining surface UV irradiance. Surface measurements of total and diffuse UV irradiance in the summer and fall of 1999 at the seven narrowband wavelength channels of an UV multifilter rotating shadowband radiometer (UVMFR‐SR) at Black Mountain, N. C., were coupled with a tropospheric ultraviolet radiative transfer model to produce values of ω. Its value ranged from 0.65 to 0.91 at 300 nm, 0.71 to 0.96 at 305.5 nm, 0.73 to 0.97 at 311.4 nm, 0.74 to 0.91 at 317.6 nm, 0.76 to 0.96 at 325.4 nm, 0.77 to 0.97 at 332.4 nm, and 0.80 to 0.99 at 368 nm. Error in this procedure decreases with increasing aerosol optical depth (AOD), from ±0.63 at AOD = 0.05 to ±0.04 at AOD = 1.0 averaged over the seven wavelengths. The current values of ω have a slightly wider variation than values reported from a previous study at the same site. The lower values of ω could indicate that, over the site, preferential absorption of UV radiation by black carbon aerosols could be occurring. More values of ω in the UV spectrum will allow for better estimation of this parameter for UV radiative transfer modeling and will lessen error in estimation of surface UV irradiances.

Ozone profile differences between Europe and New Zealand: Effects on surface UV irradiance and its estimation from satellite sensors

Substantial differences in summertime surface UV irradiance between New Zealand and Europe have been reported previously. The main contributors to these differences are differences in total column ozone, differences in Sun‐Earth separation between the Southern Hemisphere summer (December/January) and the Northern Hemisphere summer (June/July), and differences in tropospheric pollution. Differences due to higher Northern Hemisphere aerosol extinction have been well documented. Here we present a climatology of ozone profiles measured by balloon‐borne sensors launched from Lauder, New Zealand, and from Hohenpeissenberg, Germany, to show that there are also substantial differences in the shapes of ozone profiles, especially during the summer months. A radiative transfer model is then used to investigate the extent to which these profile shape differences contribute to observed UV differences. It is found that the differences in the vertical distribution of ozone amplify existing Northern and Southern Hemisphere differences in UV. Effects can be significant in the UV‐B region, but the sign of the effect depends on the solar zenith angle. Consequently, the effect on daily doses of biologically harmful UV radiation is suppressed somewhat, and the differences are responsible for differences in erythemally weighted UV of only a percent or two, which is small compared with other effects. While these effects are relatively small, differences in the vertical profile of ozone can affect the accuracy of satellite retrievals of total column ozone. This analysis shows that the lower tropospheric ozone amounts at Lauder may lead to underestimations of summertime UV from satellite by ∼4% at that site.

Version 2 total ozone mapping spectrometer ultraviolet algorithm: problems and enhancements

Satellite instruments provide global maps of surface UV irradiance by combining backscattered radiance measurements with radiative transfer models. The accuracy of the models is limited by uncertainties in input parameters representing the atmosphere and the Earth's surface. To reduce these uncertainties, we have made enhancements to the currently operational TOMS surface UV irradiance algorithm (Version 1) by including the effects of diurnal variations of cloudiness, an improved treatment of snow/ice, and a preliminary aerosol correction. We compare results of the version 1 TOMS UV algorithm and the proposed version. We evaluate different approaches for improved treatment for average cloud attenuation within a satellite pixel, with and without snow/ ice on the ground. In addition to treating cloud transmission based only on the measurements at the local time of the TOMS observations, the results from other satellites and weather assimilation models can be used to estimate atmospheric UV irradiance transmission throughout the day. A new method is proposed to obtain a more realistic treatment of the effects from snow-covered terrain. The method is based on an empirical relation between UV reflectivity and measured snow depth. The new method reduces the bias between the TOMS UV estimations and ground-based UV measurements for snow periods. We also briefly discuss the complex problem of estimating surface UV radiation in presence of UV-absorbing aerosols. The improved (Version 2) algorithm can be applied to reprocess the existing TOMS UV irradiance and exposure estimates (since November 1978) and to future satellite sensors (e.g., GOME-2, OMI on EOS/Aura, and Triana/EPIC).

A case study of the impact of boundary layer aerosol size distribution on the surface UV irradiance

The relationship between scattering characteristics of surface aerosol and surface UV irradiance was examined on the basis of the measurements carried out in June–August 1999 in Pärnu, Estonia on the Eastern coast of the Baltic Sea (58°22′27″N, 24°30′43″E). The UV radiation spectra (300–340 nm) were measured with the Ocean Optics Inc. UV spectrometer PC1000, the aerosol size distributions (3–10 000 nm) were measured with the electric aerosol spectrometer EAS. A case study was conducted for six sequential cloudless days, when the decrease of the surface UV irradiance was seemingly influenced by atmospheric aerosol. Aerosol radiative properties were calculated from the measured size distributions that represented the maritime polar (North Atlantic) and mixed maritime–continental air. The aerosol optical depths at 500 nm for the North Atlantic air were estimated to be from 0.08 to 0.13. The spectral aerosol optical depth agreed well with the Ångström law, the Ångström exponent α varied from day to day between values of 0.52–0.90. Aerosol asymmetry factor at 300 nm changed between values of 0.76 and 0.80, and was highly correlated with the mean radius of aerosol number distribution. The total aerosol UV scattering was mostly influenced by changes in aerosol with a diameter of 100–560 nm. The aerosol scattering coefficients were positively correlated with the relative humidity of air. The ground aerosol properties were used for calculating the surface UV irradiance from the radiative transfer model of Bird and Riordan (J. Climate Appl. Meteorol. 25 (1986)). The calculated UV irradiances correlated quite well with the measured ones, showing that the use of ground aerosol data for radiative transfer calculations turned out reasonable results. However, ignoring the changes in the aerosol vertical distribution resulted in overestimation of aerosol optical depth on hazy days.

Indicators of UV Exposure in Corals and Their Relevance to Global Climate Change and Coral Bleaching

A compelling aspect of the deterioration of coral reefs is the phenomenon of coral bleaching. Through interactions with other factors such as sedimentation, pollution, and bacterial infection, bleaching can impact large areas of a reef with limited recovery, and it might be induced by a variety of stressors including temperature and salinity extremes, and ultraviolet light. Under conditions of ocean warming, often associated with calm and stratified waters, photobleaching of UV-absorb-ing chromophoric dissolved organic matter (CDOM) is increased, and penetration of both UV-B and UV-A is greatly enhanced. Indices of UV-specific effects in coral tissue are needed to test whether UV increases, associated with global climate change, are harmful to corals. To address this challenge, we have evaluated UV-specific effects in corals and have characterized factors that alter penetration of UV radiation over coral reefs. An immunoblotting assay was developed to examine UV-specific lesions (thymine dimers) in coral and zooxanthellae DNA. We observed dose-dependent increases of thymine dimers in coral (Porites porites var porites) exposed to artificial solar irradiance in a solar simulator, although effects were not strictly proportional. UV measurements were made in July 1999 at Eastern Sambo reef and nearby sites, including profiling along transects from reef to shore. Results of these analyses indicate that the coral at Eastern Sambo reef (at 3-4 meters) were receiving UV-B radiation that was equivalent to 25 to 30% of surface UV irradiance. However, the water just inside the reef in Hawk Channel (located closer to land) was considerably more opaque to UV. This water photobleached with loss of UV absorbance and fluorescence when it was exposed to simulated solar radiation. These results indicate that photobleaching of the DOM and transport of near-shore water out over the reefs might play a key role in controlling UV penetration to the reef surface.

Comparison of Model Calculations with Spectral UV Measurements during the SUSPEN Campaign: The Effect of Aerosols

Spectral measurements of global solar irradiance, obtained under cloud-free conditions during the SUSPEN campaign (July 1997) in Thessaloniki, Greece, are compared with radiative transfer model calculations, showing an agreement to within 65% for wavelengths higher that 305 nm. The uncertainties in the modeled spectra were analyzed with respect to the aerosol-related model input parameters (single-scattering albedo and asymmetry factor), which were not derivable from measurements. A range of single-scattering albedo values was used to investigate its impact on surface UV irradiance through comparison of measurements with model calculations. It was found that a difference in the single-scattering albedo of 0.1 changes the model‐measurement ratio by 7%‐14%, depending on solar zenith angle. Finally, an attempt was made to relate the estimated values of singlescattering albedo to wind direction and relative humidity, which control the origin and type of the aerosols in the area.

Satellite estimation of spectral surface UV irradiance: 2. Effects of homogeneous clouds and snow

This paper extends the theoretical analysis of the estimation of the surface UV irradiance from satellite ozone and reflectivity data from a clear‐sky case to a cloudy atmosphere and snow‐covered surface. Two methods are compared for the estimation of cloud‐transmission factor CT, the ratio of cloudy to clear‐sky surface irradiance: (1) the Lambert equivalent reflectivity (LER) method and (2) a method based on radiative transfer calculations for a homogeneous (plane parallel) cloud embedded into a molecular atmosphere with ozone absorption. The satellite‐derived CT from the NASA Total Ozone Mapping Spectrometer (TOMS) is compared with ground‐based CT estimations from the Canadian network of Brewer spectrometers for the period 1989–1998. For snow‐free conditions the TOMS derived CT at 324 nm approximately agrees with Brewer data with a correlation coefficient of ∼0.9 and a standard deviation of ∼0.1. The key source of uncertainty is the different size of the TOMS FOV (∼100 km field of view) and the much smaller ground instrument FOV. As expected, the standard deviations of weekly and monthly CT averages were smaller than for daily values. The plane‐parallel cloud method produces a systematic CT bias relative to the Brewer data (+7% at low solar zenith angles to −10% at large solar zenith angles). The TOMS algorithm can properly account for conservatively scattering clouds and snow/ice if the regional snow albedo RS is known from outside data. Since RS varies on a daily basis, using a climatology will result in additional error in the satellite‐estimated CT. The CT error has the same sign as the RS error and increases over highly reflecting surfaces. Finally, clouds polluted with absorbing aerosols transmit less radiation to the ground than conservative clouds for the same satellite reflectance and flatten spectral dependence of CT. Both effects reduce CT compared to that estimated assuming conservative cloud scattering. The error increases if polluted clouds are over snow.

Variability of spectral solar ultraviolet irradiance in an Alpine environment

Seven spectroradiorneters measured simultaneous surface UV irradiances at six different sites in the vicinity of Garmisch‐Partenkirchen, Germany, during spring 1999. The measured clear‐sky irradiance variability between the sites was analyzed with respect to altitude, aerosol optical depth, solar zenith angle, effective albedo, and tropospheric ozone. For conditions of low aerosol loading the increase of irradiance per 1000 m altitude difference was 9% at 400 nm. 20% at 320 nm, and 30% at 300 nm in this season. Effective albedo differences of 0.15, 0.29, and 0.65 were found between the snow‐covered stations and the snow‐free ground station with the higher effective albedo values determined at the two mountain stations. Clean continental aerosols with a single‐scatter albedo of 0.95 were observed during this campaign. The measurements and the observed variations between the sites should enable more accurate modeling studies to be performed for an Alpine environment.