Radiation Amplification Factor Research Articles

Spring and summer time ozone and solar ultraviolet radiation variations over Cape Point, South Africa

Abstract. The correlation between solar ultraviolet radiation (UV) and atmospheric ozone is well understood. Decreased stratospheric ozone levels which led to increased solar UV radiation levels at the surface have been recorded. These increased levels of solar UV radiation have potential negative impacts on public health. This study was done to determine whether the break-up of the Antarctic ozone hole has an impact on stratospheric columnar ozone (SCO) and resulting ambient solar UV-B radiation levels at Cape Point, South Africa, over 2007–2016. We investigated the correlations between UV index, calculated from ground-based solar UV-B radiation measurements and satellite-retrieved column ozone data. The strongest anti-correlation on clear-sky days was found at solar zenith angle 25∘ with exponential fit R2 values of 0.45 and 0.53 for total ozone column and SCO, respectively. An average radiation amplification factor of 0.59 across all SZAs was calculated for clear-sky days. The MIMOSA-CHIM model showed that the polar vortex had a limited effect on ozone levels. Tropical air masses more frequently affect the study site, and this requires further investigation.

Quantifying the effects of a low-ozone event and shallow stratocumulus clouds on ultraviolet erythemal radiation exposure.

Meteorological and dosimetric ultraviolet (UV) erythemal radiation (UVER) measurements were performed in Didcot, England, on 6 and 7 April 2017. Both days were characterized by clear-sky conditions in the morning and the afternoon with development of shallow stratocumulus clouds (SSC) around noon. In addition, a low-ozone event occurred on 7 April characterized by a 34 DU (Dobson Unit) drop in total stratospheric ozone content. Compared to 6 April, the ozone mini-hole caused UVER increases of 2.67 standard erythema dose (SED) for diffuse and 4.32 SED for global radiation characterized by radiation amplification factors (RAF) of 1.62 and 1.52, respectively. The total global UVER dose reductions due to SSC coverage amount to 2.33 SED (6 April) and 2.81 SED (7 April). As innovation the RAF is decomposed into two parts, named cloud ozone factor (COF) and radiation amplification factor based on measured data (RAFm), to quantify the low-ozone event's effect and the SSC influence in independently modifying the UVER doses. Hereby, the weight of each of these two effects acting during the same low-ozone event is expressed by the new COF. In this case, the COF values range between -0.13 and -0.11 for diffuse UVER and -0.03 to -0.07 for the global UV and UV-B parts. A positive COF value (0.18) results for the global UV-A range.

Relationship between erythema effective UV radiant exposure, total ozone, cloud cover and aerosols in southern England, UK

Abstract. Evidence of an underlying trend in the dependence of erythema effective ultraviolet (UV) radiant exposure (Her) on changes in the total ozone, cloud cover and aerosol optical depth (AOD) has been studied using solar ultraviolet radiation measurements collected over a 25-year period (1991–2015) at Chilton in the south of England in the UK. The monthly mean datasets of these measures corrected for underlying seasonal variation were analysed. When a single linear trend was fitted over the whole study period between 1991 and 2015, the analyses revealed that the long-term variability of Her can be best characterised in two sub-periods (1991–2004 and 2004–2015), where the estimated linear trend was upward in the first period (1991–2004) but downward in the second period (2004–2015). Both cloud cover (CC) and total ozone (TO) were found to have a highly statistically significant influence on Her, but the influence of the AOD measure was very small. The radiation amplification factor (RAF) for the erythema action spectrum due to TO was −1.03 at constant levels of CC over the whole study period; that is, for a 1.0 % increase in TO, Her decreases by 1.03 %. Over the first period (1991–2004), the RAF related to CC was slightly higher at 0.97 compared to that for TO at 0.79. The proportion of the change in Her explained by the change in CC (47 %) was much greater than the proportion explained by changes in TO (8 %). For the second period (2004–2015), the pattern reversed, with the observed RAF related to TO being −1.25, almost double that of CC (−0.65). Furthermore, in this period the proportion of variation in Her explained by TO variation was 33 %, double that of CC at 16 %, while AOD changes had a negligible effect (1 %). When the data were examined separately for each season, for the first period (1991–2004) the greatest effect of TO and CC on Her (i.e. the largest RAF value) was found during spring. Spring was also the season during which TO and CC variation explained the greatest proportion of variability in Her (82 %). In the later period (2004–2015), the RAF and greatest influence of TO and CC were observed in winter (67 %) and the AOD effect explained a further 5 % variability in Her. This study provides evidence that both the increasing trend in Her for 1991–2004 and the decreasing trend in Her for 2004–2015 occur in response to variation in TO, which exhibits a small increasing tendency over these periods. CC plays a more important role in the increasing trend in Her for 1991–2004 than TO, whereas for 2004–2015, the decreasing trend in Her is less associated with changes in CC and AOD.

Ultraviolet radiation modelling from ground-based and satellite measurements on Reunion Island, southern tropics

Abstract. Surface ultraviolet radiation (SUR) is not an increasing concern after the implementation of the Montreal Protocol and the recovery of the ozone layer (Morgenstern et al., 2008). However, large uncertainties remain in the prediction of future changes of SUR (Bais et al., 2015). Several studies pointed out that UV-B impacts the biosphere (Erickson et al., 2015), especially the aquatic system, which plays a central part in the biogeochemical cycle (Hader et al., 2007). It can affect phytoplankton productivity (Smith and Cullen, 1995). This influence can result in either positive or negative feedback on climate (Zepp et al., 2007). Global circulation model simulations predict an acceleration of the Brewer-Dobson circulation over the next century (Butchart, 2014), which would lead to a decrease in ozone levels in the tropics and an enhancement at higher latitudes (Hegglin and Shepherd, 2009). Reunion Island is located in the tropics (21° S, 55° E), in a part of the world where the amount of ozone in the ozone column is naturally low. In addition, this island is mountainous and the marine atmosphere is often clean with low aerosol concentrations. Thus, measurements show much higher SUR than at other sites at the same latitude or at midlatitudes. Ground-based measurements of SUR have been taken on Reunion Island by a Bentham DTMc300 spectroradiometer since 2009. This instrument is affiliated with the Network for the Detection of Atmospheric Composition Change (NDACC). In order to quantify the future evolution of SUR in the tropics, it is necessary to validate a model against present observations. This study is designed to be a preliminary parametric and sensitivity study of SUR modelling in the tropics. We developed a local parameterisation using the Tropospheric Ultraviolet and Visible Model (TUV; Madronich, 1993) and compared the output of TUV to multiple years of Bentham spectral measurements. This comparison started in early 2009 and continued until 2016. Only clear-sky SUR was modelled, so we needed to sort out the clear-sky measurements. We used two methods to detect cloudy conditions: the first was based on an observer's hourly report on the sky cover, while the second was based on applying Long and Ackerman (2000)'s algorithm to broadband pyranometer data to obtain the cloud fraction and then discriminating clear-sky windows on SUR measurements. Long et al. (2006)'s algorithm, with the co-located pyranometer data, gave better results for clear-sky filtering than the observer's report. Multiple model inputs were tested to evaluate the model sensitivity to different parameters such as total ozone column, aerosol optical properties, extraterrestrial spectrum or ozone cross section. For total column ozone, we used ground-based measurements from the SAOZ (Système d'Analyse par Observation Zénithale) spectrometer and satellite measurements from the OMI and SBUV instruments, while ozone profiles were derived from radio-soundings and the MLS ozone product. Aerosol optical properties came from a local aerosol climatology established using a Cimel photometer. Since the mean difference between various inputs of total ozone column was small, the corresponding response on UVI modelling was also quite small, at about 1 %. The radiative amplification factor of total ozone column on UVI was also compared for observations and the model. Finally, we were able to estimate UVI on Reunion Island with, at best, a mean relative difference of about 0.5 %, compared to clear-sky observations.

A long term study of the relations between erythemal UV-B irradiance, total ozone column, and aerosol optical depth at central Argentina

Global ultraviolet-B irradiance (UV-B, 280–315nm) measurements made at the campus of the University of Córdoba, Argentina were analyzed to quantify the effects of ozone and aerosols on surface UV-B erythemal irradiance (UVER). The measurements have been carried out with a YES Pyranometer during the period 2000–2013. The effect of ozone and aerosols has been quantified by means of the Radiation Amplification Factor (RAF) and by an aerosol factor (AF, analogous to RAF), respectively. The overall mean RAF under cloudless conditions was (1.2 ± 0.3) %, ranging from 0.67 to 2.10% depending on solar zenith angle (SZA) and on Aerosol Optical Depth (AOD). The RAF increased with the SZA with a clear trend. Similarly, the aerosol effect under almost-constant ozone and SZA showed that, on average, a 1% increase in AOD forced a decrease of (0.15 ± 0.04) % in the UVER, with a range of 0.06 to 0.27 and no defined trend as a function of the SZA. To analyze the effect of absorbing aerosols, an effective single scattering albedo (SSA) was determined by comparing the experimental UVER with calculations carried out with the TUV radiative transfer model.

Comparison of Five Modeling Approaches to Quantify and Estimate the Effect of Clouds on the Radiation Amplification Factor (RAF) for Solar Ultraviolet Radiation.

A generally accepted value for the Radiation Amplification Factor (RAF), with respect to the erythemal action spectrum for sunburn of human skin, is −1.1, indicating that a 1.0% increase in stratospheric ozone leads to a 1.1% decrease in the biologically damaging UV radiation in the erythemal action spectrum reaching the Earth. The RAF is used to quantify the non-linear change in the biologically damaging UV radiation in the erythemal action spectrum as a function of total column ozone (O3). Spectrophotometer measurements recorded at ten US monitoring sites were used in this analysis, and over 71,000 total UVR measurement scans of the sky were collected at those 10 sites between 1998 and 2000 to assess the RAF value. This UVR dataset was examined to determine the specific impact of clouds on the RAF. Five de novo modeling approaches were used on the dataset, and the calculated RAF values ranged from a low of −0.80 to a high of −1.38.

Variability in solar irradiance observed at two contrasting Antarctic sites

The features of erythemally weighted (EW) and short-wave downwelling (SWD) solar irradiances, observed during the spring–summer months of 2007–2011 at Johann Gregor Mendel (63°48′S, 57°53′W, 7m a.s.l.) and Dome Concordia (75°06′S, 123°21′E, 3233m a.s.l.) stations, placed at the Antarctic coastal region and on the interior plateau respectively, have been analysed and compared to each other. The EW and SWD spectral components have been presented by the corresponding daily integrated values and were examined taking into account the different geographic positions and different environmental conditions at both sites. The results indicate that at Mendel station the surface solar irradiance is strongly affected by the changes in the cloud cover, aerosols and albedo that cause a decrease in EW between 20% and 35%, and from 0% to 50% in SWD component, which contributions are slightly lower than the seasonal SWD variations evaluated to be about 71%. On the contrary, the changes in the cloud cover features at Concordia station produce only a 5% reduction of the solar irradiance, whilst the seasonal oscillations of 94% turn out to be the predominant mode. The present analysis leads to the conclusion that the variations in the ozone column cause an average decrease of about 46% in EW irradiance with respect to the value found in the case of minimum ozone content at each of the stations. In addition, the ratio between EW and SWD spectral components can be used to achieve a realistic assessment of the radiation amplification factor that quantifies the relationship between the atmospheric ozone and the surface UV irradiance.

The comparative analysis of observational series of total ozone content and UV-B radiation in boreal forest zones

Results of the comparative analysis of observational series of the total ozone content and UV radiation of the 300–315-nm wavelength band at stations located in boreal forest zones of midlatitudes of Russia and Canada (Northern Hemisphere, 50° N and higher) are presented. It is shown that the ozonosphere is a primary modulator of the biologically active part of UV-B radiation in this climatic zone. Radiation amplification factors of solar UV-B spectral region are determined. It is shown that 20% of total ozone depletion increases more than twofold the dose of the short-wave part of solar UV-B radiation relative to its climatic normal.

Sensitivity of UV Erythemal Radiation to Total Ozone Changes under Different Sky Conditions: Results for Granada, Spain.

This study focuses on the analysis of the sensitivity of UV erythemal radiation (UVER) to variations in the total ozone column (TOC) under different sky conditions at Granada (southeastern Spain). The sensitivity is studied both in relative terms by means of the Radiation Amplification Factor (RAF) and in absolute terms using the Ozone Efficiency (OE). These two variables are determined for diverse sky conditions characterized by the cloud cover information given by a sky camera (in oktas) and the cloud optical depth (COD) estimated from global solar radiation measurements. As expected, in absolute terms, the TOC variations cause substantially smaller UVER changes during completely overcast situations than during cloud-free cases. For instance, the OE (SZA = 30°, TOC = 290 DU) decreases from 0.68 mW m(-2) per unit of TOC (0 oktas) to 0.50 mW m(-2) per unit of TOC (8 oktas). However, the opposite is observed when the analysis is performed in relative terms. Thus, the RAF (determined for SZA cases below 80°) increases from 1.1 for cloud-free cases (0 oktas) to 1.4 for completely overcast situations (8 oktas). This opposite behavior is also found when both RAF and OE are analyzed as functions of COD. Thus, while the OE strongly decreases with increasing COD, the RAF increases as COD increases.

Characterisation of <i>J</i>(O<sup>1</sup>D) at Cape Grim 2000–2005

Abstract. Estimates of the rate of production of excited oxygen atoms due to the photolysis of ozone (J(O1D)) have been derived from radiation measurements carried out at Cape Grim, Tasmania (40.6° S, 144.7° E). The individual measurements have a total uncertainty of 16 % (1σ). These estimates agree well with model estimates of clear-sky photolysis rates. Observations spanning 2000–2005 have been used to quantify the impact of season, clouds and ozone column amount. The annual cycle of J(O1D) has been investigated via monthly means. These means show an interannual variation (monthly standard deviation) of 9 %, but in midsummer and midwinter this reduces to 3–5 %. Variations in solar zenith angle and total column ozone explain 86 % of the observed variability in the measured photolysis rates. The impact of total column ozone, expressed as a radiation amplification factor (RAF), is found to be ~ 1.53, in agreement with model estimates. This ozone dependence explains 20 % of the variation observed at medium solar zenith angles (30–50°). The impact of clouds results in a median reduction of 30 % in J(O1D) for the same solar zenith angle range. Including estimates of cloudiness derived from long-wave radiation measurements resulted in a statistically significant fit to observations, but the quality of the fit did not increase significantly as measured by the adjusted R2.

Influence of cloud fraction and snow cover to the variation of surface UV radiation at King Sejong station, Antarctica

This study investigated how cloud fraction and snow cover affect the variation of surface ultraviolet (UV) radiation by using surface Erythemal UV (EUV) and Near UV (NUV) observed at the King Sejong Station, Antarctica. First the Radiative Amplification Factor (RAF), the relative change of surface EUV according to the total-column ozone amount, is compared for different cloud fractions and solar zenith angles (SZAs). Generally, all cloudy conditions show that the increase of RAF as SZA becomes larger, showing the larger effects of vertical columnar ozone. For given SZA cases, the EUV transmission through mean cloud layer gradually decreases as cloud fraction increases, but sometimes the maximum of surface EUV appears under partly cloudy conditions. The high surface EUV transmittance under broken cloud conditions seems due to the re-radiation of scattered EUV by cloud particles. NUV transmission through mean cloud layer also decreases as cloud amount increases but the sensitivity to the cloud fraction is larger than EUV. Both EUV and NUV radiations at the surface are also enhanced by the snow cover, and their enhancement becomes higher as SZA increases implying the diurnal variation of surface albedo. This effect of snow cover seems large under the overcast sky because of the stronger interaction between snow surface and cloudy sky.

The effect of ozone and aerosols on the surface erythemal UV radiation estimated from OMI measurements

Surface erythemal UV radiation is mainly affected by total column ozone, aerosols, clouds, and solar zenith angle. The effect of ozone on the surface UV radiation has been explored many times in the previous studies due to the decrease of ozone layer. In this study, we calculated the effect of aerosols on the surface UV radiation as well as that of ozone using data acquired from Ozone Monitoring Instrument (OMI). First, ozone, aerosol optical depth (AOD), and surface erythemal UVB radiation measured from satellite are compared with those from ground measurements. The results showed that the comparison for ozone was good with r2 of 0.92. For aerosol, there was difference between satellite measurements and surface measurements due to the insufficient information on aerosol in the retrieval algorithm. The r2 for surface erythemal UV radiation was high (∼0.94) but satellite measurements showed about 30% larger values than surface measurements on average by not considering the effect of absorbing aerosols in the retrieval process from satellite measurements. Radiative amplification factor (RAF) is used to access the effect of ozone and aerosol quantitatively. RAF for ozone was 0.97∼1.49 with solar zenith angle. To evaluate the effect of aerosol on the surface UV radiation, only clear-sky pixel data were used and solar zenith angle and total column amount of ozone were fixed. Also, RAF for aerosol was assessed according to the single scattering albedo (SSA) of aerosols. The results showed that RAF for aerosol with smaller SSA ( 0.90). The RAF for aerosol was 0.09∼0.22 for the given conditions which was relatively small compared to that for ozone. However, considering the fact that aerosol optical depth can change largely in time and space while the total column amount of ozone does not change very much, it needs to include the effect of aerosol to predict the variations of surface UV radiation more correctly.

Effects of ozone and aerosol on surface UV radiation variability

Global (direct+diffuse) spectral ultraviolet (UV, 290–363nm) and total ozone measurements made on the roof of the Main Science Building, Yonsei University at Seoul (37.57°, 128.98°E) were analyzed to quantify the effects of ozone and aerosol on the variability of surface erythemal UV (EUV) irradiance. The measurements have been made with a Brewer Spectrophotometer MKIV (SCI-TEC#148) and a Dobson Ozone Spectrophotometer (Beck#123), respectively, during 2004–2008. The overall mean radiation amplification factor, RAF(AOD, SZA) [23,24] due to total ozone (O3) (hereafter O3 RAF) shows that 1% decrease in total ozone results in an increase of 1.18±0.02% in the EUV irradiance with the range of 0.67–1.74% depending on solar zenith angles (SZAs) (40–70°) and on aerosol optical depths (AODs) (<4.0), under both clear (cloud cover<25%) and all sky conditions. For the mean AOD, the O3 RAFs(SZA) for both sky conditions increased as SZA increased from 40° to 60°, and then decreased for higher SZA 70°, where the patterns are consistent with results of the previous studies [2,10]. A similar analysis of the RAF(O3, SZA) due to AOD (hereafter AOD RAF) under clear and all-sky conditions shows that on average, a 1% increase in AOD forces a decrease of 0.29±0.06% in the EUV irradiance with the maximum range 0.18–0.63% depending on SZAs and O3. Thus, overall sensitivity of UV to ozone (O3, RAF) was estimated to be about four times higher than to the aerosol (AOD RAF). At the mean O3, the AOD RAFs(SZA) for both skies appears to be almost independent of SZAs. It is shown that the O3 RAFs are nearly independent of the sky conditions, whereas the AOD RAFs depend distinctly on the sky conditions with the larger values for all skies. Under cloud free conditions, the overall mean ratio for measured-to-modeled O3, RAF(AOD, SZA) is 1.13, whereas the ratio for AOD RAF(O3, SZA) shows 0.82 in the EUV irradiance. Overall, the RAF measurements are corroborated by radiative transfer model calculations under clear-sky conditions.

Short-term variations in surface UV-B irradiance and total ozone column at Ny-Ålesund during the QAARC campaign

Diurnal variations in total ozone column were observed to exceed 40DU during the QAARC campaign of late spring 2009 held at the Ny-Ålesund Arctic station in Svalbard. They were correlated to corresponding oscillations of surface UV-B irradiance, assuming that the strong dependence between these two variables ensures the reliability of registered total ozone oscillations. To make the comparison, an approach is proposed that allows the retrieval of diurnal variability in UV-B irradiance due to total ozone column changes, for application to the data collected during the campaign. The UV irradiance features observed with the reference spectroradiometer over a 7-day period were analyzed together with the corresponding ozone column data-sets, simultaneously obtained using two different devices. Power curves were found to provide the best-fit solutions for the retrieved UV-B variations, examined as a function of total ozone variations. Such approximations are characterized by regression coefficients of about −0.94 and by the exponent named in earlier studies as the radiation amplification factor (RAF), which was found to vary between 0.79 and 2.49. These RAF evaluations are in good agreement with previous estimates derived from both field measurements and model evaluations, which confirm the complete dependability of the short-term variations in the ozone column measured at Ny-Ålesund.

Ozone mini‐holes over Valencia (Spain) and their influence on the UV erythemal radiation

AbstractLow ozone level episodes (also known as mini‐holes) have been analysed over Valencia, Spain, during the period from 1 January 2000 to 31 December 2007. In this study, 24 episodes, lasting 1 or 2 days each, were detected. Most of them can be explained dynamically by the existence of an anticyclone over the Azores, which sends air masses coming from the Caribbean over the Iberian Peninsula. A positive anomaly in the vertical temperature profile in the high troposphere and one negative anomaly in the medium and low stratosphere are also observed in most of the analysed episodes. The total ozone column decrease is accompanied by a rise in the UV erythemal radiation levels. This increase is quantified at 1.29 times the decrease in the total ozone column. This is known as the radiation amplification factor (RAF) of UV erythemal radiation. Copyright © 2010 Royal Meteorological Society

Variations of UV irradiance at Antarctic station Concordia during the springs of 2008 and 2009

AbstractThe features of solar UV irradiance measured at the Italian-French Antarctic Plateau station, Concordia, during the springs of 2008 and 2009 are presented and discussed. In order to study the impact of the large springtime variations in total ozone column on the fraction of ultraviolet B (UV-B) irradiance (fromc.290–315 nm) reaching the Earth surface, irradiance datasets corresponding to fixed solar zenith angles (SZAs = 65°, 75° and 85°) are correlated to the daily ozone column provided by different instruments. For these SZAs the radiation amplification factor varied from 1.58–1.94 at 306 nm and from 0.68–0.88 at 314 nm. The ultraviolet index reached a maximum level of 8 in the summer, corresponding to the typical average summer value for mid latitude sites. The solar irradiance pertaining to the ultraviolet A (UV-A, 315–400 nm) spectral band was found to depend closely on variations of atmospheric transmittance characteristics as reported by previous studies. Model simulations of UV-B irradiance showed a good agreement with field measurements at 65° and 75° SZAs. For SZA = 85° the ozone vertical distribution significantly impacted model estimations. Sensitivity analysis performed by hypothetically varying the ozone distribution revealed some features of the ozone profiles that occurred in the period studied here.

Use of an improved radiation amplification factor to estimate the effect of total ozone changes on action spectrum weighted irradiances and an instrument response function

Multiple scattering radiative transfer results are used to calculate action spectrum weighted irradiances and fractional irradiance changes in terms of a power law in ozone Ω, U(Ω/200)−RAF, where the new radiation amplification factor (RAF) is just a function of solar zenith angle. Including Rayleigh scattering caused small differences in the estimated 30 year changes in action spectrum‐weighted irradiances compared to estimates that neglect multiple scattering. The radiative transfer results are applied to several action spectra and to an instrument response function corresponding to the Solar Light 501 meter. The effect of changing ozone on two plant damage action spectra are shown for plants with high sensitivity to UVB (280–315 nm) and those with lower sensitivity, showing that the probability for plant damage for the latter has increased since 1979, especially at middle to high latitudes in the Southern Hemisphere. Similarly, there has been an increase in rates of erythemal skin damage and pre‐vitamin D3 production corresponding to measured ozone decreases. An example conversion function is derived to obtain erythemal irradiances and the UV index from measurements with the Solar Light 501 instrument response function. An analytic expressions is given to convert changes in erythemal irradiances to changes in CIE vitamin‐D action spectrum weighted irradiances.

Evolution of erythemal and total shortwave solar radiation in Valladolid, Spain: Effects of atmospheric factors

Erythemal ultraviolet (UVER; 280–400 nm) and total shortwave (SW; 305–2800 nm) solar irradiances were recorded from 2000 to 2009 in Valladolid, Spain. UVER and SW values under cloudless conditions are simulated by radiative transfer (TUV 4.6) and empirical models. These model estimations are tested with experimental measurements showing a great agreement (root mean square error around 7%). The aerosol effect on UVER irradiance is determined through a model study. UVER radiation and total ozone column (TOC) temporal evolutions show a negative relationship. TOC accounts for 80% of UVER variance and its radiation amplification factor is 1.1 at zenith of 65°. Cloud effects on solar radiation are shown and quantified by the cloud modification factor. Moreover the enhancement effect cases are analysed. SW radiation proves more sensitive to clouds than UVER. Clouds are seen to attenuate and enhance solar radiation by up to 93% and 22% in the UVER range, respectively.

Effect of Stratospheric Ozone in UVB Solar Radiation Reaching the Earth’s Surface at Qena, Egypt

This study describes the relationship between UVB and total ozone column (TOC) at a subtropical location through the period from 2001 to 2005. To analyze the relationship between TOC and UVB, the dimensionless parameter UVB transmission (KtUVB) and slant total ozone column (Z) were estimated. The results show an opposite seasonal behavior for KtUVB and Z in all sky conditions at Qena, Egypt (26.200N, 32.750E, 96m above sea level). In order to quantify the UV transmission variations produced by slant path ozone changes (Z), a linear regression between these two variables for cloud–free cases were employed. The correlation (r2) of this relationship was equal to 0.7 for the daily values. In addition, this value (r2) was increased for the monthly average (0.8). This study led to the values of UVB Radiation Amplification Factor (RAF) for the daily and monthly mean values in cloudless sky conditions that were equal to 1.0014 and 1.05, respectively.

Estimation of different biologically effective irradiances at Visakhapatnam (17.7°N, 83.3°E) from standard action spectra

Measurements of solar ultraviolet radiation in the biological band (280 - 320 nm) using a ground based UV-B Photometer at VISAKHAPATNAM have been used to estimate the biological effects of the incoming radiation on various species like human, animals, plants etc. with the help of standard action spectra. A comparison is made with TOMS ozone data to find the dependence on column ozone at a subtropical Indian station Visakhapatnam (17.7°N, 83.3°E). It is found that there is no significant change in the incoming biological effective radiation in the latitude (10 - 20° North) which corresponds to very less change in the columnar ozone content for this latitude. A bilinear regression model developed for this station was used to find the RAF (Radiation Amplification Factor) which indicates the change in the biological effect with respect to the corresponding change in ozone. The values of RAF for different biological effects were evaluated and are presented in this paper. Key words: UV-B irradiance, TOMS ozone, RAF, regression model, solar zenith angle, action spectrum.