UV-B Radiation: How it is Measured and Results of Observations. James B. Kerr

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1 UV-B Radiation: How it is Measured and Results of Observations James B. Kerr Atmospheric Environment Service 4905 Dufferin Street Downsview, Ontario, Canada Abstract Many biological systems are influenced by the intensity of solar radiation in the ultraviolet (UV) region of the spectrum reaching the Earth s surface. It is important to study this radiation since it is strongly influenced by many geophysical variables, including atmospheric ozone which causes the short wavelength cutoff of the solar spectrum. Various types of instruments have been developed to measure solar UV-B radiation and several long-term monitoring networks have been established. Instruments with the required stability for making routine field measurements of spectral UV irradiance over long periods of time have only been available since the latter part of the 1980's. Establishing the long-term behaviour of UV irradiance as a function of time of year and over large areas of the Earth s surface is a difficult task. Radiative transfer model calculations are limited because variables such as clouds, ozone, haze, pollution and surface reflectivity are difficult to model accurately. Analyses of the data from the relatively new spectral irradiance records have enhanced our understanding of the behaviour of UV radiation and its dependence on stratospheric ozone under a wide variety of conditions. A brief description of the methods for measuring UV radiation will be given and results illustrating our current understanding of this radiation and its dependence on variables such as ozone and other factors will be presented. Introduction The extent to which ozone has decreased over the past two decades has been reported by many researchers and the results have been summarized in periodic international scientific assessments on ozone depletion [e.g. WMO, 1991; 1994]. These assessments have quantified the long-term trends in ozone as functions of latitude and time of year and have summarized findings which associate these changes to enhanced levels of stratospheric chlorine and bromine that are anthropogenic in origin. The long-term trend values are based on the results of analyses of stratospheric ozone measurements which have been made routinely on a global scale for nearly 4 decades. Figure 1 shows spectra of solar radiation at UV-B ( nm) and UV-A ( nm) wavelengths measured in space as well as at the Earth s surface at noon on a summer day in

2 Figure 1: Measurements of UV radiation from space and at the Earth s surface on a summer day at noon in Toronto. The sharp reduction in irradiance at shorter wavelengths is due to stratospheric ozone. The erythemal action spectrum is also shown. Toronto. It is quite apparent that the reduction of radiation by the atmosphere is significantly more pronounced at shorter wavelengths. Irradiance values at the surface are ~50%of extra-terrestrial values at 325 nm (UV-A) and decrease to less that 0.1% transmission in the UV-B at 295 nm. The main cause for this sharp reduction in the UV-B is stratospheric ozone. On the other hand, biological sensitivity generally increases quite significantly towards shorter wavelengths as illustrated in Figure 1 by the Commission internationale de l éclairage (CIE) erythemal action spectrum [McKinley and Diffey, 1987]. The quantification of long-term changes in UV-B irradiance at the earth s surface is more difficult than detecting long-term changes in ozone. A large part of the difficulty has been the lack of availability of adequate instrumentation. Broad-band instruments have been operating since the middle 1970's in the United States and have been the source of data for a trend analysis at several sites [Scotto et al., 1988]. Results of this analysis are somewhat unexpected since there was an observed decrease in UV-B irradiance over a period when stratospheric ozone was also decreasing. Reanalysis of the data by Weatherhead et al. [1997] illustrates the difficulties in maintaining a stable calibration for these instruments over a long period of time. These broad-band instruments measure only one parameter and, since surface UV irradiance depends on many variables, it is difficult to identify the cause of the measured change in radiation.

3 Analysis of spectral data can distinguish between changes due to ozone depletion and changes due to other causes. However, it is only relatively recently (since the late 1980's) that instruments have been developed with the capability of carrying out routine field measurements of spectral UV-B radiation with the required accuracy and long-term stability [Kerr and McElroy, 1993; 1994]. These relatively short records have been analysed to quantify the dependencies of UV-B radiation as functions of ozone and other variables. Instrumentation There are basically three types of ground-based instruments which measure UV-B radiation. These are the broad-band filter instruments, spectrometers and multi-filter narrow band instruments. In order to measure UV irradiance, all instrument types must be calibrated on an absolute basis. Calibration is carried out by measuring the response of the instrument to incandescent lamps with outputs traceable to calibrations made at a national standard institute such as the U.S. National Institute for Standards and Technology (NIST). Accurate calibration of an instrument requires careful regulation of the power to the lamps and accurate alignment of the lamp with the instrument. The simplest type of instrument for measuring UV-B radiation is the broad-band instrument which has been in routine operation since the 1970's. This instrument uses one or more filters with broad wavelength pass band (~10-30 nm). In many cases the transmission of the broad-band filter attempts to approximate the erythemal weighted response (shown in Figure 1), so that a measurement will be proportional to the amount of sunburning radiation. In order to calibrate these instruments on an absolute scale, it is necessary to know accurately both the filter transmission and the response of the detector as a function of wavelength. Advantages of these instruments include their ease of operation and their relatively inexpensive cost. One dis-advantage is that wavelength dependence of the filter transmission and the detector response are difficult to characterize. This causes uncertainty for determining the calibration of the instrument and for assessing the longterm stability of wavelength transmission or responsivity. Another disadvantage is that most broad-band instruments measure only one value (weighted UV irradiance), making it difficult to attribute specific causes for variations in the observed reading. Spectrometers have now been in routine operation at some sites for about 10 years. Although these records are relatively short, the data acquired by these instruments has contributed significantly to the understanding of UV irradiation and radiative transfer through the atmosphere. The advantage of this type of instrument is that the wavelength response (slit transmission function) is relatively easy to characterize. Therefore absolute calibration is carried out with greater certainty. Also, since irradiance is measured across the entire spectrum, changes due to ozone can be distinguished from changes due to other causes such as clouds, haze, pollution or.surface albedo. Disadvantages of spectral instruments are that they require careful operation to obtain results with suitable accuracy and stability and that they are relatively expensive. Several networks of spectral UV-B instruments are now in operation. Figure 2 shows a map with sites that are known to be monitoring spectral UV radiation at the ground. Data from the sites depicted by solid symbols have been reported to the World Ozone and Ultraviolet radiation Data Centre (WOUDC) in Toronto and provide the database for results reported here. The database includes data from 21 sites with about 100 station years of records that represent a reasonable coverage of the globe over a wide range of latitude and observing conditions. When data from other sites (depicted by open symbols on the map) become available, a more thorough analysis will be possible.

4 Figure 2: Location of spectral UV instruments. Recent improvements in filter technology have resulted in the development of the multi-filter narrow band instruments for measuring UV- B irradiance [Thompson et al., 1997; Bigelow et al., 1998]. The intent of this type of instrument is to use several narrow band (~2 nm) filters to sample at different wavelengths throughout the UV part of the spectrum. The filters are used either with a single detector which samples each filter in sequence or with one detector for each filter. These instruments are easier to operate and less costly than spectral instruments. They also provide more information than the broad-band instruments since they sample at several wavelengths. However, in order to derive spectral weighted values (such as the erythemal dose), it is necessary to combine the measurements with a radiative transfer model to recreate the entire spectrum. There have been several intercomparisons of ultraviolet monitoring instruments carried out since the early 1990's [e.g. Gardiner et al., 1993; 1994; Thompson et al., 1997]. These intercomparisons give a measure of agreement between different instrument types operated by different agencies using different calibration sources. Results of these intercomparisons show that there has been differences which arise from the calibration lamp reference and from variations on calibration methods. The agreement has generally improved during the 1990's as various problems are revealed and corrected. The more recent intercomparisons indicate that most instruments now agree to within about 5% provided a common calibration lamp reference is used.

5 Results of Spectral Measurements Dependence of UV Irradiance on Ozone Figure 3 shows examples that illustrate how UV irradiance values at different wavelengths in the UV vary differently with total ozone. The measurements were made at Toronto for the period from 1989 to 1996 (left) and Montreal between 1993 and 1996 (right) when the solar zenith angle (ZA) is within 2.5 o of 45 o. Radiation at shorter wavelengths in the UV-B (300 nm) is absorbed strongly by ozone and shows a significant dependence on the amount of total ozone (top panels). Energy values increase by about a factor of 10 as ozone decreases from 450 to 250 Dobson Units (DU). The middle panels show that radiation at longer wavelengths in the UV-A (324 nm), where ozone has negligible absorption, has very little dependence on ozone. The bottom panels show that the UV Index [Kerr et al., 1994; Burrows et al., 1995], which is proportional to the erythemally weighted [McKinley and Diffey, 1987] irradiance at UV-B and UV-A wavelengths, is noticeably dependent on the amount of total ozone. Figure 3 illustrates that the dependence of irradiance values on total ozone as measured in Toronto is very much the same as that seen in Montreal. In fact the dependencies of UV irradiance on ozone are similar to the results shown above for the other stations reporting to the WOUDC. The results in Figure 3 illustrate the relationship between UV irradiance and ozone only for a zenith angle of about 45 o and only at wavelengths of 300 nm and 324 nm. A more complete relationship over all wavelengths between 290 nm and 325 nm and for solar zenith angles between 25 o and 90 o has been quantified with a statistical model reported by Fioletov and Kerr [1997]. Comparison of the observations at Toronto with radiative transfer model is reported in Eck et al., [1995]; Herman et al. [1996] and Krotkov et al., [1998].

6 Figure 3: Influence of total ozone on irradiance measurements in the UV-B at 300 nm (top), in the UV-A at 324 nm (middle) and the UV Index (bottom) for Toronto (left) and Montreal (right). Measurements are for a solar zenith angle of about 45 o. Results from Toronto and Montreal are very much the same. Dependence of UV Irradiance on Other Variables Figure 3 illustrates that UV irradiance is significantly variable, even at a constant solar zenith angle (45 o ) and for a constant ozone value. The reason for the significant variability is due to other causes such as clouds, aerosols, surface reflection (albedo) and pollution. The data points in Figure 3 define fairly distinct upper limits which represent the clear sky conditions. The reduction in irradiance is due to scattering (by clouds and aerosols) and absorption (by pollution and absorbing aerosols) processes in the atmosphere. These processes can reduce the irradiance by more than 95% in some situations. These reductions are mostly due to clouds and aerosols.and are in general about the same for all wavelengths [Kerr, 1997; Fioletov and Kerr, 1996; Fioletov et al., 1997; Wardle et al., 1997], except in some cases under very heavy convective clouds or when an UV absorbing gas (such as ozone or SO 2 ) is present in the atmosphere from sources such as pollution or a volcanic eruption..

7 A few isolated points lie above the upper limits in Figure 3 and are likely due to real causes such as snow on the ground which can enhance UV irradiance values by more than 30%, depending on location [Wardle et al., 1997]. In general these enhancements are approximately the same at all wavelengths and can therefore be distinguished from ozone absorption which has strong wavelength dependence. Figure 4 shows an example from Resolute that clearly illustrates the enhancement in UV irradiance caused by snow. The data in Figure 4 are in the UV-A at 324 nm where there is negligible effect from ozone absorption. The diagram shows measured values.as functions of time of day and time of year expressed as the percentage of the value expected under!clear sky" conditions. The clear sky values were determined statistically from the upper limit (see middle panels in Figure 3 for ZA = 45 o ) observed at all stations for all days without snow and for all zenith angles. From October to May, when there is snow on the ground and the sky is usually clear, most of observations shown in Figure 4 are more than 100% of the clear sky value with up to 40% enhancement. From June to September, when there is usually no snow and the sky is often cloudy, the readings are lower and more variable. Figure 4: Measurements made at 324 nm relative to the expected!clear sky" value. Snow on the ground between September and May can enhance values by up to 40%. There is no influence of ozone since ozone has negligible absorption at 324 nm.

8 Present UV-B Climatology There are now about 100 station years of spectral UV irradiance data stored in the WOUDC. Many of the stations have records more than five years long. These data have been used to determine average UV values measured over the length of the records at each site. Examples of how these UV-B!climatologies" may be graphically summarized are shown in Figure 5. In these example, the average values of the observed UV Index are shown as a function of time of day and time of year at Churchill, Canada (59N), San Diego (32N), Toronto (44N) and Sapporo, Japan (43N). Inspection of the diagrams in Figure 5 reveals several interesting behaviours of UV-B irradiance. At San Diego, Toronto and Sapporo the UV Index is lower near the spring equinox (March 21) than it is near the fall equinox (September 21). This is due to the fact that there is more ozone during spring than there is in fall. In Churchill, however, the UV Index is higher in spring than it is in the fall. This is because the UV levels are enhanced by snow on the ground Figure 5: UV Index!climatology" plots for Churchill, San Diego, Toronto and Sapporo. Graphs show values averaged over several years..which usually disappears in May or June. Comparison of the four station climatologies clearly shows higher levels of UV irradiance at lower latitudes. It also illustrates the longer hours of sunshine during summer and shorter hours during winter at higher latitudes. Comparison

9 of the climatology at Toronto with that at Sapporo indicates that there can be significant differences in UV levels at two stations with similar latitude. These differences are due to different ozone and/or cloud conditions. The measured climatologies that are evolving from the growing data base are useful to compare with short term variations. Figure 6 shows the UV Index for individual years at Palmer, Antarctica (64S, 1993), San Diego (32N, 1993), Toronto (43N, 1993) and Mauna Loa, Hawaii (20N, 1997). The observed day-to-day fluctuations are much larger than one might infer from.the climatology diagrams shown in Figure 5. These fluctuations result in larger peak values than are seen in the climatological averages. For example, the climatology for Toronto shows a peak Index of less than 7 in June and July, but the individual observations made in 1993 (when ozone was a record low) show peak values greater than 9.5. From the perspective of biological effects, the episodes of extreme values on days with low ozone and clear skies may be just as important to consider as changes in the climatological averages. Figure 6 shows that Palmer under the ozone hole in October has UV Index values greater than 11, suggesting very high sun burning potential. These values are more than those seen on a summer day in San Diego in southern California. The record at Mauna Loa illustrates that the combination of low ozone values in the tropics with the sun directly overhead and the high altitude (3500 m) results in UV Index values peaking near 16. Figure 6: UV Index values for Palmer, Antarctica (1993), San Diego (1993), Toronto (1993) and Mauna Loa, Hawaii (1997). Palmer Station had higher values under the Antarctic ozone hole than San Diego did in summer. UV Index values at Mauna Loa are one of the highest observed (up to 16) under the low ozone values of the tropics with the sun nearly overhead and at high altitude (3500 m).

10 Estimating Past UV-B Climatology In order to determine long-term changes in UV-B irradiance that may have occurred since the onset of ozone depletion in the late 1970's it is necessary to estimate levels of UV-B irradiance that existed before routine spectral measurements were made. Estimates of past UV-B levels are the product of a thorough understanding of the dependancies of spectral UV-B irradiance on geophysical variables and applying these relationships with the aid of radiative transfer models to archived measurements of the other variables (e.g. ozone, cloud cover, optical depth, etc.). Work with the goal of reconstructing past UV-B climatology has been under way in recent years. The estimated past climatologies may then be compared with those of recent years to determine long-term changes. Several studies have been done [e.g. Frederick et al., 1993; Madronich, 1992] that calculate changes in UV-B irradiance from long-term changes in total ozone measured by ground-based or satellite instruments. In deriving estimated changes in UV-B irradiance most of these studies have assumed that the effects of clouds and other variables have not changed over time and are therefore representative of changes under clear sky conditions. Kerr and McElroy [1993; 1994] reported results that quantify changes in spectral UV-B irradiation to changes in ozone under all types of weather conditions derived from measurements made at Toronto between 1989 and Applying the reported relationship between changes in ozone and irradiation to the known long-term trends in ozone [WMO, 1991; 1994] a long-term trend of 11% per decade would be determined for irradiation at 300 nm for summer months (May to August). This trend value has since been verified at Toronto through extension of the record both into the past (to 1986) and the future (to 1996). More recently Herman et al. [1996] have estimated past UV-B irradiance values at the surface from measurements made by the Total Ozone Mapping Satellite (TOMS). The TOMS instrument uses backscattered radiance measurements at several wavelengths in the UV to determine total ozone from space. The backscattered data may also be used to estimate cloud thickness and therefore derive the attenuation of surface UV-B irradiance due to clouds. Comparison of the TOMS estimated UV-B surface irradiance with actual ground-based irradiance measurements has been a key part of development of this approach [Eck et al., 1995]. Although there are still problems to overcome with this method the potential to provide a realistic estimation of surface UV-B irradiance values on a daily basis with global coverage back to the late 1970's is quite promising. The possibility to extend the record further back in time at sites where total ozone and cloud cover are measured also exists, although results of such analyses are yet to be reported. Conclusion Measurements of spectral UV irradiance are important to help our understanding of the behaviour of UV irradiance and to quantify relationships with the many variables that affect radiation at the earth s surface. Accurate replication of past and present global climatologies of UV is important for studies on the impacts of changing UV levels and can only be achieved through a better understanding of the relationships.

11 Spectral UV irradiance measurements have been made on a routine operational basis at network sites since the late 1980's and early 1990's and have proven to be quite reliable. The data-base for spectral UV irradiance measurements has been growing over the past few years and is expected to continue to grow in future years both with increasing length of the records as well as an increased number of stations measuring and reporting data. Analyses of the data records have yielded information and results which were not known 10 years ago. The analysis has matured from isolated studies that define results pertinent only to a specific site to more general studies that use data from a wide range of sites to define results on a global scale. Acknowledgements Dr. Vitali E. Fioletov carried out a significant part of the analysis reported here. Drs. D.I. Wardle and C.T. McElroy contributed fruitful scientific discussion. References Bigelow, D.S., J.R. Slusser, A.F. Beaubien and J.H. Gibson, The USDA ultraviolet radiation monitoring program, submitted J. Geophys. Res., Burrows, W., M. Vallee, D.I. Wardle, J.B. Kerr, L.J. Wilson and D.W. Tarasick, The Canadian UV-B and total ozone forecast model, Met. Apps., 1, , Burrows, W., Cart regression models for predicting UV radiation at the ground in the presence of cloud and other environmental factors, J. Appl. Meteorol., 36, , Correll, D.L., C.O. Clark, B. Goldberg, V.R. Goodrich, D.R. Hayes, W.H. Klein and W.D. Schecher, Spectral ultraviolet-b radiation fluxes at the earth s surface: long-term variations at 39 o N, 77 o W, J. Geophys. Res., 97, , Eck, T.F., P.K. Bhartia and J.B. Kerr, Satellite estimation of spectral UV-B irradiance using TOMS-derived total ozone and UV reflectivity, Geophys. Res. Lett., 22, , Fioletov, V.E. and J.B. Kerr, Numerical relationship between UV irradiance, total ozone, and other variables from analysis of Brewer spectral UV-B measurements archived at the World Ozone and UV Data Centre, Proc. Quadrenniel Ozone Symp., Fioletov, V.E., J.B. Kerr and D.I. Wardle, The relationship between total ozone and spectral UV irradiance from Brewer spectrophotometer observations and its use for derivation of total ozone from UV measurements, Geophys. Res. Lett., 24, , Frederick, J.E., P.F. Soulen, S.B. Diaz, I. Smolskaia, C.R. Booth, T. Lucas and D. Neuschuler, Solar ultraviolet irradiance observed from Southern Argentina: September 1990 to March 1991, J. Geophys. Res., 98, , 1993.

12 Gardiner, B.G., A.R. Webb, A.F. Bais, M. Blumthaler, I. Dirmhim, P. Forster, D. Gillotay, K. Henriksen, M. Huber, P.J. Kirsch, P.C. Simon, T. Svenoe, P. Weihs amd C.S. Zerefos, European intercomparison of ultraviolet spectroradiometers, Env. Tech., 14, 25, Gardiner, B.G. and P.J. Kirsch, eds., Second European intercomparison of ultraviolet spectro-radiometers: report to the Commission of the European Communities, Air Pollution Research Report 49, Report EUR 15449, Herman, J.R., P.K. Bhartia, J. Ziemke, Z. Ahmad and D. Larko, UV-B radiation increases ( ) from decreases in total ozone, Geophys. Res. Lett., 23, , Kerr, J.B., C.T. McElroy, D.I. Wardle and D.W. Tarasick, The Canadian ozone watch and UV-B advisory programs, in Ozone in the troposphere and stratosphere, Proceedings of the Quadrennial Ozone Symposium 1992, NASA Conference Publication 3266, Berlin: Springer-Verlag, , Kerr, J.B. and C.T. McElroy, Evidence for large upward trends of ultraviolet-b radiation linked to ozone depletion, Science, 262, , Kerr, J.B. and C.T. McElroy, Analyzing ultraviolet-b radiation: is there a trend?, Science, 264, , Kerr, J.B., Observed dependencies of atmospheric UV radiation and trends, NATO ASI Series, Vol. I 52, Eds. C.S. Zerefos and A.F. Bais, Berlin: Springer-Verlag, , Krotkov, N.A., P.K. Bhartia, J.R. Herman, V. Fioletov and J. Kerr, Satellite estimation of spectral surface UV irradiance in the presence of tropospheric aerosols 1: Cloud-free case, Submitted J. Geophys. Res., Madronich S., Implications of recent total ozone measurements for biologically active ultraviolet radiation reaching the Earth s surface, Geophys. Res. Lett., 19, 37-40, McKinley, A.F. and B.L. Diffey, A reference spectrum for ultraviolet induced erythema in human skin, in Human Exposure to Ultraviolet Radiation: Risks and Regulations, Edited by W.R. Passchler and B.F.M. Bosnajokovic, Elsevier, Amsterdam, Scotto, J.,G. Cotton, F. Urbach, D. Berger, and T. Fears, Biologically effective ultraviolet radiation: measurements in the United States , Science, 239, , Thompson, A., E.A. Early, J. DeLuisi, P Disterhoft, D. Wardle, J. Kerr, J. Rives, Y. Sun, T. Lucas, T. Mestechkina, P. Neale, The 1994 North American interagency intercomparison of ultraviolet monitoring spectroradiometers, J. Res. NIST, 102, , Wardle, D.I., J.B. Kerr, C.T. McElroy and D.R. Francis, eds., Ozone Science: a Canadian perspective on the changing ozone layer, Environment Canada Report CARD 97-3, 1997.

13 Weatherhead, E.C., G.C. Tiao, G.C. Reinsel, J.E. Frederick, J.J. Deliusi, D. Choi and W. Tam, Analysis of long-term behavior of ultraviolet radiation measured by Robertson-Berger meters at 14 sites in the United States, J. Geophys. Res., 102, , WMO, Scientific Assessment of Stratospheric Ozone: 1991, World Meteorological Organization Global Ozone Research and Monitoring Project, Report No. 25, Geneva World Meteorological Organization, WMO, Scientific Assessment of Stratospheric Ozone: 1994, World Meteorological Organization Global Ozone Research and Monitoring Project, Report No. 37, Geneva World Meteorological Organization, 1994.