Sasha Madronich

Changes in biologically active ultraviolet radiation reaching the Earth's surface

Since publication of the 1998 UNEP Assessment, there has been continued rapid expansion of the literature on UV-B radiation. Many measurements have demonstrated the inverse relationship between column ozone amount and UV radiation, and in a few cases long-term increases due to ozone decreases have been identified. The quantity, quality and availability of ground-based UV measurements relevant to assessing the environmental impacts of ozone changes continue to improve. Recent studies have contributed to delineating regional and temporal differences due to aerosols, clouds, and ozone. Improvements in radiative transfer modelling capability now enable more accurate characterization of clouds, snow-cover, and topographical effects. A standardized scale for reporting UV to the public has gained wide acceptance. There has been increased use of satellite data to estimate geographic variability and trends in UV. Progress has been made in assessing the utility of satellite retrievals of UV radiation by comparison with measurements at the Earths surface. Global climatologies of UV radiation are now available on the Internet. Anthropogenic aerosols play a more important role in attenuating UV irradiances than has been assumed previously, and this will have implications for the accuracy of UV retrievals from satellite data. Progress has been made inferring historical levels of UV radiation using measurements of ozone (from satellites or from ground-based networks) in conjunction with measurements of total solar radiation obtained from extensive meteorological networks. We cannot yet be sure whether global ozone has reached a minimum. Atmospheric chlorine concentrations are beginning to decrease. However, bromine concentrations are still increasing. While these halogen concentrations remain high, the ozone layer remains vulnerable to further depletion from events such as volcanic eruptions that inject material into the stratosphere. Interactions between global warming and ozone depletion could delay ozone recovery by several years, and this topic remains an area of intense research interest. Future changes in greenhouse gases will affect the future evolution of ozone through chemical, radiative, and dynamic processes In this highly coupled system, an evaluation of the relative importance of these processes is difficult: studies are ongoing. A reliable assessment of these effects on total column ozone is limited by uncertainties in lower stratospheric response to these changes. At several sites, changes in UV differ from those expected from ozone changes alone, possibly as a result of long-term changes in aerosols, snow cover, or clouds. This indicates a possible interaction between climate change and UV radiation. Cloud reflectance measured by satellite has shown a long-term increase at some locations, especially in the Antarctic region, but also in Central Europe, which would tend to reduce the UV radiation. Even with the expected decreases in atmospheric chlorine, it will be several years before the beginning of an ozone recovery can be unambiguously identified at individual locations. Because UV-B is more variable than ozone, any identification of its recovery would be further delayed.

The Role of Solar Radiation in Atmospheric Chemistry

Solar radiation at visible and ultraviolet wavelengths drives the chemistry of the atmosphere, by photo-dissociating relatively stable molecules into highly reactive radical fragments. Knowledge of photo-dissociation rate coefficients (J values) is crucial to understanding the behavior of global stratospheric and tropospheric ozone, the atmospheric lifetimes of gases such as carbon monoxide, methane, and non-methane hydrocarbons, and the formation of oxidants at urban and regional scales. J values depend on molecular parameters (absorption cross sections and photo-dissociation quantum yields) that are specific to the photo-reaction of interest, and on the availability of solar radiation at any specific location in the atmosphere. Advances in computer modeling of atmospheric radiative transfer now allow rapid calculation of J values for use in photo-chemistry models, and routinely include the effects of molecular absorbers and scatterers, clouds, aerosols, and surface reflections, for any location and time of the year. However, actual atmospheric conditions needed as input to the calculation are often not available. Direct measurements of J values, while in principle preferable, are technically difficult and limited in their temporal/spatial coverage, but generally support the theoretical calculations at least under optimal conditions (e. g., cloud free skies).

Ozone depletion and climate change: Impacts on UV radiation.

The Montreal Protocol is working, but it will take several decades for ozone to return to 1980 levels. The atmospheric concentrations of ozone depleting substances are decreasing, and ozone column amounts are no longer decreasing. Mid-latitude ozone is expected to return to 1980 levels before mid-century, slightly earlier than predicted previously. However, the recovery rate will be slower at high latitudes. Springtime ozone depletion is expected to continue to occur at polar latitudes, especially in Antarctica, in the next few decades. Because of the success of the Protocol, increases in UV-B radiation have been small outside regions affected by the Antarctic ozone hole, and have been difficult to detect. There is a large variability in UV-B radiation due to factors other than ozone, such as clouds and aerosols. There are few long-term measurements available to confirm the increases that would have occurred as a result of ozone depletion. At mid-latitudes UV-B irradiances are currently only slightly greater than in 1980 (increases less than ~5%), but increases have been substantial at high and polar latitudes where ozone depletion has been larger. Without the Montreal Protocol, peak values of sunburning UV radiation could have been tripled by 2065 at mid-northern latitudes. This would have had serious consequences for the environment and for human health. There are strong interactions between ozone depletion and changes in climate induced by increasing greenhouse gases (GHGs). Ozone depletion affects climate, and climate change affects ozone. The successful implementation of the Montreal Protocol has had a marked effect on climate change. The calculated reduction in radiative forcing due to the phase-out of chlorofluorocarbons (CFCs) far exceeds that from the measures taken under the Kyoto protocol for the reduction of GHGs. Thus the phase-out of CFCs is currently tending to counteract the increases in surface temperature due to increased GHGs. The amount of stratospheric ozone can also be affected by the increases in the concentration of GHGs, which lead to decreased temperatures in the stratosphere and accelerated circulation patterns. These changes tend to decrease total ozone in the tropics and increase total ozone at mid and high latitudes. Changes in circulation induced by changes in ozone can also affect patterns of surface wind and rainfall. The projected changes in ozone and clouds may lead to large decreases in UV at high latitudes, where UV is already low; and to small increases at low latitudes, where it is already high. This could have important implications for health and ecosystems. Compared to 1980, UV-B irradiance towards the end of the 21st century is projected to be lower at mid to high latitudes by between 5 and 20% respectively, and higher by 2-3% in the low latitudes. However, these projections must be treated with caution because they also depend strongly on changes in cloud cover, air pollutants, and aerosols, all of which are influenced by climate change, and their future is uncertain. Strong interactions between ozone depletion and climate change and uncertainties in the measurements and models limit our confidence in predicting the future UV radiation. It is therefore important to improve our understanding of the processes involved, and to continue monitoring ozone and surface UV spectral irradiances both from the surface and from satellites so we can respond to unexpected changes in the future.

Implications of recent total atmospheric ozone measurements for biologically active ultraviolet radiation reaching the Earth's surface

Recent satellite measurements of total atmospheric ozone were analyzed to deduce the changes in biologically active ultraviolet (UV) radiation reaching the Earths surface from 1979 to 1989. The calculated increases are on average substantially larger than earlier estimates, particularly at mid and high latitudes of both hemispheres.

The Atmosphere and UV-B Radiation at Ground Level

Ultraviolet (UV) radiation emanating from the sun travels unaltered until it enters the earth’s atmosphere. Here, absorption and scattering by various gases and particles modify the radiation profoundly, so that by the time it reaches the terrestrial and oceanic biospheres, the wavelengths which are most harmful to organisms have been largely filtered out. Human activities are now changing the composition of the atmosphere, raising serious concerns about how this will affect the wavelength distribution and quantity of ground-level UV radiation.

Measurements and model simulations of the photostationary state during the Mauna Loa Observatory Photochemistry Experiment: Implications for radical concentrations and ozone production and loss rates

Simultaneous measurements of [NO2], [NO], [O3], and the NO2 photo-dissociation rate coefficient, J2, were made during a one-month field study in the spring of 1988 at Mauna Loa, Hawaii, and were used to evaluate the photostationary state ratio, ϕ = J2[NO2]/k1[NO][O3]. Over 5600 measurements were made for clear sky conditions, allowing a detailed comparison with photochemical theory. Values of ϕ determined from the observations were consistently higher than unity, approaching 2.0 for high sun, and indicated peroxy radical mixing ratios near 60 pptv. High sun values of ϕ were independent of NOx (NO + NO2), but correlated well with ozone and water vapor through the expression ϕ−1 = (0.11 ± 0.21) + (1.59 ± 0.64) × 10−3 × ([H2O]/[O3])½. A photochemical box model is shown to give good agreement with the values of ϕ, the peroxy radical concentrations, and the correlations with physical and chemical environmental variables determined from the observations. The rate of photochemical production of ozone was estimated from measurements of ϕ, and the rate of photochemical ozone destruction was estimated from the box model. For free tropospheric air samples characteristic of altitudes near 3.4 km, the 24-hour average net ozone production rate is shown to be −0.5 ppbv/d (net ozone destruction), and is determined primarily by photolytic destruction.

Effect of anthropogenic aerosols on biologically active ultraviolet radiation

Aerosols from anthropogenic sources contribute significantly to the scattering of solar radiation in the atmosphere over most populated areas. By using observed values of visual range we estimate that in non-urban areas of the industrialized countries the amount of biologically active solar radiation (UVB, 280 to 315 nm) reaching the surface has decreased by a range of 5 to 18% since the industrial revolution, primarily due to aerosols formed from emissions of sulfur dioxide (SO2). The UVB reduction in the industrialized countries may have offset partly or fully the UVB increases associated with current stratospheric ozone depletion at NH continental mid-latitudes. However, this offset is not expected to continue because the SO2 emissions are leveling off in the industrialized countries.

Enhanced absorption of UV radiation due to multiple scattering in clouds: Experimental evidence and theoretical explanation

Measurements of spectral ultraviolet-B irradiance under optically thick clouds show strongly enhanced attenuation by molecular and particulate absorbers. The atmospheric photon path is enhanced owing to the presence of a highly scattering medium, leading to an amplification of absorption by tropospheric ozone and aerosol. Calculations with discrete ordinate and Monte Carlo models show that photon paths in realistic water clouds may be enhanced by factors of 10 and more compared to cloudless sky. Model calculations further show that UV spectra measured under thick clouds can be well simulated within 10%, indicating that the involved processes are quantitatively described by current models. These findings are of important consequence for all ground-based remote sensing applications which take advantage of measuring scattered radiation in order to infer atmospheric trace gas abundances. These algorithms, for example, the calculation of total ozone from global irradiance or zenith sky radiance, are subject to large errors, when neglecting the influence of cloud scattering on the derived data. In the present study, errors of more than 300 Dobson Units (DU) have been found, if such methods were applied without care in the presence of thick clouds.

Theoretical Estimation of Biologically Effective UV Radiation at the Earth’s Surface

Models of atmospheric transmission allow the estimation of spectral and biologically-weighted ultraviolet (UV) radiation reaching the Earth’s surface. The theory of radiative transfer is well established, but information about the atmosphere (e.g., ozone profiles, cloud morphology), which is required as input to the models, is often incomplete. Still, model sensitivity studies provide many useful insights that, when combined with measurements, give us a more complete understanding of the complex UV environment.

Changes in CH4 and CO growth rates after the eruption of Mt. Pinatubo and their link with changes in tropical tropospheric UV flux

Trace gas measurements from air samples collected weekly at a globally distributed network of sampling sites revealed sharp increases in the growth rates of CH4 and CO in the tropics and high southern latitudes immediately following the eruption of Mt. Pinatubo on June 15, 1991. The eruption emitted ∼20 Mt SO 2 into the lower stratosphere. Calculations made with a radiative transfer model show that UV actinic flux in the wavelength region 290-330 nm was attenuated by ∼12% immediately after the eruption due to direct absorption by SO 2 , and that it was perturbed for up to I year after the eruption due to scattering by sulfate aerosols. We suggest that the decreased UV flux decreased the steady-state [OH] and led to the observed anomalously large growth rates for CH 4 and CO during late-1991 and early-1992.