UV variability and climatology from the re-evaluated ... - CiteSeerX

L P

A

PHYSIC IC S

BORATORY LA

ARISTOTLE UNIVERSITY OF THESSALONIKI - AUTH LABORATORY OF ATMOSPHERIC PHYSICS - LAP

ATMOSP H

ER

OF

A.U.T.H.

Davos, UV conference, 18-20 September, 2007

UV variability and climatology from the re-evaluated spectral records of seven European stations A.F Baisa, H. Slaperb, J. Kaurolac, W. Josefssond, U. Feistere, M. Janouchf, C. Meletia, S. Kazadzisa, N. Kouremetia, K. Lakkalac, K. Garanea, C. Topalogloua, a

b

c

Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Greece, National Institute for Public Health and the Environment, The Netherlands, Finnish Meteorological Institute, Finland d

e

f

Swedish Meteorological and Hydrological Institute, Sweden, Deutscher Wetterdienst, Lindenberg, Germany, Czech Hydrometeorological Institute, Czech Republic

Introduction

Table 1. Instrument information

Long term changes in surface UV irradiance have been studied extensively for different locations and time periods using ground based measurements (e.g., Bernhard et al., 2006; Josefsson, 2006; Lakkala et al., 2003; Trepte and Winkler, 2004; e.g., Zerefos et al., 2001), or a combination with satellite UV retrievals (Fioletov et al., 2004). In the 1990s the most prominent factor influencing UV-B radiation was the ozone depletion, particularly in the Antarctic region, and consequently surface UV irradiance has been shown to increase. Partly the observed increases in surface UV radiation are attributed to the cleaning of the atmosphere since the late 1980s (Pinker et al., 2005). In late 1990s and beginning of 2000s ozone depletion has been shown to slowdown, and at some sites total ozone has been observed to increase. However, surface UV radiation continues to increase. The aim of this work is to provide a general picture of the surface UV irradiance variability over 7 European stations during the last two decades and suggest possible linkages to the evolution of other UV influencing factors.

Location

Data and Methodology

JOKIOINEN, 60.8°N

5.19 / 52.12 14.12 / 52.20 16.15 / 58.58 23.49 / 60.81 26.63 / 67.36

CIE

50o sza

40

0

-40

-40

40

+19.07%/dec

20 0 -20

40

LINDENBERG, 52.2°N

40

0

+0.88%/dec

+10.71%/dec

JOKIOINEN, 60.8°N

40

0

0

-40

-40

40

+32.83%/dec

NORRKOPING, 58.6°N

40

0

0

-40

-40

-9.34%/dec

40

+8.88%/dec

LINDENBERG, 52.2°N

40

0

0

0

-40

-40

-40

BILTHOVEN, 52.1°N 20 0 -20 HRADEC KRALOVE, 50.1°N 20 0 -20 THESSALONIKI, 40.5°N 20 0 -20

+2.61%/dec

40

40

0

0

-40

-40

+5.98%/dec

40

HRADEC KRALOVE, 50.1°N

+1.17%/dec

40

0

0

-40

-40 THESSALONIKI, 40.5°N

+0.84%/dec

40

+7.30%/dec

40

0

0

-40

-40

1990 1992 1994 1996 1998 2000 2002 2004 2006

Year

+4.19%/dec

BILTHOVEN, 52.1°N

Period 01 / 1990 – 12 / 2004 01 / 1994 – 05 / 2005 01 / 1996 – 12 / 2004 01 / 1995 – 12 / 2004 07 / 1996 – 12 / 2004 01 / 1996 – 12 / 2005 04 / 1990 – 12 / 2004

60o sza +4.49%/dec

SODANKYLA, 67.3°N

-1.94%/dec

20 0 -20 NORRKOPING, 58.6°N

Instrument Spectral range (nm) Brewer 290-325 MKII Brewer 290-325 MKIV Dilor 290-380 2XY.50 Brewer 290-325 MKIV Brewer 290-365 MKIII Brewer 290-363 MKIII Brewer 290-325 MKII

15.83 / 50.18

Local Noon SODANKYLA, 67.3°N 20 0 -20

% DEPARTURES

The methodology that was used in this work aimed to derive estimates on ultraviolet (UV) variability at different time scales and at different locations, limited by the availability of UV monitoring stations with sufficient data sets. The data that were used for the analysis were obtained from the European Ultraviolet Data Base (EUVDB) that is currently located at the Finnish Meteorological Institute (FMI). Specifically, seven European UV monitoring stations were selected. The selection criteria were mainly based on the time period that each of the EUVDB station is covering. The stations that were selected are providing the longest spectral UV irradiance time series within Europe. In table 1 more details are presented for the monitoring stations used here. UV variability was calculated for different wavelength bands, solar zenith angle ranges and time integrals. The spectral irradiance data were first standardized to 1 nm spectral resolution using the SHICrivm algorithm. The same algorithm was used to simulate wavelength regions that are not recorded by some instruments, but are necessary for the calculation of certain wavelength integrals (e.g., the range 365-400 nm needed for the calculation of UV-A integral in MKIII Brewer spectroradiometers and the range 325-400 nm needed for the calculation of UV-A integral in MKII Brewer spectroradiometers). The wavelength bands that were chosen were: UV-B integral (280-315 nm) that are available from all instruments and CIE weighted integral, (available from all instruments using wavelength extension). Also, single wavelengths at: 305, 324 nm, 350 nm For all single-wavelength analyses the mean irradiance in the wavelength range reported above ±1nm was used, to avoid effects from variations caused by possible wavelength structure in the spectral measurements. The time integrals that were used are the following: Daily integrals of UV irradiance: They were chosen as the most representative quantity for assessing effects from UV exposure. Significant uncertainties may be introduced in the calculation of daily integrals if the frequency of measurements is relatively low or due to large gaps in the data sets during a day. For reducing such uncertainties we set limitations for the spacing of data within a day taking into account the usual measuring schedules of each station: The following criteria were used for the construction of daily integrals of irradiance. - Measurements in the day are less than 1.5 hours apart - One data point recorded at least within 50 minutes from the true local noon is available - Data are available for at least 60% of the daylight time (from sunrise to sunset) during a day Monthly means of UV irradiance: The mean for a particular month is calculated if at least 15 days are available. In most cases the missing days in a month are distributed more or less evenly and therefore the monthly means can be considered representative. For each station, three months (April, July and October) were considered in the analysis of the long term trends to minimize effects from seasonally dependent parameters (ozone, cloud cover, solar zenith angle range). Local noon irradiance: The average irradiance within ±1 hour from the true local noon was considered as the local noon irradiance, since it would be rare to find measurements recorded exactly at local noon. Irradiance at specific solar zenith angles: Data at specific solar zenith angles (sza) were used to reduce the effects from imperfect exposure of the entrance optics of the spectroradiometers due to obstructions and the effects from imperfect angular response. For this purpose the solar zenith angles of 50°, 60° and 75° were chosen. (Where a measurement at a specific sza is defined as the measurement recorded within ±1 degree from this sza). However not all sza were available in each site for the entire year, depending on the latitude of the site. Therefore, in order to obtain at least one uninterrupted series in the year for each stations, the above mentioned large sza were selected. This increases the uncertainty of the data, due to low values of irradiance and the increasing influence of the angular response errors and the imperfect exposure of the entrance optics. The selected data series at 75°, 60° and 50° sza was a compromise between obtaining series of data with sufficient accuracy and short gaps. In addition to the mean values at single wavelengths, the CIE weighted and the UVB integrals at different sza and time scales, as well as the minimum and maximum values were calculated for each station. The minimum and maximum irradiance for each month is defined as the mean of the highest 20% of the measurements.

Longitude (E) Latitude (deg) 22.95 / 40.63

Thessaloniki, Greece HradecKralove, Czech Republic Bilthoven, The Netherlands Lindenberg, Germany Norrkoping, Sweden Jokioinen, Finland Sodankyla, Finland

SODANKYLA, 67.3°N

+2.05%/dec

JOKIOINEN, 60.8°N

-1.89%/dec

NORRKOPING, 58.6°N

Figure 1. Long-term monthly maxima of irradiance daily integrals for 305 nm, 324 nm, CIE-weighted and UV-B at 7 European stations. Station Ids are listed above.

+12.02%/dec

LINDENBERG, 52.2°N

7.709%/dec

BILTHOVEN, 52.1°N

+8.58 %/dec

HRADEC KRALOVE, 50.1°N

-2.05%/dec

THESSALONIKI, 40.5°N

+13.42%/dec

1990 1992 1994 1996 1998 2000 2002 2004 2006 1990 1992 1994 1996 1998 2000 2002 2004 2006

Year

Year

Figure 2. Monthly mean CIE-weighted irradiance for 7 European stations at local noon and at 50° and 60° solar zenith angle. Linear trends are superimposed on the monthly data.

Figure 3. Long-term monthly mean irradiance at 305 nm, 324 nm and erythemal irradiance (CIE) at 7 European stations for three solar zenith angles (50°, 60°, 75°) and for local noon are listed in Table 1.

Figure 4 and 5. Linear trend of monthly mean and monthly maximum of 305 nm, 324 nm and CIE-weighted irradiance at local noon (right) and solar zenith angle of 60 degrees ( left) for the month of July . Station Ids are listed in Table 1.

Figure 6 and 7. Mean and maximum irradiance at 305 nm, 324 nm, and erythemal irradiance (CIE) at 7 European stations, at three solar zenith angles (50°, 60°, 75°) and at local noon from all months andApril ( (left) and July and October (right).

Conclusions For the annual course of the long-term monthly means of the daily integrals of irradiance at 305 nm and 324 nm, and of the CIE-weighted and UV-B integrals, Thessaloniki has by far the highest values, while the Scandinavian stations of Sodankyla, Jokioinen and Norrkoping the lowest. The same pattern appears also for the maximum values (Figure 1). At 324 nm (a wavelength less affected by ozone) and especially in the summer, the maximum values of the northern stations converge towards the values of the central European stations as a result of the increasing length of the day. The June maximum in Sodankyla (24 hours of daylight) approaches the maximum value at Thessaloniki (difference of less than 20%), while at 305 nm the maximum at Thessaloniki is more than twofold higher. From all irradiance spectra in each station monthly mean changes of irradiance have been calculated. Figure 2 shows the changes in monthly CIE irradiance for local noon and for the solar zenith angles (sza) of 50° and 60°. In this analysis we have considered only linear changes in surface irradiance, as more appropriate for comparing stations with different lengths in their measurement records. In most cases the calculated changes are positive. The highest changes were found in Norrkoping for all sza and for local noon. The observed changes can be attributed to changes in ozone but also to changes in cloud cover and aerosols. The changes in maximum CIE irradiance are almost independent of the cloud variability, hence are mainly affected by ozone and aerosol changes. The changes in total ozone at Thessaloniki for the same period of irradiance measurements cannot explain the observed positive change in CIE irradiance. However, measurements of the aerosol optical depth have shown a decreasing tendency which may explain part of the observed increasing trend in irradiance. The long-term mean and maximum irradiance for each month was calculated in each station from the entire period of measurements. The analysis was carried out for the three sza’s and the three spectral bands mentioned in the methodology. Figure 3 show the monthly mean irradiance. Local noon irradiances show the expected pattern due to the different latitude of the stations. At 324 nm the southern stations show higher mean monthly irradiance during the summer and autumn and this can be explained by the lower probability of cloudy skies than at the northern stations. On the contrary, the maximum irradiance is higher at northern stations for almost all months, and especially for February, March and April. A possible explanation is the lower aerosol at these three sites and the effect from snow albedo during the winter and spring months. The effect from snow is evident also in the mean monthly irradiance shown in Figure 3. The seasonal pattern seen in the irradiance at specific sza is caused by the annual variations in clouds and ozone. The effect from the ozone annual variation (higher irradiance in the autumn) is clearer in the maximum monthly irradiance where the effect of clouds is reduced substantially, and is more pronounced in the irradiance at 305 nm where the ozone absorption is higher. Figures 4 and 5 show for each station the linear trend in the monthly mean and the monthly maximum irradiance at different wavelengths and wavelength bands, calculated for the month of July at local noon and at 60 degrees sza. With the exception of Hradec Kralove, in most cases and stations the trends are positive. Hradec Kralove show mainly negative trends that reach -20% in some cases. The smallest trends appear at Thessaloniki. Thessaloniki, Hradec Kralove and Sodankyla present the most consistent picture with respect to wavelength. The upper row of Figure 6 shows mean and maximum values from all measurements. Local noon, mean and maximum irradiances show clearly the latitude dependence (at Thessaloniki the mean erythemal irradiance at local noon is 4 times higher than at Sodankyla).At specific sza there are no major differences between stations. The second row of Figure 1 shows the same mean and maximum values only for month April. For all sza's and wavelengths it is clear that stations affected by snow (mostly Sodankyla and also Jokioinen and Norrkoping) show higher values from all other stations. In Figure 7 the same results are presented for July and October. The high probability of clear sky conditions in July in Thessaloniki leads to very high mean irradiance at 324 nm for all sza, while maximum irradiances are in the same levels. For October (second row in Figure 7) only at 75° sza there are data at all stations, with comparable irradiance levels.

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