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E L S E V I E R Joul-ilal of Photochemistry and Photobiology B: Biology 31 (1995) 63-68

Continual increase in biologically effective dose of solar UV radiation determined by spore dosimetry from 1980 to 1993 in Tokyo

Nobuo Munakata Radiobiology Division, National Cancer Centre Research Institute, Tsukiji 5-1-1, Chuo-ku, Tokyo 104, Japan

Abstract

The genotoxic activity of solar UV radiation was monitored by measuring the inactivation of UV-sensitive spores of Bacillus subtilis; this has been performed since 1980 in Tokyo. The relationship between the cumulative values of the half-day inactivation dose and the half-day global insolation was determined by regression analysis. Data collected in 127 afternoons were grouped into three successive periods ( 1980- 1986, 1989-1991 and 1992-1993). The ratio of the UV dose to the insolation of the last group was the highest, and that of the first group was the lowest. The ratio of the observed half-day inactivation dose to that expected from the relationship was regressed to daily values of the ozone column recorded at the nearest Tsukuba observatory. The relative inactivation dose was related to the ozone column by a power function with a constant exponent of -2.1. However, the inclusion of this relationship in the regression analysis between the inactivation dose and insolation could not explain completely the differences observed between the three groups. It was suggested that certain atmospheric factors other than the change in the stratospheric ozone concentration over the Kanto plain were responsible for this continual increase.

Keywords: Biologically effective dose; Solar UV radiation; Spore dosimetry

1. Introduction

Predicted changes in solar UV radiation in our environment due to stratospheric ozone depletion have raised concern and debate in the public arena and scientific community. The paucity of quantitative and experimental data on the subject demands more active contributions by photobiologists. The use of biological systems to estimate, monitor and evaluate the amount of solar UV radiation received on the Earth's surface is indispensable. To fulfil this need, a dosimetric system has been developed using Bacillus subtilis spores and has been employed since 1980 in Tokyo [1-3] . Previous analysis of the data obtained up to 1991 led to an unexpected conclusion that the cumulative half-day inactivation dose (ID) relative to the half-day insolation (INS) increased sig- nificantly in the late 1980s [3]. Additional data collected in 1992 and 1993, and analysed in this report, lead to another unexpected conclusion that the increase in the biologically effective dose of solar UV radiation has been continuous. In this paper, the causes of this change are investigated by taking into account ozone column data and other meteorological variables in the relationship between ID and INS.

2. Experimental details

The methodology of spore dosimetry using UV-sensitive spores of Bacillus subtilis strain TKJ6312 has been described

1011-1344/95/$09.50 © 1995 Elsevier Science S.A. All fights reserved SSDI1011-1344(95)07170-9

previously [1-3], and was performed on the roof of the National Cancer Centre Research Institute situated in down- town Tokyo near Tokyo bay (35°40'N, 139°46'E). Survivals were obtained for the exposures covering an entire afternoon from local solar noontime to sunset. The absolute natural logarithm of the fractional survival was termed the inactiva- tion dose (ID). The ID values of each interval were summed to obtain a cumulative value of ID for a half day (half-day ID), which provided the basis for the analyses. Hourly global insolation (INS), visibility, cloudiness, temperature and humidity were taken from the daily records of the Tokyo Meteorological Observatory (35°41 'N, 139046 'E) . The INS (MJ m - 2 ) is a measurement of the global solar irradiance using an electric thermopile. The daily average value of the total ozone column in Dobson units determined by the Brewer spectrophotometer located at the Aerological Observatory (36°03'N, 140°08'E) in Tsukuba was available for each day of measurement. Since this station is in the Kanto plain within a distance of 53 km northeast of Tokyo, it was considered that the data were applicable to the Tokyo sky with regard to the stratospheric ozone layer. Concurrent measurements with an erythema UV dose and intensity meter (MED meter, model 5D, Solar Light Co., Philadelphia) have been made since 1990. The regression analyses were performed with a Kaleida software program (Synergy Software, Reading, PA)

64 N. Munakata / Journal of Photochemistry and Photobiology B: Biology 31 (1995) 63-68

for Macintosh computers. The long-term stability of the response to UV radiation of the spore samples was checked several times during the course of the study. No difference in the sensitivity to germicidal UV (254 nm) irradiation was observed between freshly prepared samples and those stored in water at 4 °C for up to 15 years.

3. Results and discussion

Spore dosimetry experiments were performed on 39 days in 1992 and 1993, and the data compiled in Table 1 were added to those of 35 days from 1980 to 1986 and 53 days from 1989 to 1991 [3]. Summary comparisons were made

Table 1 Summary of solar UV monitoring from 1992 to 1993 •

chiefly between the three sets of data (group 1 from 1980 to 1986, group 2 from 1989 to 1991 and group 3 from 1992 to 1993).

To gain an overview of the collected data, the values of the half-day ID are plotted on a logarithmic scale against the month of the year in Fig. 1. In this type of presentation, the data points at the profile maxima are under clear skys, since the values of the half-day ID can be reduced to almost zero under heavy clouds in any season. On examination, Fig. 1 shows that the profile for group 2 is above that for group 1 as noted previously. Furthermore, the values for group 3 exhibit a profile clearly above that for group 2. Thus most of the monthly maxima obtained in the earlier years are surpassed by the data added in 1992 and 1993.

Date Day OC SE CD VB HDINS HDMED HDID SNIR

19920221 52 34 i 0.691 1.0 7.0 4.16 2.21 5.75 0,060 19920226 57 382 0.719 0.0 20.0 7.43 3.28 9.38 0,070 19920423 114 347 0.922 0.0 30.0 11.49 9.32 33.55 0,203 19920521 142 383 0.965 0.0 8.0 12.22 10.38 36.20 0,163 10020629 181 323 0.976 9.5 5.0 6.90 6.80 29.89 O. 115 19920730 212 299 0.953 0.5 4.0 10.32 ! 0.94 49.16 0.271 19920805 218 286 0,946 10.0 4.5 3.73 4.95 30.13 0.203 19920806 219 282 0.944 0.9 4.0 10.63 12.56 53.60 0.274 19920807 220 283 0.943 1.0 10.0 10.50 15.24 52.39 0.286 19920811 224 304 0.936 10.0 10.0 4.50 4.51 i 6.24 0.119 19920817 230 282 0.924 7.0 20.0 I 1.22 17.02 57.63 0.321 19920819 232 281 0.919 4.0 25.0 9.62 i 6.12 54.91 0. 326 19920824 237 278 0.907 7.0 30.0 8.35 14.25 52.72 0.282 19930224 55 295 0.704 0.5 60.0 .59 6.06 30.12 0.153 19930303 62 329 0.737 2.0 12.0 8.08 4.60 13.47 0.070 19930317 76 334 0.795 i.0 30.0 9.70 6.92 34.19 0.229 ! 9930330 89 321 0.846 3.0 10.0 9.50 7.69 37.76 0.210 19930407 97 323 0.873 9.5 8.0 9.72 7.16 31.95 0.201 19930596 126 315 0.943 9.0 25.0 8.55 10.06 47.70 0.347 19930511 131 357 0.951 0.5 30.0 12.71 12.91 49.85 0.321 19930520 140 384 0.962 9.5 5.0 4.63 4.40 18.91 0.131 19930526 146 349 0.967 5.0 20.0 5.54 5.78 23.15 0.244 19930527 147 356 0.968 1.0 20.0 12.01 10.42 42.52 0.230 19930610 161 352 0.975 2.0 12.0 11.70 13.14 51.02 0.279 19930708 189 303 0.973 9.5 15.0 5.56 6.48 34.72 0.159 19930728 209 275 0.957 5.0 25.0 10.81 15.58 63.35 0.418 19930819 231 277 0.921 10.0 3.5 5.14 7.47 32.40 0.122 19930823 235 287 0.911 8.0 15.0 8.64 I 1.64 43.20 0.290 19930825 237 265 0.906 9.5 8.0 8.57 1 !.88 52.90 0.316 19930830 242 283 0.891 9.5 5.0 6.56 8.60 42.35 0.177 19930831 243 285 0.888 9.5 4.5 4.95 7.32 37.94 0.206 19930916 259 302 0.838 5.0 6.0 6.48 7.64 36.50 0.208 19930928 271 275 0.790 8.0 8.0 7.57 7.30 34.73 0.24 I 19931109 313 261 0.607 0.5 30.0 5.96 3.75 18.12 0.126 19931119 323 272 0.571 6.0 50.0 4.83 3.15 14.51 0.110 19931125 329 296 0.553 0.0 4.0 3.88 1.54 4.96 0.027 19931202 336 271 0.534 7.0 ! 5.0 3.81 2.13 9.40 0.074 19931215 349 255 0.516 9.0 40.0 2.94 ! .44 5.55 0.021 19931227 361 286 0.515 0.5 15.0 4.86 2.20 8.20 0.044

• Descriptions of columns: Date, year month day; Day, numbered day of a year; OC, daily representative value of total ozone column in Dobson units recorded at the Aerological Observatory in Tsukube; SE, solar elevation (sin 0) at solar noontime; CD, cloudiness factor at 12:00 (0 + and 10- in the record are shown as 0.5 and 9.5 respectively); VB, visibility (km) at 12:00; HDINS, half-day insolation (MJ m-=); HDMED, half-day minimal erythema dose; HDID, half-day inactivation dose; SNIR, inactivation rate constant (rnin-1) at solar noontime.

N. Munakata / Journal of Photochemist~ and Photobiology B: Biology 31 (1995) 63-68 65

1 0 2 I 1 1 I I 1 1 I I 1 t

@

.~ 10 ] ~,_ - ~ + .o ,~ •

.= 10 ° ,

.

1 0 " ] i i i ~ ~ , , ~ ,

JanFebMarAprMayJun JulAugSepOct NovDe c

Fig. 1. Year-round change in the values of half-day ID observed from 1980 to 1993. Data obtained from 1980 to 1986 (o), 1989 to 1991 (<~) and 1992 to 1993 ( • ).

I l l O . O

- 1 " S l ° o =

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, , o ¢1

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Half-day Insolation (MJ/m')

Fig. 2. Relationship between half-day ID and half-day INS for the data obtained from 1980 to 1986 (o) (line 5 of Table 2), 1989 to 1991 (C,) (line 6 of Table 2) and 1992 to 1993 ( • ) (line 7 of Table 2 )

Since the choice of the experimental day was arbitrary, without any fixed rules, and was dependent primarily on the weather (any days with heavy clouds or when rain was expected were avoided), a bias may arise if experiments were conducted on clearer days in recent years or vice versa. How-

Table 2

ever, the average values of the half-day INS (MJ m-2) are not significantly different between the three groups (8.57+_2.81 for group 1, 7.94+3.25 for group 2 and 7.75 + 2.83 for group 3), suggesting that the choice was not significantly biased with regard to insolation.

As described previously [3], to make quantitative com- parisons of these data, we can analyse the correlations between the values of the half-day ID and half-day INS. The ratio of the UV dose to the insolation seems to address our major needs, since we are less concerned with the absolute amounts than with the relative portions of biologically effec- tive UV dose within the total radiation received on the Earth's surface. Fig. 2 shows a plot of the half-day ID vs, the half- day INS. Clearly, there is a positive correlation between the half-day ID and the half-day INS. Both exponential and power functions can be used to fit the data; however, since it is natural to extrapolate to zero on both axes, a power function regression was preferred.

When the values of the half-day ID were regressed to those of the half-clay INS by a power function, the three sets of data yielded distinct lines (Table 2), Over the whole range of insolation (less than 15 MJ m - z ) attainable at this site, the line for group 3 is above that for group 2, which is distinctly above that for group 1. These regression relationships were simplified to be dependent on one parameter to ease compar- ison. This was performed firstly by regressing all the ID values to the INS values as shown in line 1 of Table 2. In the next step, the data set of each group was regressed with a fixed value of the exponent obtained from the total regression (b = 1.539). Fig. 2 exhibits the resulting lines of the con- strained regression, and the parameters are compiled in lines 5, 6 and 7 of Table 2. The parameter a represents the propor- tionality between the values of the half-day ID and INS, and serves as a simple index of the relative biologically effective UV dose. The values normalized to group 1 are 1.34 for group 2 and 1.90 for group 3.

Trends of the relative UV dose can also be exhibited as the residuals of the ID values from the regression relationship

Summary of regression analyses: values (y) of half-day ID are fitted to half-day INS (x) in power function relationships (y = ax b, R = correlation coefficient)

Line" Data set No. a b R

I 1980-1993 127 0.859 1.539 0.704 2 1980-1986 35 0.280 1.879 0.852 3 1989-1991 53 0.837 1.531 0.868 4 1992-1993 39 2.004 1.328 0.729 5 ~ 1980-1986 35 0.662 1.539 0.842 6 a 1989-1991 53 0.889 1.539 0.868 7 a ! 992-1993 3~, 1.260 1.539 0.655 8 1980-1993 127 0.768 1.619 0.777 9 1980-1986 35 0.285 1.904 0.882

10 1989-1991 53 0.775 1.592 0.922 11 1992-1993 39 1.440 1.513 0.861 12 a 1980-1986 35 0.602 1619 0.865 13 " 1989-1991 53 0.769 1.619 0.922 14 a 1992-1993 39 1.139 1.619 0.839

• Lines 5-7 and 12-14 are from constrained regressions.

66 N. Munakata /Journal of Photochemistry and Photobiology B: Biology 31 (1995) 63-68

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0 , 1 ' ' ' ' I . . . . I ' ' ' ' 250 300 350 400

Ozone Column (DU)

Fig. 3. Relationship between half-day ID and ozone column. The ratio of the observed ID to that estimated from the regression relationship is plotted against the values of the daily ozone column. Data obtained from 1980 to 1986 (o), 1989 to 1981 (O) and 1992 to 1993 ( • ). A line is drawn from the power function regression of 127 data points (y = 1.66 × 105x- 2.1 t; cor- relation coefficient = 0.422).

(Table 2, line 1) obtained for all the data against the date since 1980 as shown in Fig. 3. The scatter is large, but a positive trend is evident.

Clearly, the power regressions of half-day ID to half-day INS leave a large unexplained scatter. The next attempt to understand the data involved the determination of meteoro- logical parameters that may reduce the scatter. To do this, the ratios of the observed values of the half-day ID to the expected values from the regression relationship were plotted against meteorological variables. In this analysis, no significant cor-

relations were noted with regard to temperature, humidity, visibility or cloudiness. Noontime solar elevation seemed to exhibit a weak positive correlation. However, the incorpo- ration of this correlation resulted in little statistical improve- ment in the regression relationship (data not shown).

One of the remaining factors is the total ozone column. Since no continuous measurements were available in Tokyo, it was an inevitable compromise to use the data collected at the Aerological Observatory in Tsukuba. The daily represen- tative values of the total ozone column, expressed in Dobson units (DU), exhibit periodic seasonal, biannual and sun cycle perturbations as shown in Fig. 3. When these periodicities are removed, small decreasing trends (about 1% per decade) from 1970 to 1990 are observed at this site consistent with global ozone loss [4]. However, the average values ( + SD) of the ozone column for the days of the three groups subjected to this analysis were 311.6 (-I-31.0), 307.8 (+33 . 1 ) and 307.2 (+35 . 9 ) for groups 1, 2 and 3 respectively, and the differences between them were considered to be negligible.

It was then examined whether the deviations and scatter from the regression relationship could be explained by vari- ations in the ozone column for each experimental day. To examine this possibility, the ratio of the values of the observed ID to those expected from the regression (lines 2, 3 and 4 in Table 2) was calculated for each data point, and this ratio was plotted against the value of the ozone column. As shown in Fig. 4, there is a negative correlation between the ID ratio and the ozone column. The three groups of data seem to exhibit similar correlations. The relationship can be fitted to

40

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5 0 0 1 0 0

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Fig. 4. Upper panel: analysis of the t r end o f ha l f -day I D s ince 1980, Res idua ls o f ha l f -day ID f r o m es t imat ion wi th the r eg ress ion re la t ionship ( l ine 1 o f Tab l e

2) are plo t ted aga ins t the date. A line is drawn from the linear regression of 127 data points (y = - i 1.8 + 0 ,00412x; correlation coefficient = 0.611 ). L o w e r

panel: dally representative v a l u e s o f the ozone c o l u m n in D o b s o n units at T s u k u b a ( a b o v e ) and dai ly g lobal insolat ion ( M J m - 2 ) at T o k y o ( b e l o w ) .

N. Munakata / Journal of Photochemistry and Photobiology B: Biology 31 (1995) 63-68 67

any simple function, but a power function regression was employed. Since the three data sets exhibit essentially iden- tical patterns, all three sets were mixed to derive the regres- sion relationship ID(Obs/Exp) = 1.66× 105(OC)-~1 (R = 0.422). This relationship shows that the ratio is almost unity at 300 DU; a 10% decrease to 270 DU increases the ID ratio by 23%, and when the ozone column is halved to 150 DU, the ID ratio increases fourfold. Experimentally derived ozone amplification factors are unavailable, but this seems to concur well with the results of model calculations within the normal range of the ozone column (250-400 DU) [5,6].

The results obtained by the application of the regression formula are shown in Table 2, lines 8-10. The relationships can be simplified further using constrained regression with a fixed value of the exponent. The exponent (b) obtained by regression of all the data to the relationship was 1.619. This value was then fixed, and constrained regression was applied to each group of data. The resulting relationships are shown in Table 2, lines 12-14. The increases in the correlation coef- ficients suggest significant improvements in fitting.

By taking the variations of the ozone column into account, it is still evident that the biologically effective dose of solar UV radiation relative to insolation increased significantly in the late 1980s, and this trend continued through 1993, The ratio of parameter a is 1 : 1.28 : 1.89 for groups 1, 2 and 3 respectively. When the trend analysis was repeated for the residuals of the ID values corrected for the ozone column, essentially identical results to those shown in Fig. 3 were obtained (data not shown). Thus changes in the ozone col- umn do not seem to be the major factor responsible for the increase.

An important caveat against the above conclusion is that the ozone column measurement and spore dosimetry were performed at different locations. The problem here seems to be the vertical distribution of ozone. Although this has been studied well at Tsukuba, and the stratospheric ozone far out- weighs the tropospheric ozone contribution there, such infor- mation is not available for the sky over Tokyo. Tropospheric ozone, together with chemical pollutants and aerosol parti- cles, has been suggested to be disproportionately more effec- tive in reducing the UV exposure on the ground [7].

Since the increase in solar activity, as demonstrated by sun spot activity, occurred in the late 1980s (after the minimum was reached in 1986), this factor has been proposed as a possible reason for the observed increase in the relative UV dose. However, the further increase after the maximum was attained in 1989 seems to exclude this as the cause of the continual trend. On the other hand, the Pinatubo eruption, which has caused measurable decreases in the ozone column, may have affected the UV transparency [ 8].

The major factor remaining as a possible cause of the increasing trend is the change in atmospheric transparency due to the decrease in certain types of polluting chemicals in Tokyo. The atmospheric concentrations of major pollutants, such as sulphur oxides and carbon monoxide, exhibited sig- nificant decreases in the early 1980s at observatory stations

in downtown Tokyo, but these concentrations have shown no definite trends in later years [9]. On the other hand, other pollutants such as nitrous oxides still seem to be on the increase. It remains to be investigated how the concentrations of air pollutants are related to the UV transparency.

Biological dosimetry alone is not effective in pinpointing the causes of the remarkable variability in the UV dose observed in this work. This must be accompanied by detailed atmospheric measurements, the most pertinent being the solar irradiance spectrum at UV wavelengths. Unfortunately, no serious efforts have been carried out to obtain the irradiance spectra in Tokyo. However, at four meteorological stations, including the Aerological Observatory in Tsukuba, continu- ous hourly measurements with Brewer spectrophotometers have been performed since 1990 [ 10]. It should be inform- ative to perform concurrent measurements of the spectra and spore dosimetry at these locations. Some efforts in this direc- tion have been initiated, and results will be published else- where.

4. Conclusions

The increasing trend of the biologically effective dose of solar UV radiation in Tokyo observed in the late 1980s has been confirmed to have continued through 1993. The cause is not clear, but the most probable explanation is the changing composition and concentration of atmospheric pollutants. Since the stratospheric ozone loss is expected to continue in the near future, we may expect further drastic changes in environmental UV radiation in the coming decade. This cer- tainly suggests the need for concerted efforts to monitor bio- logically effective UV doses, and to determine how these changes will affect life on Earth. It seems that we have been unintentionally thrown into the midst of large-scale experi- ments of environmental change incurred by various human activities.

Acknowledgements

Daily representative values of the ozone column at the Aerological Observatory in Tsukuba were made available by Dr. T. Ito (Ozone Monitoring Office, Meteorological Agency). The author is grateful to Drs. K. Hieda (Rikkyo University) and K. Nozu (Japan Association for Sunlight Protection) for helpful suggestions and criticism.

References

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[2] N. Munakata, Genotoxic action of sunlight upon Bacillus subtilis spores: monitoring studies at Tokyo. Japan. J. Radiat. Res., 30 (1989) 338-351.

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[ 3 ] N. Munakata, Biologically effective dose of solar ultraviolet radiation estimated by spore dosimetry in Tokyo since 1980, Photoehem. Photobiol., 58 (1993) 386-392.

[4] R. Stolarski, R. Bojkov, L. Bishop, C. Zeferos, J. Staehelin and J. Zawodny, Measured trends in stratospheric ozone, Science, 256 (1992) 342-349.

[5] R, Setlow, The wavelengths in sunlight effective in producing skin cancer: a theoretical analysis, Proc. Natl. Acad, Sci. USA. 71 (1974) 3363-3366.

[6] J.E. Frederick, H.E. Smell and E.K. Haygood, Solar ultraviolet radiation at the earth's surface, Photochem. Photobiol., 50 (1989) 443-450.

[7] C. Briihl and P.L Crutzen, On the disproportionate role of tropospheric ozone as a filter against solar UV-B radiation, Geophys. Res. Lett., 16 (1989) 703-706.

[8] A.M. Vogelmann, T.P. Ackerman and R.P. Turco, Enhancements in biologically effective ultraviolet radiation following volcanic eruptions, Nature, 359 (1992) 47--49.

[9] Environmental Agency of Japan, Survey of Air Pollution in Japan, Gyosei, Tokyo, 1994 (in Japanese).

[ 10] T. lto, UV-B observation network in the Japan Meteorological Agency, in A. Shima et al, (eds.), Frontiers of Photobiology, Elsevier Science, 1993, pp. 515-518.