Ultraviolet Radiation Flux Research Articles

Variability in ultraviolet total optical depth during the Southern California Ozone Study (SCOS97)

Formation of photochemical air pollution is governed in part by the solar ultraviolet actinic radiation flux, but wavelength-resolved measurements of UV radiation in polluted urban atmospheres are rarely available. As part of the 1997 Southern California Ozone Study, cosine weighted solar irradiance was measured continuously at seven UV wavelengths (300, 306, 312, 318, 326, 333 and 368 nm) at two sites during the period 1 July to 1 November 1997. The first site was at Riverside (260 m a.s.l.) in the Los Angeles metropolitan area, which frequently experiences severe air pollution episodes. The second site was at Mt Wilson (1725 m a.s.l.), approximately 70 km northwest of Riverside, and located above much of the urban haze layer. Measurements of direct (i.e ., total minus diffuse) solar irradiance were used to compute total atmospheric optical depths. At 300 nm, optical depths (mean±1 S.D.) measured over the entire study period were 4.3±0.3 at Riverside and 3.7±0.2 at Mt Wilson. Optical depth decreased with increasing wavelength, falling at 368 nm to values of 0.8±0.2 at Riverside and 0.5±0.1 at Mt Wilson. At all wavelengths, both the mean and the relative standard deviation of optical depths were larger at Riverside than at Mt Wilson. At 300 nm, the difference between the smallest and largest observed optical depths corresponds to over a factor 2 increase in the direct beam irradiance for overhead sun, and over a factor 7 increase for a solar zenith angle of 60°. Principal component analysis was used to reveal underlying factors contributing to variability in optical depths. PCA showed that a single factor (component) was responsible for the major part of the variability. At Riverside, the first component was responsible for 97% of the variability and the second component for 2%. At Mt Wilson, 89% of the variability could be attributed to the first component and 10% to the second. Dependence of the component contributions on wavelength allowed identification of probable physical causes: the first component is linked to light scattering and absorption by atmospheric aerosols, and the second component is linked to light absorption by ozone. These factors are expected to contribute to temporal and spatial variability in solar actinic flux and photodissociation rates of species including ozone, nitrogen dioxide, and formaldehyde.

Influence of ultraviolet-B radiation, stratospheric ozone variability, and thermal stratification on the phytoplankton biomass dynamics in a mesohumic lake

Terrestrial ultraviolet radiation (UVR) is highly variable in both space and time, and phytoplankton in the mixed layer may be exposed at irregular intervals to significant daily doses. The influence of the natural UVR on phytoplankton dynamics was investigated in a small mesohumic lake, Lac Cromwell, in the Laurentian Hills by means of a time-intensive (about 60 days) daily study of the relationship between UVR flux and phytoplankton biomass. Following the onset of lake stratification, at which time the epilimnion became shallower than 2.5 m, the study revealed a strong negative relationship between ultraviolet-B radiation (UVB) and algal biomass at the surface (r 2 = 0.61) and at 1 m (r 2 = 0.38). Although this relationship held throughout the stratified period, chlorophyll a concentration declined particularly rapidly (-65%) during a short-lived ozone-thinning period. There was a major shift in the community taxonomic composition during the same period from a typical diatom-chrysophyte spring bloom towards a dinoflagellate-dominated community that was followed by cyanobacteria. Here, we present evidence that the impact of the temporary increase in UVB was intensified by a concurrent lack of mixing, indicating that turbulence and thermal stratification are key components modulating UVB effects in lakes.

The ultraviolet history of the terrestrial planets — implications for biological evolution

A radiative transfer model is employed to investigate the comparative surface ultraviolet (UV) radiation histories of Earth, Mars and Venus from 4.5 Ga to the present and thus their comparative theoretical photobiological histories. Earth probably began with a period of higher ultraviolet radiation fluxes during the anoxic Archean. During the early Proterozoic UV fluxes declined as oxygen partial pressures and thus ozone column abundance rose, but the ozone column became subject to stochastic depletion events caused principally by impact events and possibly large-scale volcanism and less frequently, close cosmic events such as supernovae. In contrast Mars has been subject to a history dominated by a slow increase in solar luminosity and a reduction in partial pressures of CO 2, both of which have resulted in an increase in UV flux. The UV radiation history of Venus has been dominated by the greenhouse effect through which high partial pressures of CO 2 made the surface UV radiation environment clement. These distinct histories influence the potential comparative evolutionary photobiology of the three planets. On Earth, life transitioned from the Archean, when tolerance to UV radiation, particularly for exposed organisms, must have been high to a more photobiologically clement era. In this latter era the predominant evolutionary selection pressure is one that allows for tolerance of sudden and unpredictable increases in UVB radiation above seasonal and diurnal maxima caused by exogenous perturbation of the ozone column. In the case of Mars, the UV radiation flux has increased over time. Today the biologically effective irradiances to DNA are not considerably different from those that are calculated for Archean Earth. If the planet suffered an atmospheric collapse then it may have been subject to an ultraviolet crisis at some point in its past when DNA-weighted irradiance would have increased three to five-fold. Venus transitioned into a photobiologically clement era soon after late bombardment. The lifeless surfaces of Mars and Venus, when in the former case DNA-weighted irradiances are not much greater than Archean Earth and in the latter case, insignificant, are testament to the unimportance of UV radiation as an evolutionary selection pressure when other physical factors, particularly lack of liquid water, become limiting to life. Understanding the comparative evolutionary differences in surface UV flux of the terrestrial planets can help us understand the influence, and lack of influence, of UV radiation in determining their suitability as abodes for life at different stages in their past.

Inhibition of phytoplankton photosynthesis by solar ultraviolet radiation: studies in Lake Titicaca, Bolivia

1. Lake Titicaca is a large, high altitude (3810 m a.s.l.) tropical lake (16°S, 68°W) that lies on the border of Bolivia and Perú, receiving high fluxes of ultraviolet radiation (UVR) throughout the year. Our studies were conducted during September of 1997 with the main objective of studying the impact of solar UVR upon phytoplankton photosynthesis.2. Water samples were taken daily and incubated in situ (down to 14 m depth) under three radiation treatments to study the relative responses to PAR (Photosynthetic Available Radiation, 400–700 nm), UV‐A (320–400 nm), and UV‐B (280–320 nm) radiation.3. Photosynthetic inhibition by UVR in surface waters was about 80%, with UV‐A accounting for 60% and UV‐B for 20%; the inhibition by high levels of PAR was less than 20%. The inhibition due to UVR decreased with depth so that there were no significant differences between treatments at 8.5 m depth.4. The amount of inhibition per unit energy received by phytoplankton indicates that even though there was a significant inhibition of photosynthesis due to UVR, species in Lake Titicaca seem to be better adapted than species in high latitude environments.5. The cellular concentration of UV‐absorbing compounds, a possible mechanism of photoadaptation, was low in phytoplanktonic species. However, they were abundant in zooplankton, suggesting a high rate of bioaccumulation through the diet.

Global surface ultraviolet radiation climatology from TOMS and ERBE data

A global climatology of biologically active solar ultraviolet radiation (UVR) at the Earth's surface is derived using NASA total ozone mapping spectrometer (TOMS) measurements of column ozone abundance and NASA Earth Radiation Budget Experiment (ERBE) measurements of solar reflectance from the Earth‐atmosphere system. These two sources of satellite data are used as input to a delta‐Eddington radiative transfer model to estimate climatological cloud opacity and thereby demonstrate how surface UVR varies with geography and season. The surface UVR fluxes are spectrally resolved to enable weighted integration with any biological action spectrum. Solar elevation is shown to be more important than total column ozone abundance in governing the variability of surface UVR over large geographic areas, although some regions with pronounced local minima in ozone (30 Dobson units or more) will cause noticeable enhancements of integrated UV‐B (280–315 nm) flux relative to UV‐A (315–400 nm). The greatest variability in surface UVR within a given climate zone is induced by cloud cover. During summer, regions that show lower surface UVR fluxes relative to their surrounding regions include the eastern United States (versus the western United States), India, China (in the vicinity of the Yangtze River), and Japan (relative to the surrounding oceans). Cloud cover over tropical rainforest areas reduces the surface UVR flux relative to ocean areas at the same latitudes. The UVR cloud transmission derived from the TOMS and ERBE data correlates with an independent climatology of global cloud coverage. The UVR mapping method, based on the TOMS and ERBE data, allows a direct investigation of diurnal variability and a rigorous calculation of the biologically relevant integrated daily dose of UVR. However, it is shown that a UVR mapping method based on TOMS data alone, which is limited to only local noon satellite measurements, can make defensible estimates of the integrated daily UVR dose and the instantaneous local noon UVR surface flux.

New TOMS instrument measures ozone and aerosols

New Total Ozone Mapping Spectrometer (TOMS) instruments launched in 1996 have provided new and better information about ozone distribution, sulfur dioxide concentrations following large volcanic eruptions, the distributions of ultraviolet‐absorbing aerosols (including ash plumes) in the troposphere, and the flux of ultraviolet radiation reaching the Earth's surface. The instruments gathered important data on ozone depletion in the Antarctic in the austral springs of 1996 and 1997 and in the Arctic in the Northern Hemisphere spring of 1997. Ash clouds associated with forest fires in the United States and in Indonesia, as well as the eruption of the Soufriere Hills volcano in Montserrat in December 1997, were also observed.NASA's Earth Probe (EP) satellite, launched in July 1996, carries one TOMS instrument. A second TOMS instrument flew on the Japanese Advanced Environmental Orbiting Satellite (ADEOS), known as “Midori,” launched in August 1996 from Japan. Unfortunately, because the ADEOS spacecraft failed in June 1997, only 10 months of data are available from the ADEOS TOMS instrument (see http://mentor.eorc.nasda.go.jp/ADEOS). The Earth Probe satellite flew in a 500 km Sun‐synchronous orbit, which was chosen to complement the 800 km altitude of the ADEOS spacecraft by providing improved horizontal resolution for viewing aerosol sources (26 km x 26 km at nadir instead of the 42 km x 42 km resolution of ADEOS). Because of the lower orbit, data gaps did not allow for full daily maps of the sunlit Earth from EP TOMS. In order to restore the complete global coverage lost with the ADEOS failure, the EP spacecraft was boosted to a 740 km orbit in early December 1997.

The impact of aerosols on solar ultraviolet radiation and photochemical smog.

Photochemical smog, or ground-level ozone, has been the most recalcitrant of air pollution problems, but reductions in emissions of sulfur and hydrocarbons may yield unanticipated benefits in air quality. While sulfate and some organic aerosol particles scatter solar radiation back into space and can cool Earth's surface, they also change the actinic flux of ultraviolet (UV) radiation. Observations and numerical models show that UV-scattering particles in the boundary layer accelerate photochemical reactions and smog production, but UV-absorbing aerosols such as mineral dust and soot inhibit smog production. Results could have major implications for the control of air pollution.

Past ultraviolet radiation environments in lakes derived from fossil pigments

Natural levels of ultraviolet (UV) radiation can harm organisms inshallow aquatic ecosystems in which concentrations of photo-protective dissolved organic carbon are low1,2,3. These compounds can be removed as a result of acidic precipitation and climate changes, an effect which may have recently been manifested in up to 200,000 boreal lakes4,5. Unfortunately, meteorological and biological monitoring studies are usually too brief to record the magnitudes of past changes in UV radiation fluxes and their effects. Here we demonstrate that certain fossil pigments in lake sediments can be used to document historical changes in the UV radiation environment of lakes. These pigments are produced by benthic algae when exposed to UV radiation and show sedimentary concentrations that are correlated to the depth of penetration of UV radiation within lakes. Analysis of fossil profiles from the sediments of two mountain lakes suggests that past UV radiation penetration has sometimes been—at least in these mid-latitude lakes—greater than during the period of anthropogenic stratospheric ozone depletion.

ON THE POSSIBLE OBSERVATIONS OF INTERGALACTIC OR HALO CLOUDS IN OPTICAL EMISSION

The discussion of the problems of observation of tenuous intergalactic and/or halo clouds in recombination emission is presented. We elaborate the possibility that clouds are illuminated only by metagalactic ultraviolet radiation flux, and assume some plausible values for the parameters determining the physical conditions in such objects. A simple, semiqualitative approach may give specific observational predictions for the two major signposts of fluorescent emission: Lyα and Hα lines. We conclude that Hα detection of Lyman-limit systems at low redshift is within present observational capabilities. Open questions for further research are put in a convenient framework.

Effects of ultraviolet radiation on periphyton in an alpine lake

Many alpine lakes have low concentrations of ultraviolet (UV) radiation‐absorbing dissolved organic matter, yet receive higher UV radiation flux than low‐elevation lakes. We tested whether ambient UV radiation affected periphyton development in a small alpine lake in Banff National Park, Canada. After 30 d, total periphytic biomass and chlorophyll accrual on artificial substrates were enhanced ∼100% (t‐test, P < 0.05) by removal of UV radiation (<400 nm). The inhibitory effect of UV radiation was species‐specific, significantly suppressing (Bonferroni‐adjusted t‐test, P < 0.05) Achnanthes minutissima Kützing but not other colonists. Although taxa apparently differed in their sensitivity to UV radiation, periphyton communities remained dominated by early successional taxa, especially A. minutissima (75% of total biovolume). In contrast, natural epilithic communities were dominated by cyanobacteria (Anabaena subcylindrica Borge, Calothrix sp.). These findings suggest that ambient UV radiation at alpine sites can suppress periphyton development by inhibiting littoral diatom production during the short ice‐free season (July–September).

Global Climate Change and Emerging Infectious Diseases

Climatic factors influence the emergence and reemergence of infectious diseases, in addition to multiple human, biological, and ecological determinants. Climatologists have identified upward trends in global temperatures and now estimate an unprecedented rise of 2.0 degrees C by the year 2100. Of major concern is that these changes can affect the introduction and dissemination of many serious infectious diseases. The incidence of mosquito-borne diseases, including malaria, dengue, and viral encephalitides, are among those diseases most sensitive to climate. Climate change would directly affect disease transmission by shifting the vector's geographic range and increasing reproductive and biting rates and by shortening the pathogen incubation period. Climate-related increases in sea surface temperature and sea level can lead to higher incidence of water-borne infectious and toxin-related illnesses, such as cholera and shellfish poisoning. Human migration and damage to health infrastructures from the projected increase in climate variability could indirectly contribute to disease transmission. Human susceptibility to infections might be further compounded by malnutrition due to climate stress on agriculture and potential alterations in the human immune system caused by increased flux of ultraviolet radiation. Analyzing the role of climate in the emergence of human infectious diseases will require interdisciplinary cooperation among physicians, climatologists, biologists, and social scientists. Increased disease surveillance, integrated modeling, and use of geographically based data systems will afford more anticipatory measures by the medical community. Understanding the linkages between climatological and ecological change as determinants of disease emergence and redistribution will ultimately help optimize preventive strategies.

Effects of clouds and stratospheric ozone depletion on ultraviolet radiation trends

ANTHROPOGENIC depletion of ozone in the lower stratosphere has been of global environmental concern for two decades, but the environmentally relevant quantity—the flux of solar ultraviolet radiation (UVR) reaching the Earth's surface—remains poorly quantified on a global basis. The three most important parameters governing surface UVR fluxes and trends are solar elevation, total vertically integrated ozone abundance and cloud opacity. Here we use global satellite measurements of total ozone abundance and cloud reflectance to examine how the trends in UVR resulting from established trends in total ozone abundance1,2 compare with the potentially large natural variability in UVR that results from variations in cloud opacity. We find that throughout many temperate regions—including large parts of continental Europe, North and South America, New Zealand, Australia and southern Africa—interannual variability in cloud opacity is sufficiently small that by the end of this century, trends in summer average local-noon UVR dose rates relevant to mammalian skin cancer or plant damage should be significant with respect to cloud variability.

A reliable and efficient two-stream algorithm for spherical radiative transfer: Documentation of accuracy in realistic layered media

We present a fast and well documented two-stream algorithm for radiative transfer and particle transport in vertically inhomogeneous, layered media. The physical processes considered are internal production (emission), scattering, absorption, and Lambertian reflection at the lower boundary. The medium may be forced by internal sources as well as by parallel or uniform incidence at the top boundary. This two-stream algorithm is based on a general purpose multi-stream discrete ordinate algorithm released previously. It incorporates all the advanced features of this well-tested and unconditionally stable algorithm, and includes two new features: (i) corrections for spherical geometry, and (ii) an efficient treatment of internal sources that vary rapidly with depth. It may be used to compute fluxes, flux divergences and mean intensities (actinic fluxes) at any depth in the medium. We have used the numerical code to investigate the accuracy of the two-stream approximation in vertically inhomogeneous media. In particular, computations of photodissociation and warming/cooling rates and surface fluxes of ultraviolet and visible radiation for clear, cloudy and aerosol-loaded atmospheres are presented and compared with results from multi-stream computations. The O3 +hv → O(1D) + O2 and O3 +hv → O(3P) + O2 photodissociation rates were considered for solar zenith angles between 0.0–70.0° and surface albedos in the range 0.0–1.0. For small and moderate values of the solar zenith angle and the surface albedo the error made by the two-stream approximation is generally smaller, <10%, than the combined uncertainty in cross sections and quantum yields. Surface ultraviolet and visible fluxes were calculated for the same range of solar zenith angles and surface albedos as the photodissociation rates. It was found that surface ultraviolet and visible fluxes may be calculated by the two-stream approximation with 10% error or less for solar zenith angles less than 60.0° and surface albedos less than 0.5. For large solar zenith angles and/or large surface albedos, conditions typical at high latitudes, the error made by the two-stream approximation may become appreciable, i.e. 20% or more for the photodissociation rates in the lower stratosphere and for ultraviolet and visible surface fluxes for large surface albedos. The two-stream approximation agrees well with multi-stream results for computation of warming/cooling rates except for layers containing cloud and aerosol particles where errors up to 10% may occur. The numerical code provides a fast, well-tested and robust two-stream radiative transfer program that can be used as a ‘software tool’ by aeronomers, atmospheric physicists and chemists, climate modellers, meteorologists, photobiologists and others concerned with radiation or particle transport problems. Copies of the FORTRAN77 program are available to interested users.

Vibrationally excited H2, HCl, and NO(+) in the diffuse clouds toward Zeta Ophiuchi

Absorption lines from vibrationally excited H2 and from HCl were detected in the ultraviolet spectrum of Zeta Oph with the Hubble Space Telescope (HST). Improved upper limits on NO(+) absorption were obtained as well. The data were analyzed with an updated version of a chemical model for diffuse clouds (van Dishoeck & Black 1986) as a guide to understanding this gaseous environment. The comparisons suggest that the flux of ultraviolet radiation impinging on the cloud surface is approximately 1-2 times the average interstellar radiation field, which is lower than once believed.

Ultraviolet radiation at Edmonton, Alberta, Canada

The ultraviolet radiation flux measured over hourly time intervals is used to study the variation over a four-year period. The monthly average total radiation ranged from 859 watt-hours per square meter in December to a value of 7772 in July. The average radiation flux based on the monthly hours of record had a 3.5 to 15.1 watts per square meter range. The amount and type of cloud cover significantly affected the total monthly radiation and during this period the ratio of maximum to minimum monthly total radiation varied from 1.03 to 1.54. An Angström form of relationship between ultraviolet radiation and hours of bright sunshine is developed.

Thermal processes in molecular gas

The dynamics of the cold atomic and molecular gas, on which we focus here, is strongly affected by non equilibrium heating and cooling processes. We give two different examples, in which the breaking of the thermal balance is due respectively to variations of the incident ultraviolet radiation flux, and non equilibrium abundances of H2 molecules in molecular clouds envelopes. Fluctuations of the ultraviolet radiation flux in clumpy molecular cloud envelopes result in the formation or the destruction of dense regions. Large density contrasts, greater than one order of magnitude, are easily achieved in cloud regions of moderate visual extinction. Condensation or expansion develop on quite short time scales, of the order of a few tenth of million year, and induce collective motions which can feed turbulence.Another example of the importance of out of equilibrium thermochemical processes is furnished by the study of the H — H2 transition layers in molecular clouds envelopes. They turn out to be unstable against convection-like motions, driven by the energy released by H2 photodestruction. The gas velocities involved in these motions are, again, typical of the observed turbulent velocity in clouds envelopes.

Thermal processes in molecular gas

The dynamics of the cold atomic and molecular gas, on which we focus here, is strongly affected by non equilibrium heating and cooling processes. We give two different examples, in which the breaking of the thermal balance is due respectively to variations of the incident ultraviolet radiation flux, and non equilibrium abundances of H2 molecules in molecular clouds envelopes. Fluctuations of the ultraviolet radiation flux in clumpy molecular cloud envelopes result in the formation or the destruction of dense regions. Large density contrasts, greater than one order of magnitude, are easily achieved in cloud regions of moderate visual extinction. Condensation or expansion develop on quite short time scales, of the order of a few tenth of million year, and induce collective motions which can feed turbulence.Another example of the importance of out of equilibrium thermochemical processes is furnished by the study of the H — H2 transition layers in molecular clouds envelopes. They turn out to be unstable against convection-like motions, driven by the energy released by H2 photodestruction. The gas velocities involved in these motions are, again, typical of the observed turbulent velocity in clouds envelopes.

Clues on the hot star content and the ultraviolet output of elliptical galaxies

view Abstract Citations (414) References (146) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Clues on the Hot Star Content and the Ultraviolet Output of Elliptical Galaxies Greggio, Laura ; Renzini, Alvio Abstract The relatively strong ultraviolet radiation flux emitted by elliptical galaxies shortward of ~ 2000 A is certainly due to the presence of hot stars. Yet, so far it has not been firmly established whether such hot stars are young, massive objects being currently formed in these galaxies, or whether old, low-mass stars suffice to produce the observed UV radiation, as well as the observed correlation with the galaxy metallicity. In this paper we investigate in great detail under which conditions the old-star option can give acceptable solutions. Several kinds of hot star candidates have been considered, including post-red giants, hot horizontal-branch stars and their AGB- manque progeny, post-early-AGB stars, and post-AGB stars of the helium- and hydrogen-burning varieties. Among binary star candidates we have considered post-red giants produced by a Roche-lobe overflow, and accreting white dwarfs. We present numerical simulations showing how sensitive is the production of the various kinds of hot stars to small differences in the assumptions concerning the helium enrichment and the mass-loss trends with increasing metallicity. These numerical simulations and additional arguments tend to disfavor classical post- AGB stars as the most important UV producers, at least among the galaxies with the strongest UV upturns. Also binary stars seems insufficient, unless stable mass transfer onto white dwarf primaries will turn out to be possible for a wider range of initial binary parameters than generally accepted. Rather, this study tends to favor hot HB stars, AGB-manque, and post-EAGB stars, whose production, however, is extremely sensitive to ill-known parameters. For this reason we conclude that stellar evolution theory alone cannot currently solve the problem of the origin of the UV output of elliptical galaxies. We therefore list in detail for each of all the above candidates the major observable characteristics which can help future space observations to discriminate among the various options, with particular reference to resolvable stellar systems such as galactic globular clusters, M32, and the bulges of the Milky Way and M31. Finally, we also discuss the predicted evolution with lookback time (redshift) and argue that the UV upturn of ellipticals is likely to be the most rapidly evolving feature in the spectrum of these galaxies, possibly allowing the detection of evolutionary effects already at rather modest redshifts (z <~ 0.2). Publication: The Astrophysical Journal Pub Date: November 1990 DOI: 10.1086/169384 Bibcode: 1990ApJ...364...35G Keywords: Elliptical Galaxies; Galactic Evolution; Hot Stars; Stellar Evolution; Ultraviolet Radiation; Asymptotic Giant Branch Stars; Binary Stars; Galactic Bulge; Metallicity; Stellar Luminosity; Stellar Models; Astrophysics; GALAXIES: EVOLUTION; GALAXIES: STELLAR CONTENT; STARS: EVOLUTION; ULTRAVIOLET: GENERAL full text sources ADS | data products SIMBAD (2) MAST (1)

Solar radiation (190–230 nm) in the stratosphere: Implications for photoelectric emissions from instrumentation at balloon altitudes

Photoelectric contamination of conductivity measurements using the relaxation time method and electric field measurements using double‐probe systems have been observed well below maximum balloon altitudes. The solar wavelengths responsible for the photoemissions are primarily those in the 190–230 nm range. At these wavelengths, the atmosphere is partially transparent, allowing a large fraction of the incident solar photons to penetrate deep into the stratosphere. The effects of photoemissions on the electrical measurements have been observed at altitudes as low as 25 km, even for low solar angles. In the relaxation time measurements the photoelectric effects are manifested by a large perturbation of the positive conductivity measurement as a function of altitude and solar angle, with a ratio of the measured positive and negative conductivities greater than 1 above 25 km. We employ a photocurrent model to investigate the contamination of the relaxation time measurements. The model result shows that the solar flux in the range of 190–230 nm is more than sufficient to account for our conductivity observations. In the double‐probe measurements of electric fields, the photoelectric effects appear as perturbations to the probe potentials. As a result of the large flux of energetic solar photons in the stratosphere, many of the requirements for satellite double‐probe systems to reduce photoelectric contamination of electric field measurements are also applicable to balloon‐borne systems. On the basis of our analysis, we recommend that the surfaces of double‐probe instruments in the stratosphere be coated with a material of highly uniform work function in the optimum range of 4.6–5.4 eV. Our observations demonstrate the presence of a large flux of ultraviolet radiation in the stratosphere which is capable of photoemissions from aluminum surfaces and surfaces coated with high work function materials. As a consequence, many types of electrical or chemical measurements made in the sunlit stratosphere may suffer contamination by photoemissions.

Possible long-term changes in biologically active ultraviolet radiation reaching the ground.

Abstract— Three scenarios for long‐term changes in atmospheric ozone over the time period 1960 to 2030 lead to different projections for the ultraviolet radiation flux at the earth's surface. Biologically effective fluxes for damage to DNA and generalized damage to plants vary by a factor of 10 or more with latitude and season irrespective of possible changes in ozone. The natural latitudinal gradient in radiation corresponds to spatial changes in biologically effective fluxes which are large compared to temporal changes expected from trends in ozone over the time period analyzed. In an extreme scenario of ozone change, based on an assumed increase in chlorofluorocarbon release rates of 3% per year after 1980, the annually integrated effective flux for damage to DNA increases by 13.5% at latitude 40°N between 1960 and 2030. With chlorofluorocarbon release rates held fixed at their 1980 values, the corresponding radiation increase is only 2.3%. In a scenario where atmospheric chlorine remains fixed at its 1960 value, trends in atmospheric methane and nitrous oxide imply a decrease in biologically effective flux at 40°N of 5.3% between 1960 and 2030.