Pergamon Atmospheric Environment Vol. 31, No. 18, pp. 2971 2976, 1997 ~ 1997 Elsevier Science Lid

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NEW MAXIMUM UV IRRADIANCE LEVELS OBSERVED IN CENTRAL EUROPE

G. S E C K M E Y E R , B. M A Y E R , G. B E R N H A R D , R. E R B , A. A L B O L D , H. J , A G E R

a n d W.R. S T O C K W E L L

Fraunhofer Institute for Atmospheric Environmental Research (IFU), Kreuzeckbahnstr. 19, 82467 Garmisch-Partenkirchen, Germany

(First received 30 May 1996 and in final form 3 February 1997. Published July 1997)

Abstract--Simultaneous measurements of UV irradiance have been made from two adjacent sites (47.5°N, I I.I°E) in Garmisch-Partenkirchen (730m a.s.l.) and the Zugspitze (2964 m a.s.l.). New maximum erythemally weighted irradianee levels were observed in Garmisch-Partenkirchen, Germany during March 1996 and June 1995. The March episode was associated with a polar stratospheric cloud while the June episode was associated with an unusually low total ozone column (for June) and broken clouds. The June maximum may be especially significant because it was preceded by a minimum of UV irradiance caused by heavy cloud cover, thus the absolute change in the UV irradiance was very extreme. Even though the maximum irradiance levels increased, these measurements show that the monthly mean spectral UV irradiation over Germany in 1995 was not significantly different compared to previous years. This,is important because an increase in the number of extreme fluctuations from low UV irradiation followed by very high UV irradiation may be much more dangerous for the biosphere than a small gradual increase in average UV dose, because natural adaptation mechanisms may not be able to cope with these extreme fluctuations.

The monthly erythemally weighted irradiation is between 25 and 90% higher on the Zugspitze than on the lower Garmisch-Partenkirchen site. The difference between the two sites cannot be characterized by a single number because the average monthly ratio of the irradiances was very variable with respect to both time and wavelength. The variability in the differences between the two sites indicates that the differences are caused by a combination of several factors including Rayleigh scattering, cloud effects, air pollutants (e.g. tropospheric ozone), aerosols and albedo. These results show that although UV irradiation measure- ments from one site (e.g, Garmisch-Partenkirchen) can be used in the assessment of biological effects in the immediate neighbourhood, data from a single site are not sufficient to extrapolate even to different altitudes near the measurement site and certainly should not be used to extrapolate conclusions to the atmospheric chemistry of the global troposphere. © 1997 Elsevier Science Ltd.

Key word index: Ultraviolet irradiance, ozone column, polar stratospheric cloud.

INTRODUCTION

The objective of our research is to understand the influence of stratospheric and tropospheric ozone concentrations, aerosols, albedo and altitude above sea level on UV irradiance at the ground. Although the anticorrelation between the total ozone column and UV irradiance is well established (McKenzie et al., 1995; Seckmeyer and McKenzie, 1992; Kerr and McElroy, 1993; Seckmeyer et al., 1994) the effect of clouds, aerosols and other factors on UV irradiances are not as well understood. To quantify the contribu- tion of these parameters it is very helpful to investi- gate the difference in spectral UV-irradiance at two different altitudes. However, there are few simulta- neous measurements of ultraviolet irradiance at two different altitudes at nearby sites for cloudless skies (Blumthaler et al., 1994). Simultaneous measurements of UV irradiance at two different altitudes are now

available from our sites in Garmisch-Partenkirchen and the Zugspitze over a longer time period and for all weather conditions. These simultaneous measure- ments allow the determination of the variability in UV irradiance due to clouds, aerosols, tropospheric ozone and albedo. The effect of clouds may couple with the effect of reductions in stratospheric ozone to produce extremely high levels of ultraviolet irra- diance. High levels of ultraviolet radiation have been reported before for other sites (McKenzie et al., 1995; Bodhaine et al., 1996), but these sites are situated either in the southern hemisphere or in a different climate (e.g. Hawaii). Biological organisms may be well adapted for their particular climate. Howeveri if extreme maxima in the levels of ultraviolet irradiances in Europe are followed (or preceded) by episodes o f low ultraviolet irradiance levels, such sudden changes in the ultraviolet irradiance may have important con- sequences for the troposphere and the biosphere.

2971

2972 G. SECKMEYER et al.

MEASUREMENT METHODS

Instruments for the measurement of ultraviolet ir- radiance that are sufficiently accurate for the deter- mination of long-term trends were not available until the end of the 1980s (McKenzie et al., 1995; Seck- meyer and McKenzie, 1992). The IFU instruments are part of the Network for the Detection of Stratospheric Change (NDSC) and their accuracy has been demon- strated through several intercomparison studies (McKenzie et al., 1993; Seckmeyer et al., 1995; Blum- thaler et al., 1994; Gardiner and Kirsch, 1996). Our site is illustrated in Fig. 1; one stationary spectro- radiometer is located at Garmisch-Partenkirchen (47.5°N, l l . I°E, 730 m) and the second instrument is located 8 km away from the Garmisch-Partenkirchen site on the Zugspitze (2964 m). An additional mobile instrument is used for quality assurance and quality control.

The spectroradiometer and its input optics is de- scribed in detail elsewhere (Seckmeyer et al., 1996; Bernhard and Seckmeyer, 1996). Spectrally resolved direct and global (direct plus diffuse) UV irradiance are measured. The spectroradiometer is composed of a cosine-response diffuser coupled by quartz fiber optics to a double monochromator (model DTM300 from Bentham Instruments) with a spectral resolution of 0.5 nm. The diffuser has been designed so that the response of the radiometer is independent of the po- larization of the incident radiation. The system is maintained at a constant temperature, 20+0.5°C, and it uses its own independent power supply.

Spectra are scanned between 285 and 410 nm and the time between two consecutive scans is about 6 min. The system is calibrated at least once a week with a tungsten-halogen lamp standard. The calibration uncertainty is __+5% and the wavelength accuracy was determined to be better than 0.05 nm.

Differences between the spectral UV irradiance simultaneously measured at the Zugspitze and Garmisch-Partenkirchen were determined over a five month period during 1995. From the recorded spec- tral global irradiance, the erythemally weighted ir- radiation (McKinlay and Diffey, 1987) was calculated.

RESULTS

The monthly erythemally weighted irradiation is between 25 and 90% higher on the Zugspitze com- pared with the lower Garmisch-Partenkirchen site (Fig. 2). In contrast to previous studies (McKenzie et al., 1995), it is not possible to characterize the differ- ence between the two sites by a single number because the average monthly ratio of the irradiances was very variable with respect to both time and wavelength. The variability in the differences between the two sites indicates that the differences are caused by a combina- tion of several factors including Rayleigh scattering, cloud effects, air pollutants (e.g. tropospheric ozone) and albedo. The influence of these variables also af- fects the time series (Fig. 3) of daily erythemally weighted irradiation beginning in 1992 (Seckmeyer

Fraunhofer Institute for Atmospheric Environmental Research

UV measurements at IFU \ '\\ ' \

'Y \ \'\, UVB Back.scaffered \ '\,,\ .... , radiation ~

\~ Stratospheric Ozone ,,\

~peclToradiometer~. ~, ~" Sun-tracker

GARMISCH Stationary reference spectroradiometer

Fig. 1. Description of measurement sites.

UV lrradiance levels 2973

, |

2.4 i - - A u g u s ,

........ September 2.2 October

" " " November - -~ . . . . . December

1 . 6 - . . - . . . , . .

1.4 ~ "~ "" ' '~ '-'~'~:" :.-',. :.- -~--~...r....o'~-...-:,*,.....~.'..t,~.~

1 . 2 ' " ' '

1 . 0 ' ' ' 300 320 340 360 380 400

Wavelength [nm]

Fig. 2. Ratios of monthly average spectral daily doses between Zugspitze (2964 m) and Garmisch-Parten- kirchen (730 rn) measured in 1995.

6

4

3

- - - v -

June 19

Jun 1Oct 1Feb 1Jun 1Oct 1Feb 1Jun 1Oct 1Feb 1Jun 1Oct 1Feb 1Jun 92 92 94 93 93 94 94 94 95 95 95 96 96

Date

Fig. 3. Time series of erythemally weighted daily irradiation measured in Garmisch-Partenkirchen (data in 1992 are from Neuherberg), Germany.

et al., 1994). The time series remains too short for the determination of possible trends (McKenzie et al., 1995) but it shows the dominating effects of solar zenith angle and clouds which introduce a high addi- tional variability to the measured irradiance. Figure 4 shows the histogram of erythemally weighted irra- diance for different solar zenith angles. It can be seen that only for low solar zenith angles there exists

a high chance of finding values with more than 140 mW/(m 2 nm).

The effect of stratospheric ozone depletion on UV irradiance can be demonstrated by measurements during episodes with abnormally low total ozone col- umns. On 3 March 1996 a minimum in the total ozoaae of only 200 DU was observed over northern Scotland (University of Thessaloniki, 1996). During the night

2974 G. SECKMEYER et al.

200 180 160

.o 140 o ~ 120

100 80

40 z 21)

0 0

250

I Solar zenith angle 300

50 100 150 200 250

Erythemal Irradiance [mW / (m 2 m-n)]

o 200 o o~ 150 8

100

50

500

II Solar zenith angle 500

IiillJllllljM,,,,o 20 4o 60 80 100 12o 14o

Erythemal Irradiance [mW / (m 2 nm)]

450

400 350 300 250

O 200 "~ 150

50

0

I Solar zenith angle 700

!1 IM[ [III 5 10 15 20 25 30 35 40

Erythemal Irradiance [mW / (m 2 nm)]

Fig. 4. Histogram of erythemally weighted irradiance for 30 ° , 50 ° and 70 ° solar zenith angle.

between 5 and 6 March, LIDAR measurements detec- ted a polar stratospheric cloud (PSC) over Garmisch- Partenkirchen for the first time during the last 20 years. Extremely low temperatures of 190-193 K at 30 hPa were reported by the Munich radiosonde for the period 4 to 6 March. The PSC was found at 50hPa and was associated with a temperature of 195-196 K. The observations suggest that an extreme vortex distortion occurred that advected air masses containing ozone, possibly depleted by heterogeneous chemistry involving the PSC. This situation occurred just before two days nearly cloudless sky over Gar- misch-Partenkirchen: 6 and 8 March, 1995. The total ozone column was measured to be 255 DU in the morning of 6 March and the daily average was 282 DU. This was in good agreement with a measure- ment of 286 DU obtained at the nearby Hohenpeis- senberg (H. Claude and U. Kfhler, personal communication); the measurement method for total ozone at our sites is based on the independently derived direct irradiance determination in Garmisch- Partenkirchen, and has been described (Mayer and

Seckmeyer, 1996). On 8 March the total ozone col- umn returned to more normal values of a daily aver- age of 377 DU at Garmisch-Partenkirchen. The long- term average for March at Hohenpeissenberg is 366 DU.

Figure 5 shows the effect of the change in ozone column on the erythemally weighted irradiance. The effect of the ozone column on the spectral distribution is even more significant (Fig. 6). The UV-A irradiance is not influenced by the total ozone column but the UV-B is greatly enhanced on 6 March relative to 8 March. The erythemally weighted irradiation is 45% higher while the DNA damaging irradiation (Caldwell, 1986) is 85% higher on 6 March.

D I S C U S S I O N

In Figs. 5 and 6 it can be recognized that reduced ozone columns lead to higher UV irradiance. How- ever, the irradiance in March is much smaller than in June, as can be expected from the higher solar zenith angles in spring. During the summer, record levels of UV irradiance due to the combined effects of high solar zenith angle, cloud effects and relatively low total ozone columns were observed. Such extreme conditions occurred during June 1995 when a record low in total ozone column for June, 293 DU was measured at Hohenpeissenberg (Claude and KShler, 1995). The record low in total ozone column was coupled with reflection in broken clouds and it pro- duced extremely high values of erythemally weighted irradiance. A comparison with model results (not shown here) indicate that the broken clouds produced a higher daily irradiation. Although there is no con- tinuous UV observations available before 1990, it is very probable that this combination has not occurred within the last 30 years. This maximum is especially significant because it was preceded by a minimum of UV irradiance caused by heavy cloud cover; thus the absolute change in the UV irradiance was very ex- treme. Especially in spring such extreme changes may have a great impact on biological systems (Bornman and Teramura, 1993). The monthly mean spectral UV irradiation over Germany in 1995 was not signifi- cantly different compared to previous years even though the maximum levels increased. The only ex- ception occurred during 1993 when reduction in the total ozone column led to an increase in the mean UV-B dose.

C O N C L U S I O N S

The quantitative separation of the contribution of the parameters that cause the wavelength-dependent difference between the Zugspitze and the Garmisch site is not yet performed and should be studied in future. However, it is already clear that the difference is due to a combination of parameters including

UV Irradiance levels 2975

300

250

200 . ~

150

100

~ 50

j June 19, 1995 (298DU)

~ J u n e 1995 13 20, (3 DU)

4 6 8 10 12 14 16 18 20

Time [CET]

Fig. 5. Diurnal variation of erythemally weighted irradiance on 6 and 8 March as well as on 19, 20 and 29 June 1995 measured in Garmisch-Partenkirchen, Germany.

10 2

-ff ¢-q

o

° . . .q

° . . .~

lO t

10 °

10 -n

10 .2

10 -3

10 -4

10 -5 290

June 29, 1 9 ~

J u n e ~ ~ ~ i

295 300 305 310 315 320 325 Wavelength [nm]

330

Fig. 6. Spectral UV irradiation showing the influence of different total ozone columns: 19 June 1995 (298 DU); 20 June 1995 (313 DU), 29 June 1995 (331 DU); 6 March 1996 (282 DU); 8 March 1996 (377 DU).

tropospheric ozone concentration, cloud effects, Rayleigh scattering, aerosol influence and albedo differences.

Photolysis rates in the atmosphere are determined by integrating the product of the actinic flux, absorption cross sections and quantum yields over the wavelength range of the incoming radiation

(Madronich, 1987). This actinic flux differs from UV irradiance because the UV irradiance represents the flow of energy to a flat horizontal surface while the actinic flux is a spherically integrated quantity. How- ever, the changes of erythemal weighted UV irradia- tion would be expected to reflect changes in the photolysis rate of ozone for the O(~D) producing

2976 G. SECKMEYER et al,

channel, because the spectral response functions are similar for both processes. Thus, the observed extreme changes in the erythemal weighted UV irradiation will cause extreme day to day variations in HO and HO2 radical concentrations. Extreme variation in HOx concentrations will greatly disturb the oxidizing capacity of the troposphere and these extreme variations might also have an adverse impact upon the biosphere.

For northern mid-latitudes the essential question is Whether the natural adaptat ion mechanisms of hu- mans, animals and plants are sufficient to cope with a possibly increasing number of sudden changes in UV irradiance at any time of the year. The other important question is how these changes affect the oxidizing capacity of the troposphere. The signifi- cance of these sudden changes seems to be much higher than a small and long-term increase of UV irradiation for which the natural adaptat ion mecha- nisms of the biota may be sufficient.

Acknowledgements--This work has been funded by the Ger- man Ministry of Education, Science, Research and Techno- logy (BMBF), the Deutsche Bundesstiftung Umwelt (DBU) and the Commission of European Communities. We thank U. K6hler and H. Claude for their support in providing total ozone columns.

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