DECREASINGOZONE CAUSES HEALTH CONCERN How Canada Forecasts Ultraviolet-B Radiation 1
arly in 1992, the U S . National Aeronautics and Space Administration [NASA] reported that sateiure and aircraft measurements indicated high levels of reactive chlorine over a wide area at middle and high latitudes i n the Northern Hemisphere. NASA suggested that these large values had a strong potential for depleting the ozone layer. In Canada, the NASA announcement followed several years of had news about the diminishing ozone layer in polar regions and midlatitudes. In the 1980s: the ozone layer over Toronto [Canada's most populous city) decreased hv 4.2%. The Toronto measurements agreed with those of the Total Ozone Mapping Spectrometer [TOMS] aboard the Nimbus 7 satellite [which was launched in October 1978 and provides daily maps of the global distribution of total ozone]. which showed similar decreases at midlatitudes over the entire Northern Hemisphere. Long-term trends pointed to continuing ozone depletion over the mid-latitudes, where most of Canada's people live. Each new report of the shrinking ozone column fueled speculation about the damaging effects of increased levels of UV-B radiation at the Earth's surface, bath on human health and on other biological systems. The ozone layer is the planet's sunscreen; it is largely responsible for filtering out the most damaging UV radiation-the rays responsible for causing sunburn, skin cancers, and cataracts. 514 A
The NASA findings therefore triggered widespread concern io Canada. Almost immediately, ozone information programs. which had been on the back burner for some time, became priorities. The anxious climate prompted a blitz of activity at Environment Canada. In less than four months, two new products were launched and became widely available coast to coast. In March, Environment Canada introduced Ozone Watch, a weekly bulletin that gives the public an up-to-date assessment of the state of the ozone layer over Canada. In May, the new UV (Ultraviolet] Index program brought forecasts of UV levels as part of daily weather reports across the country. Environment Canada was able to implement these new programs quickly in I992 because the concept and the tools were in place as a result of some 40 years of study and monitoring ofthe ozone layer. Canada has one of the world's longest records of ozone study, and three years before the NASA discovery, routine groundbased monitoring of UV readings at t h e Earth's surface began i n Toronto. Ozone Watch
The development of Ozone Watch was relatively straightforward. Using historical measurements of
JAMES E. KERR Environment Canada Downsview, ON, Canada M3H 5T4
Environ. Sci. Technol.. VoI. 28. No. 12. 1994
ozone depth from Canada's countrywide network, scientists developed graphs to depict how ozone depths have changed [Figure I]. The bulletin also uses maps to display gains or losses in present-day ozone levels, over historic averages [Figure 2 ) . The gain-loss figures depend on comparison of today's ozone depths t o baseline data, which in most cases stretch back about two decades prior to 1980 [Figure 3). The figures compare current measurements of total ozone [averaged over two weeks of daily measurements] with historical values. It was possible to put the Ozone Watch program in operation in less than two months after the NASA report because the building blocks were in place. Canada's record of ozone study stretches back to 1948, when Canadians carried out their first experimental ozone measurements. In the early 1960s, Canada took responsihility for the World Ozone Data Centre for the World Meteorological Organization (a role it still plays). The center collects global data on the thickness of the ozone layer and publishes Ozone Data for the World. In 1957, Canada's Ozone Monitoring Network began operation, and by 1964 five sites (Toronto, Edmonton, Goose Bay, Churchill, and Resolute) were measuring total ozone with Dobson spectrophotometers. During the 198os, the Dobsons were replaced by the Canadian-designed Brewer spectrophotometers, an automated instrument that enabled more precise and frequent measurement.
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The network more than doubled in the 1980s and early 1990s as aresult of increased concern surrounding the discovery of the Antarctic ozone hole. Canada now monitors ozone from the ground at the original five sites as well as new mid-latitude sites in Saskatoon, Saturna, Winnipeg, Montreal, and Halifax, and at Alert and Eureka in the Canadian Arctic. Eureka is the new Arctic stratospheric ozone research laboratory that is monitoring stratospheric constituents as part of the World Meteorological Organization's Network for the Detection of Stratospheric Change (see Table 1).
The Brewer instrument measures the intensity of radiation falling on a horizontal diffusing surface. Each spectral measurement consists of the average of a forward and backward wavelength scan, which takes
Measuring W - B Measurement of spectral W - B is a relatively new activity for Canada, but it began routinely in Toronto in March 1989. The Brewer ozone spectrophotometer can provide detailed and highly accurate data on both ozone and UV. UV is now measured at many sites around the world, but most sites are equipped with simple broadband instruments that give only a single number for the entire UV-B range, rather than individual measurements at each wavelength. At the Canadian stations, the Brewer instrument measures radiation at 0.5-nm wavelength intervals, for wavelengths between 290 and 325 nm with a spectral resolution of
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about 8 min IO compiere. Nieasurements are made once or twice each hour throughout the day from sunrise to sunset. The Toronto measurements were the first near a populated area, but between 1990 and 1992 the UV-B measurements began to be made routinely at other Canadian sites. The daily record of measurements of UV-B a n d total ozone over Toronto showed how day-to-day fluctuations in UV-B vary with changes in column ozone. The measurements confirmed what scientists had predicted. For example, radiation at wavelengths between 320
and 325 (W-A) does not vary appreciably with changes in the ozone column because radiation at these wavelengths is not significantly absorbed by ozone. However, radiation at shorter wavelengths (UV-B), which is of more concern to people because of its detrimental health effects, was found to vary significantly with changes in ozone depth. The measurements that showed the correlation took the ozone-W-B relationship out of the realm of speculation and allowed Canadian scien-tists to develop the formula to relate changes in UV-B to changes in ozone. For most Canadians, the UV Index is simply a number from 0 to 10; the higher the number, the higher the risk for exposure to UV-B. The UV forecasts are issued daily, nationwide, from 45 locations and advise Canadians on outdoor habits depending on the number. How the index works The aim of the W Index is to provide the public throughout Canada with a forecast of the intensity of the sunburning component of W - B radiation for the next day. Scientists felt that simply reporting the current day's UV was not sufficient. The Index was developed on the assumption that a prepared public will take steps to protect themselves from the sun. In mid-latitudes, UV-B poses a greater health risk
Envimn. Sci. Technol.. Vol. 28, No. 12. 1994 515A
than it ever has in the past, but that is only partly caused by diminishing ozone. In Canada, as many other places, people now spend more time relatively uncovered in the sun, both at home and on southern vacations in winter. Because the Index was designed to be useful to the general public, it is nondimensional. The UV Index in Canada runs from 0 to 10: Index values in the dark of night are 0; at high noon, on a clear day in July, they may exceed 9. (For times of day other than high noon, index values are determined using noontime forecasts and solar elevation.) In the simplest terms, a very high UV-B Index number (greater than 9) means a very high risk of sunburn from UV-B. If the Montreal forecast warns that the day’s UV Index will climb over 9,for example, Montrealers know that the UV-B will be extremely strong and the risk of sunburn very high. The message is clear: Protect yourself. The scale was chosen so that the typical sea-level tropical values
would be in the 10-12 range (for local solar noon under clear sky conditions). Since the Index was developed, scientists have found that tropical values might be as high as 14,but the values important to Canada are all on target. The very simple scale was deliberate. The Index was designed as an Environment-Health p r o d u c t , marking a departure from strictly scientific weather reports. The daily bulletins contain the forecast UV Index values issued by Environment Canada as well as a message regarding UV-B and health effects from Health Canada. To design a scale that would be publicly useful, scientists consulted with Health Canada and many health groups (including t h e Canadian Cancer Society, the Canadian Dermatology Association, the Canadian Public Health Association, the Canadian Ophthalmology Association, the Canadian Association of Optometrists, public health agencies, and instrument manufacturers). Although the Index is designed to
Ozone depth less than normal Insufficient data
516A Environ. Sci. Technol., Val. 28. No. 12, 1994
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work all year round, it is most widely used in summer. Although ozone losses are greatest in winter, UV-B levels in Canada are still low, primarily because of the low angle of the sun, resulting in a longer path of sunlight through the ozone. In a Canadian winter, the light is weak, and short winter days mean less exposure time.
How the Index is produced Despite its simple public profile, the Index is a continuing scientific challenge. In broad brush strokes, the formula is straightforward. The intensity of UV radiation reaching the ground depends on the solar pathlength through the atmosphere and the total ozone that is encountered along that path. The pathlength is calculated using welldefined geometry and accurate ephemeral information regarding the position of the sun. (Forecasts apply to local solar noon, when the sun is highest in the sky.) After determining total ozone levels in a vertical column, scientists can cal-
tology and latitudinal predictors. This “total ozone field” is adjusted by comparing the current day’s measurements with those that had been forecast. That is, today’s observations are used to correct tomorrow’s forecast. Current ozone measurements are immediately available on Environment Canada’s computer network 11 Brewer instruments in the field automatically transmit real-time measurements from most sites. Once the ozone column’s depth is predicted, the expected UV-B can be forecast. Even with complete accuracy, however, there are complications, such as haze and cloud. In 1992, the UV Index forecast UV-B only for sunny conditions. Now, the forecasts take weather conditions into consideration, but it is still difficult to predict cloud cover accurately and modify UV forecasts accordingly.
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culate the erythemal [sunburning) component of UV-B flux in milliwattslmeterzlnm. This is the formula on which the UV-B Index is built, but because the UV Index was designed to be nondimensional, these UV-B flux numbers are simply divided by 25 milliwatts/meterZlnm to give a numerical range of 0 to about 10. The formula was developed using three years of near-simultaneous measurements of total ozone and UV-B at Toronto. For the Index, the UV wavelengths of primary importance were the erythemal units. The Brewer radiation measurements are weighted according to the McKinley-Diffey Action Spectra, which weights the intensity of radiation at each wavelength according to how effectively it causes sunburning.
(Forecasts of UV-B are t h u s weighted fluxes over the wavelength range from 290400 nm). With a relationship among total ozone, zenith angle, and W - B established, the basic problem boiled down to forecasting total ozone thickness as accurately as possible. Day-to-day changes are critical. In summer, variations are relatively small: Typical variation might be 5-10%. Day-to-day variations are higher in winter, but the W Index has concentrated on summer because of the health risks of the stronger summer sun. Prediction must take place for each location across the country. Past total ozone values are correlated to meteorological variables forecast by the Canadian Meterological Centre, including ozone clima-
Widespread use Canada’s operational weather forecast service is organized with a central prediction facility in Montreal, the Canadian Meteorological Centre (CMC),which produces a variety of forecasts for all of Canada. For each of the 45 forecast locations, the CMC calculates the total ozone depth, converts it to forecast clear sky W flux,calculates the W Index, and modifies it where necessary to account for forecast sky conditions. A bulletin is prepared giving the location, cloud description, forecast UV Index, category (low, medium, high, extreme], and the time at which the UV Index will be above 4. To meet the deadlines of morning newspapers [which are printed late at night), forecasts are prepared 18 hours i n advance and supplemented by 12-hour forecasts made later in the evening. Since its development in Canada, the Index has generated considerable attention internationally. Programs based on Canada’s index have been developed and recently implemented in England and the United States. Because more and more countries are interested in developing similar products, t h e World Meterological Organization is looking closely at the Canadian experience to develop international standards. A good monitoring capability must be in place to provide information for accurate forecasts of UV-B. Both satellite and groundbased real-time ozone data have become more readily available in
Environ. Sa. Technol.,VoI. 28. No. 12, 1994 517A
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recent years. The Brewer spectraDhotometer is now being- used in 25 countries. Althoueh there is considerable work to d;' in improving prediction of the ozone column, there is currently a good correlation between prediction and reality. In summary, the development and implementation of the new public awareness programs in Canada to help inform the general public on the state of the ozone layer and provide quantitative information regarding UV-B levels came from an extension of programs that were already in place in 1992. The monitoring a n d prediction of weather as well as the monitoring of ozone and UV-B radiation were all operational at the time. The new programs resulted from the enhancement of these existing operations and a focused scientific effort to improve the understanding of links between meteorology, atmospheric ozone, and UV-B radiation. The cost to provide the new programs has been relatively small compared with the overall cost of the existing operations: however, the potential benefits are large if these new programs help reduce detrimental health effects by modifying people's behavior regarding sun exposure. It is worth noting, in conclusion, that the NASA findings that got Canadians moving did not ultimately result in major decreases in ozone over Canada during 1992. For Canadians, however, the impetus not only focused attention on the ozone layer, but gave a boost to programs that are helping people to make healthful lifestyle choices.
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REFER TO KEY NO. 1
518A Environ. Sci. Technol.. VoI. 28, No. 12. 1994
James B. Kerr is head of Ozone Research and Monitoring for the Atmospheric Environment Service of Environment Canada and has been involved in the study of the ozone layer over Canada for 25 years. He is co-inventor of the Brewer oznne spectrophotometer. His recent work includes the completion of the first long-term study to demonstmte thot the thinning of the ozone layer has led to increoses in ultmviolet levels at the Earth's surfoce.
