Introduction
DOE's interest in atmospheric ozone and Ultraviolet (UV) radiation ensures that energy policies in such areas as power production and refrigeration are based on sound, well justified science. One of DOE's primary goals is to assess the impact of energy policy decisions on Earth's climate. Because of the stratosphere's sensitive response to tropospheric perturbations, it can act as an indicator of serious climate change issues. Measured changes in stratospheric ozone and UV radiation can potentially serve as barometers of climate change effects. For example, tropospheric forcing from anthropogenic activity can have a significant effect on ozone levels through stratospheric-tropospheric exchange processes, which in turn will cause changes in the radiative forcing of the atmosphere and affect the amount of UV radiation penetrating the troposphere. Therefore, recognizing changes in stratospheric dynamics and chemistry, which can provide an early warning of changes in Earth's climate system, is important for making intelligent policy decisions. Future changes in the energy production from emerging countries (e.g., China) are expected to have a dramatic effect on global change, so it is imperative that we understand global as well as local effects of energy policies.
The impact of climate change, either naturally or anthropogenically driven, on the ozone layer and the feedback effects of ozone perturbations on climate change bring ozone and UV radiation to the center of policy decisions. UV radiation, the penetration of which is determined primarily by the amount and distribution of stratospheric ozone, is the driving force of atmospheric chemistry and is detrimental to biological systems. The effects of UV radiation on human health, ecosystems and agriculture demand a clear understanding of ozone trends and UV changes. Changes in stratospheric properties such as ozone, temperature, aerosol content, and dynamics are the result of cumulative natural and anthropogenic processes occurring in the troposphere. The Earth's atmosphere, rather than existing in "layers," is a complicated and interconnected system of physical processes and feedback mechanisms. Thus a total comprehension of this system involves an integration of the knowledge and understanding gained in the three scientific and technical research areas (Atmospheric Chemistry, ASCOT, and the Ozone Project) of the Atmospheric Science Program (ASP) supported by DOE.
Atmospheric aerosol particles, due to their scattering and absorption properties, are significant contributors to the extinction of both visible and UV-B radiation reaching the Earth surface. Of particular interest to public health is the possibility that anthropogenic aerosol over populated industrialized areas may influence the intensity of biologically active ultra-violet radiation. A quantitative assessment of these effects requires the establishment of the relationships between the air quality parameters such as mass concentrations and aerosol optical properties and the intensity of ground level UV-B radiation.
The great public interest in ozone and UV radiation is accompanied by some concern about the reliability and interpretation of the available data. Determining the reliability of existing data is necessary for making appropriate energy policy decisions. Other federal agencies are in the process of providing specific data for analysis pertinent to the ozone/UV questions. The scientists in the Atmospheric Science Program work closely with other agencies to provide DOE with the information necessary to support DOE's mission. The DOE ozone and UV group has worked toward understanding the strengths and weaknesses of the available data with the goal of producing reliable information for scientific advances and public policy decisions. Six primary areas have been identified where advances in the past few years have been made, and where further study is needed.
The six specific areas impacted by the research of the Ozone/UVB section of the ASP are:
1. Stratospheric Dynamics and Chemistry. We are developing an understanding of the relative importance of dynamics and chemistry in determining the observed stratospheric ozone concentrations. This understanding can help bridge the gap between GCM model predictions of ozone levels and the observed ozone distributions. Studies of variability in total hemispheric ozone may provide an indication of the effects of global scale dynamical and chemical changes. Preliminary results indicate that the statistics of total northern hemisphere ozone variations may be different in the last half of the 1980's compared to the first half. Studies of this general nature may provide early indications of potentially important global climate change.
2. Quality of Ozone Data. The quality of available ozone data, both column amounts and vertical profiles, is being assessed. Both ground-based measurements and various satellite measurements have been used to determine the magnitude of errors in measuring ozone. Differences between satellite and ground-based measurements are being explored and are becoming better understood. This work is necessary to determine reliable trends in available ozone data.
3. Ozone Trends. Trends in ozone may arise as a consequence of long-term changes in either the chemistry or dynamics of the stratosphere. Attribution of observed trends to specific influences requires continuing development of physically based multi-dimensional models.
4. Longtime Aerosol Variability. Efforts are underway to reconstruct a more extensive stratospheric aerosol time series, including profiles, concentrations and optical properties, using available ground-based, satellite and lidar sensing data as well as dust-sonde data. This historical database is essential for the interpretation of long-term ozone trends and stratospheric ozone processes.
5. UV Modeling. A Global UV climatology is being developed based on modeling of atmospheric transmission in conjunction with satellite data of ozone, cloudiness, etc. Such a model is required for tropospheric chemistry modeling as well as for evaluating surface UV levelstheir climatologies and their trends. The model will be utilized with the existing and emerging UV monitoring networks.
6. UV Measurements. Analysis of available UV data have shown that prior results which showed a decrease in UV radiation are not a valid interpretation of the data. New analyses of the UV data through the early 90's are producing results which are justified by the scientific understanding of the instruments. These results show an increase in UV radiation, particularly in the past five years. The increase in UV is consistent with what is expected based on ozone measurements.
INVESTIGATOR
RESEARCH AREA
1
2
3
4
5
6
J. DeLuisi
D
D
D
D
D
W. Komhyr
D
D
D. Heath
U
D
U
U
D
L. Hood
D
U
D
N. Laulainen
U
D
U
D
S. Madronich
U
U
U
U
D
U
M. Newchurch
U
D
D
U
T. Novakov
D
R. Reck
D
U
U
U
D. Rusch
D
D
D
D
H. Schneider
D
U
U
U
U
U
E. Weatherhead
U
U
D
D
Based on discussions of the DOE ASP group on ozone and UVB research, the following table shows the relationships between the projects. A 'D' denotes a developer and a user of the data or model, a 'U' denotes a user of a data set developed by another research project. The numbers 1 through 6 correspond to the six areas of research enumerated above.
Recommendations for future research:
To fulfill DOE's need for useful information, reliable future measurements must be available for analysis. Support for current ozone measurements must be maintained, and support for new measurements generated, in order to avoid a clear threat to the continuity of ozone measurements necessary for future analyses. As important as future measurements is the support of analysis of the data to assure quality information. Without strong quality assurance and analysis, the data are not appropriate for use in determining future energy decisions.
Ozone in the upper regions of the troposphere is an important greenhouse gas. Upper tropospheric ozone responds both to stratospheric intrusions and tropospheric processes. The DOE program can give a sense of the variability of the rate at which ozone is transported from the stratosphere, thereby altering the tropospheric ozone distribution. We need to recognize that the contributions of other countries to the chemistry of the upper tropospherefor example China with its rapid growthrequires further investigation into the processes on a global scale.
The scientists of DOE's Atmospheric Science Program have concern about the existence of federal global monitoring systems for assessing ozone profiles worldwide. Relative to this concern, the ASP Ozone/UVB group recommends maintaining the Dobson network to provide a continuous ozone profile record. This network has been in continuous monitoring use since 1957. The continuation of this work is necessary for trend evaluation and ground truthing of future satellite work.
The northern polar region requires further investigations. Because of global circulation processes, climate change effects at mid-latitudes can only be understood from a global perspective, and should not be studied in isolation from other latitudes. Ozone perturbations due to climate change effects should be more extreme in the polar regions than in the midlatitudes, and thus more easily understood. Increasing our understanding of the processes which control ozone concentrations in the polar regions is thus necessary for enhancing our knowledge of the processes which dominate in the midlatitudes. In addition, the importance of methane to the concentrations of ozone in the stratosphere needs further investigation. Related to this, the concentrations of water vapor are important to ozone concentrations; however little is known about this area.
It is necessary to increase the confidence of our understanding of troposphere and surface UV radiation. Opportunities must be followed to utilize recent advances in modeling, satellite observations, and ground-based UV monitoring, toward the development of a global UV climatology and the reliable predictions of trends. The quality of historical, current, and future UV measurements must be understood in detail. Important issues remain concerning model accuracy and efficiency, as well as measurement uncertainties. Thus, intercomparisons of different models and measurements are necessary.
External Collaborations
Hood plans to collaborate with Xuexi Tie of NCAR to evaluate further the relative importance of chemistry and dynamics in producing midlatitudinal ozone trends.
Rusch is collaborating with the POAM II (Polar Ozone and Aerosol Measurement) science team to evaluate high latitude ozone perturbations and to help define the role of dynamics in determining the global ozone distribution.
Harrison, Lala, and Laulainen are collaborating with Yankee Environmental Systems, Inc., in the development and manufacture of the UVB-RSS instrument and the USDA UV-B Monitoring and Trends Network (Jim Gibson, Colorado State University) for field testing and application of the instrument. Ambler Thompson of NIST will provide independent evaluation and calibration services of the UVB-RSS.
Madronich is collaborating with Weatherhead on the comparisons between UV models and measurements, and has in addition numerous collaborations with the US and international community interested in atmospheric chemistry, UV radiation, and environmental consequences.
Heath is collaborating with McPeters, Hilsenrath, and Bhartia at GSFC; W. Komhyr at CIRES; J. Deluisi at NOAA; J. Burrows at U. Bremen; R. Hoekstra at TPD-TNO Delft; and the GOME Science Advisory Group.
Newchurch is collaborating with the ATMOS science team to investigate chemical issues including the ozone deficit problem, chlorine partitioning, nitrogen partitioning, and polar processes. He also plans to collaborate with Xuexi Tie using the NCAR 3-D model to investigate Noy partitioning and its effect on ozone.
Research summaries
Investigating the Umkehr Ozone Profile Record
John DeLuisi
NOAA Air Resources Laboratory
Umkehr observations of the vertical profile of ozone up to 50 km, beginning in the late 1950s, were the only source of data for trend analysis of ozone changes in the photochemical region near 40 km until satellite data became available starting in 1979. The Umkehr ozone profile trend analysis performed in the early 1980s on the historical data set indicated a significant reduction in ozone concentration near 40 km. Confirmation of ground-based and satellite trend results, published in 1992, has greatly improved the credibility of the data derived from both methods of observations. While the Umkehr observation produces laudable results, the method is sensitive to stratospheric aerosol interference (especially during elevated aerosol loading conditions after a significant volcanic eruption) and this requires information on the profiles and optical properties of the stratospheric aerosol to account for the interference. The method to correct Umkehrs for stratospheric aerosol is tedious and complex and has not been fully developed up to the level that it should be. The research funded by the Department of Energy will provide a comprehensive data set on stratospheric aerosols back to 1958, will evaluate the uncertainties in the method of correction, and will provide a measure of the uncertainties in the application of Umkehr data for trend analysis.
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Analysis of Shadowband Radiometric Data with Emphasis on the Coupled Effects of Particulate Scattering and Ozone Absorption in the UV-B
Halstead Harrison
University of Washington
The absorption and scattering of the earth's atmosphere can be inferred from measurements of the solar irradiance at the earth's surface. At certain wavelengths these measurements may be used to determine the total ozone and water vapor, above the observer. At other wavelengths one may infer the amounts and sizes of aerosol particles. In general, with measurements at more wavelengths one may extract more information, but because the aerosol's scattering spectra are broad and smooth there are limits on the information that may be extracted, and on the precision of the aerosol size-distributions that may be inferred from solar radiometry. This project, in part, is concerned with the optimal extraction of information about the ozone, water vapor, and aerosols of the earth's atmosphere. This task contributes importantly to the understanding of effects of aerosol scattering on the earth's climate.
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Preflight and Postflight Spectroradiometric Calibration Errors for SBUV and TOMS type instruments
Don Heath
Research Support Instruments, Inc.
The goals of this project are to determine pre-flight and post-flight spectroradiometric errors for SBUV and TOMS type ozone monitoring instruments using new spectroradiometric calibration techniques to establish a common radiometric calibration scale among instruments; and to use surface-based comparisons of direct sun Langley observations and zenith sky Umkehr observations with the world standard Dobson instrument No. 83 combined with forward model calculations using satellite overpass data to determine the sources of biases between satellite based and surface based ozone soundings. The new spectroradiometric calibration techniques were applied to the SSBUV space shuttle instrument which is used to recalibrate NOAA's SBUV/2 ozone monitoring instruments in orbit. This reduced the altitude dependent bias with the SAGE instrument, which varied from 0 to >15 percent, to a random bias of less than 3 percent. The surface based comparison measurements between the SBUV/2 and SSBUV instruments with the world standard Dobson instrument No. 83 have shown no evidence of any significant scattered light effects or solar zenith angle (seasonal) biases. Zenith sky radiance (Umkehr) comparisons suggest the presence of an uncorrected polarization effect in the Dobson measurements which does not appear to seriously affect the ozone profile inversions because the inversion algorithm uses zenith sky radiances which are normalized to a solar zenith angle of 60 degrees.
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Re-evaluation of Total and Umkehr Ozone Data from Ten NOAA/CMDL Dobson Spectrophotometer Observatories
David Hofmann and Walter Komhyr
NOAA Climate Monitoring and Diagnostic Laboratory
Because the natural variability of atmospheric ozone is large, highly reliable Dobson spectrophotometer total ozone data are needed for ozone trend determinations (to +/-1% per decade), and for ground truth for satellite ozone observations. High quality ozone data, furthermore, shed light on underlying chemical and physical processes that cause ozone to change, thereby affecting UVB irradiance at the ground as well as climate. To optimize the quality of over 400 station-years of NOAA, U.S. Department of Commerce, Dobson spectro-photometer total ozone data obtained since the early 1960s at 25 stations, work in re-evaluating the data began in early 1992. Re-evaluation of the data has now been completed. Nearly 125 station-years of the data, including Umkehr data from 7 stations, were re-evaluated with funding support received from the DOE. Currently, the data are being prepared for archiving at the WMO World Ozone Data Centre in Canada.
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Statistical Analysis and Interpretation of Stratospheric Ozone and Temperature trends with Satellite Data
Lon Hood
University of Arizona
Our work currently focuses on investigating the origin of stratospheric ozone decreases that have occurred at northern midlatitudes during the past 15 years. These decreases range from a few percent in summer to more than 10 per cent in winter and spring. In particular, we are investigating whether long-term changes in stratospheric dynamics may be responsible for a substantial part of the ozone decrease in addition to direct chemical losses resulting from man-made chlorofluorocarbons. Preliminary results indicate that a major fraction of the observed ozone losses in winter are a consequence of dynamical changes. Work is continuing to confirm this conclusion and to investigate the origin of the dynamical changes.
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New UV-B Spectral Radiometer (UVB_RSS)
Nels Laulainen
Pacific Northwest National Laboratory
The objective of this project is to develop and test a new, low-cost ultraviolet-B (UV-B) rotating shadowband spectroradiometer (UVB-RSS) with sufficiently high performance characteristics to detect trends and provide status monitoring of surface UV-B radiation. A prototype instrument, based on the design specifications of the multi-filter rotating shadowband radiometer (MFRSR) and the rotating shadowband spectroradiometer developed for the DOE Quantitative Links and Atmospheric Radiation Measurement (ARM) Programs, is being assembled with particular emphasis on minimizing stray and out-of-band light and maximizing wavelength passband stability and through-put. Once the prototype instrument is completed (expected by mid-1996), it will undergo rigorous benchtests in the laboratory and then will be deployed in the field (for example, the reference Brewer UV-B spectro-radiometer site in Toronto is a likely site for field evaluation of operational performance characteristics and intercomparison against a world standard instrument). The final phase of the project will be to harden the design and transfer the technology to Yankee Environmental Systems, Inc., for commercialization. This instrument fills a measurement niche currently unavailable for UV-B trends and effects work; namely, it has a spectral resolution of the order of 2 nm in the wavelength range 300-330 nm in the price range of $20-30K and can be operated by technicians with a modest amount of training. Broadband UV-B instruments, while relatively inexpensive ($5-10K) and easy to use, have the problem of calibration and data interpretation (the effective wavelength changes as the air mass and extinction change). The standard high resolution instruments are generally rather complex instruments to use, requiring highly skilled operators, and are relatively expensive (>$120K).
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Ultraviolet Radiation Climatology of the Earth's Surface and Lower Atmosphere
Sasha Madronich
National Center for Atmospheric Research
The state of the UV radiation environment must be understood: UV radiation is known to have harmful effects on living tissues, and controls tropospheric chemistry at all scales from urban to global. This project is developing a global UV climatology, by assimilating satellite-derived data into a numerical computer model to create realistic maps of UV radiation at the surface and throughout the troposphere. Theoretical improvements of the numerical model's accuracy and efficiency are under way, and versions will be made available to the scientific community. Comparisons of predicted and measured surface UV levels will provide the basis for understanding how accurately we can predict UV levels into the next century and for the duration of the ozone reduction problem.
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Sage II Umkehr Ozone Comparisons and Aerosol Effects: An Empirical and
Theoretical Study
Michael J. Newchurch
University of Alabama
Ozone trends derived from Umkehr observations agree with satellite-derived trends in the mean; however, the station-to-station variability of Umkehr trends is relatively high. Detailed comparisons of individual Umkehr and SAGE observations of ozone vertical profiles reveal that the new Umkehr algorithm produces profiles in much better agreement with SAGE than the previous Umkehr algorithm did. A bias of 7% between SAGE and Umkehr (SAGE high) column ozone over a large number of stations remains unexplained at this time. SAGE/Umkehr relationships empirically quantify the relative proportions of a priori and atmospheric information. Along with the time series of SAGE-a priori and Umkehr-a priori ozone differences, these relationships provide an assessment of the quality of the improved a priori profiles and of the information content of derived ozone trends from Umkehr observations.
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Measurement of Aerosol Properties
Tihomir Novakov
Lawrence Berkeley Laboratory
Atmospheric aerosol particles due to their scattering and absorption properties influence the extinction of solar radiation reaching the earth surface. An effect of anthropogenic aerosol in populated areas of industrialized countries may be to reduce the intensity of biologically active ultra-violet radiation. In order to establish the magnitude of this effect, scattering and absorption cross sections in the UV-B region have to be determined. We have measured the absorption cross section of ambient suburban aerosols in the wavelength range from 250 to 700 nm by an optical transmission method. The absorption cross section due to aerosol black carbon ("soot"), the principal light absorption species, was found to increase from about 9.2 m2 g-1 at 700 nm to 22.3 m2 g-1 at 250 nm, following an inverse wavelength dependence.
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Trends and Evolution of Minimal Ozone Amounts, 1979-1992
Ruth Reck
Argonne National Laboratory
This work focuses on understanding the daily temporal and spatial variability in total ozone and the related consequences for various regions of the globe using ozone values that were measured by the Total Ozone Mapping Spectrometer (TOMS), flown aboard the Nimbus 7 satellite for the years 1979-1991. The purpose is to identify the most important changes in ozone from day to day and possible causes, with special emphasis on the days of extremes. The maximum in global average daily ozone data decreased 17.4% over the 12 years of measurements while the change in the minimum was not significant. The measured values of TOMS data showed a 7.4% larger amount of ozone in the northern hemisphere compared with the southern hemisphere. The future work will address the causes of this hemispheric imbalance and the related variability.
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Confirmation Analysis of Sage I and Sage II Data
David W. Rusch
University of Colorado
The objective of this study is to confirm and/or modify the analysis technique and the ozone densities and trends measured by the Stratosphere Aerosol and Gas Experiments-I, and -II (SAGE-I and -II). The SAGE instruments have long been recognized as critical to defining long-term changes in lower stratospheric ozone. However, it is known that the current data reduction algorithm does not properly account for aerosol interference, especially at times of high aerosol loading. We are developing a method of analysis of SAGE data which involves an end-to-end simulation, and an inversion method, aimed primarily at refining the treatment of aerosols, that quantitatively determines the correlated errors within the system. The newly generated ozone densities will be examined for trends by a method which will produce realistic uncertainties in the computed ozone changes.
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Trend Analysis of Ozone and Ultraviolet-B Radiation
Elizabeth C. Weatherhead
University of Colorado
A variety of groups are presently collecting ozone and UV data around the world by both satellite and ground based measurements. Prior results from these groups have produced conflicting and confusing results. The cause of these problems is difficulties in instrumentation and analysis techniques. This group is dedicated to the analysis of these ozone and UV data for reliable results, particularly with respect to trends. Statisticians and atmospheric scientists are working together to develop the techniques necessary for coming up with reliable trend estimates. A careful re-analysis of the RB UV monitoring network have revealed problems which make the data insufficient for trend analysis. This analysis can give guidance to future UV monitoring with respect to UV trends.