factors affecting solar ultraviolet irradiance measured since 1990 at thessaloniki, greece

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Factors affecting solar ultravioletirradiance measured since 1990 atThessaloniki, GreeceC. Meleti a , A. F. Bais a , S. Kazadzis b , N. Kouremeti a , K.Garane a & C. Zerefos c da Laboratory of Atmospheric Physics, Aristotle University ofThessaloniki , Thessaloniki, Greeceb Finnish Meteorological Institute, Climate Change Unit , Helsinki,Finlandc Biomedical Research Foundation, Academy of Athens , Athens,Greeced Laboratory of Climatology and Atmospheric Environment,National and Kapodistrian University of Athens , Athens, GreecePublished online: 29 Jul 2009.

To cite this article: C. Meleti , A. F. Bais , S. Kazadzis , N. Kouremeti , K. Garane & C. Zerefos(2009) Factors affecting solar ultraviolet irradiance measured since 1990 at Thessaloniki, Greece,International Journal of Remote Sensing, 30:15-16, 4167-4179, DOI: 10.1080/01431160902822864

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Factors affecting solar ultraviolet irradiance measuredsince 1990 at Thessaloniki, Greece

C. MELETI*†, A. F. BAIS†, S. KAZADZIS‡, N. KOUREMETI†, K. GARANE†

and C. ZEREFOS§¶

†Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki,

Thessaloniki, Greece

‡Finnish Meteorological Institute, Climate Change Unit, Helsinki, Finland

§Biomedical Research Foundation, Academy of Athens, Athens, Greece

¶Laboratory of Climatology and Atmospheric Environment,

National and Kapodistrian University of Athens, Athens, Greece

Factors affecting the solar spectral ultraviolet (UV) irradiance at Thessaloniki,

Greece were investigated using measurements with single- and double-

monochromator Brewer spectroradiometer, which started operating respectively

in 1989 and 1993 and continue up to the present. The two data records were quality

controlled, homogenized and finally merged into one dataset, which was used in

the analysis. Subsets of these data corresponding to different solar zenith angles

(SZAs) and to cloud-free skies were used to quantify the long-term changes in

surface UV irradiance at different wavelengths, and the importance of the factors

responsible for these changes is discussed. It is shown that the calculated UV

changes vary with SZA due to the different atmospheric path of the photons and

the dependence of the diffuse to direct irradiance ratio on the SZA. The effect of

total ozone and aerosols on UV irradiance is examined and the corresponding

radiation amplification factors (RAFs) at the various wavelengths are calculated.

The observed changes in UV irradiance due to ozone are smaller than those

expected for the changes in total ozone, suggesting that the influence of the

ozone is masked by other factors. An important finding of this study is that the

improvement in air quality at Thessaloniki, during the period under examination,

is the main reason for the observed increase in solar UV irradiance.

1. Introduction

The decrease in stratospheric ozone and the character of its expected recovery havebeen the subject of intense research. A worldwide decline in the total ozone column

over the past few decades has been ascertained using ground-based measurements,

as well as satellite observations (e.g. WMO 2003). With the implementation of the

Montreal Protocol, the production of ozone-depleting substances has been reduced

by 95%, resulting in decreasing rates of the concentrations of these chemicals in the

atmosphere. Recently reported evidence for the first stage of ozone recovery (i.e. a

slowdown in the ozone depletion rate) in the upper stratosphere at 35–45 km

(WMO 2007) has confirmed the positive effect of the Montreal Protocol and itsamendments. The direct consequence of the stratospheric ozone recovery is a

decrease in the ultraviolet (UV) irradiance at the Earth’s surface. A number of

*Corresponding author. Email: [email protected]

International Journal of Remote SensingISSN 0143-1161 print/ISSN 1366-5901 online # 2009 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/01431160902822864

International Journal of Remote Sensing

Vol. 30, Nos. 15–16, August 2009, 4167–4179

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mailto:[email protected]

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models predict the rates and the timing of the return to pre-1980 ozone and UV

values, but not very accurately because of the complexity of the atmosphere’s

system and the interactions between ozone variability and climate change

(Kondratyev and Varotsos 1995, Andrady et al. 2009). Long-term changes in UV

radiance may provide evidence for the success of the implementation of theMontreal Protocol.

As stratospheric ozone is the major atmospheric absorber of solar UV radiation,

the observed changes in ozone are expected to lead to inverted changes in UV levels

at the ground level with relevant impacts on humans and the biosphere

(e.g. Kondratyev and Varotsos 1996, NRPB 2002). However, local changes in

meteorological conditions and atmospheric composition may mask the expected

effect of the ozone recovery on the UV radiation received at the Earth’s surface.

Apart from the total ozone column, clouds, aerosols, ground albedo and altitudealso influence the ground-level UV radiation. Long-term ozone measurements exist

and can be used for partly explaining UV variability in the past using alternative

methodologies (e.g. Efstathiou et al. 1998, Lindfors et al. 2007, Feister et al. 2008).

Attempts to attribute the observed long-term changes in surface solar UV irradiance

to different factors have been reported (e.g. Krzyscin and Puchalski 1998,

Chubarova and Nezval 2000, Kylling et al. 2000, Fioletov et al. 2001, Arola et al.

2003, den Outer et al. 2005, 2006).

The short temporal record and the sparse spatial coverage of UV radiationmonitoring sites continue to cause difficulties in determining UV trends or in

establishing a global UV climate using UV radiation measurements alone. Due to

the complexity of the problem, it is important to develop and validate methods for

investigating the relationships between the measurements of ground-level UV radia-

tion and those of ozone, clouds, aerosols and ground albedo. The effect of ozone is

quantified through the concept of the radiation amplification factor (RAF)

(Madronich 1992) or by comparison of the changes in irradiance at two distinct

wavelengths, one with strong and one with negligible ozone absorption. However,the effects from changes in surface albedo, aerosols or clouds are more difficult to

distinguish and quantify. Aerosols affect surface UV radiation directly, through

scattering and absorption (e.g. Coakley et al. 1983, Charlson et al. 1992), semi-

directly, by changing atmospheric thermodynamics and cloud formation

(e.g. Ackerman et al. 2000, Koren et al. 2004), and also indirectly, by changing the

cloud microphysics (e.g. Twomey 1977, Rosenfeld and Lensky 1998). Krzyscin and

Puchalski (1998) showed that, in highly polluted areas, absorption of solar UV

radiation by urban anthropogenic aerosols may mask the surface UV irradianceincrease associated with low total ozone episodes. Other studies have shown that a

decline in aerosol concentration of up to 60% has led to a statistically significant

increase in solar irradiance under cloud-free skies since the 1980s (Ruckstuhl et al.

2008, Philipona et al. 2009).

As long-term measurements of UV levels are able to confirm the continuing

success of the Montreal Protocol (Andrady et al. 2009), in this work we attempt to

separate and explain the effect of ozone and aerosols. To eliminate the effect of

clouds, we used irradiance measurements in the period 1991–2006 performed undercloud-free conditions at Thessaloniki, Greece. The time series presented are of the

longest duration worldwide and the high quality of the measurements has been

confirmed by various intercomparison campaigns (Bais et al. 2001, Grobner et al.

2005, Garane et al. 2006).

4168 C. Meleti et al.

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2. Data and Methodology

The UV monitoring station of Thessaloniki has one of the longest records of spectral

UV irradiance measurements, conducted by single- and double-monochromator

Brewer spectroradiometers operating on a regular basis at the Laboratory of

Atmospheric Physics, University of Thessaloniki, Greece. For the single monochro-

mator the data record spans 1989–2006 while for the double monochromator the data

are from 1993 to 2006. In this study we are concerned with spectral measurements of

solar UV irradiance on a horizontal surface that have been derived by combiningspectral measurements from both spectroradiometers. The instruments are positioned

on the roof of the Physics Department building (latitude 40.634�N, longitude 22.956� E,

altitude 60 m asl), which is located in the centre of the city of Thessaloniki. The horizon

of their input optics is free to the south and west. Buildings and local obstructions block

the east side up to an angle of 10� and the north side up to an angle of 30�. These

obstacles reduce the diffuse irradiance by up to 2% for both instruments. More details

on the instrument characteristics, the calibration history and the quality control of the

measurements can be found in Garane et al. (2006).The total ozone record spans from 1982 to the present and is derived from the single

monochromator using direct spectral irradiance measurements at five wavelengths in

the UV-B region, nominally at 306.3, 310.0, 313.5, 316.8 and 320.1 nm (Kerr et al.

1981). From these direct irradiance data the aerosol optical depth (AOD) is derived at

all six wavelengths and has been available since 1984. The retrieval of the AOD,

especially at the shorter UV-B wavelengths, is difficult and is associated with sub-

stantial uncertainties (Marenco et al. 1997), requiring accurate and precise calibration

and quality control procedures. The extraterrestrial flux at the six wavelengths hasbeen determined by applying the Langley extrapolation method. The days used for

the Langley method are dispersed throughout the whole period of measurements, and

the extraterrestrial fluxes for each day were determined by linear interpolation. Then

the AOD is calculated as the residual optical depth after subtracting from the total

atmospheric optical depth the optical depths due to molecular scattering and due to

absorption by O3 and SO2. Details on the methodology can be found in Meleti and

Cappellani (2000), Cheymol and De Backer (2003) and Grobner and Meleti (2004).

For the selection of the cloud-free days we used the methodology described inVasaras et al. (2001), which is based on the variability of shortwave solar irradiance

measurements (300–3000 nm) from a collocated pyranometer. In case of missing

measurements, the cloud cover at Thessaloniki International Airport ‘Macedonia’

as reported by the Hellenic National Meteorological Service was used instead.

Interpretation of the measurements was assisted by radiative transfer calculations,

performed with the libRadtran (Library for Radiative transfer) code developed by

Mayer and Kylling (2005). The spectral irradiance in the wavelength range 300–400

nm was calculated for total ozone ranging between 240 and 500 m-atm-cm and forSZAs ranging between 20� and 70�. In these calculations a constant AOD of 0.45 at

350 nm was used, representative of the mean annual value at Thessaloniki.

3. Results

3.1 UV Irradiance and Total Ozone

Spectral UV irradiance measurements at Thessaloniki started in September 1989 with

the single-monochromator Brewer spectroradiometer. During the first year of

Remote sensing and the Montreal Protocol 4169

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operation data were acquired only near local noon. Measurements at SZAs of 50� and

63� were added to the monitoring schedule in October 1990. Since August 1992

spectral irradiance has also been monitored at 5� of SZA intervals from sunrise to

sunset. The same measurement protocol is followed by the double-monochromator

Brewer spectroradiometer, which started operating in May 1993.Although both instruments were calibrated with the use of the same calibration unit

and standard lamps of spectral irradiance, their spectral irradiance measurements

differ because of the different instrumental characteristics. To homogenize the time

series of UV irradiance, the almost coincident measurements of the two instruments

under clear skies were compared. The relationships derived for the various SZAs were

applied and the corresponding datasets were merged into one dataset.

As clouds are a major factor in modifying the irradiance at the surface, the present

study focused on cloud-free measurements to investigate separately the effect of ozoneand aerosol. We analysed the time series of the spectral UV irradiance at selected

wavelengths and for SZAs ranging from 20� to 70� with a step of 5�, and at 63�

(minimum SZA during the whole year). Most of the data subsets cover a period of 16

years, from 1990 to 2006. As an example, figure 1 shows the irradiance at 305 nm

(strong absorption by ozone) and 324 nm (weak absorption by ozone) at SZA = 63�.The statistics of the monthly mean irradiance at these wavelengths and of the corre-

sponding total ozone are given in table 1 for different SZAs.

The estimated long-term variability of the irradiance at selected wavelengths and ofthe total ozone as a function of SZA is shown in figure 2. Evidently, the higher the

wavelength, the larger the observed increase in irradiance. The different time periods

and seasons corresponding to the different SZAs result in different estimates of the

long-term changes, which are positive in all but one case. This is in contradiction to

the changes expected by the long-term variability of total ozone, and hence it must be

1.0

3.0

5.0

Sol

ar ir

radi

ance

(m

w m

–2 n

m–1

)

280

320

360

400

1991 1993 1995 1997 1999 2001 2003 2005 2007Year

50

100

150

305 nm

280

320

360

400

Total ozone (m

-atm-cm

)

324 nm

(a)

(b)

Irradiance

Total ozone+ 9.2%/dec

+ 5.0%/dec

+ 1.1%/dec

+ 1.1%/dec

Figure 1. Monthly mean spectral UV irradiance at (a) 305 nm and (b) 324 nm measured atThessaloniki at solar zenith angle (SZA) 63� (solid line-circles) and the corresponding monthlymean total ozone column (broken line-diamonds). The straight lines represent linear regres-sions on the data.


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attributed to other factors affecting UV radiation. It should be mentioned here that

the significance of the changes in irradiance at 310, 315 and 320 nm lies between the

significance of 305 and 324 nm, and increases with wavelength.

Figure 2 suggests that the changes in irradiance at the UV-A wavelengths increase

with SZA. At these wavelengths the ozone absorption is weak, and because the

different SZAs correspond to different seasons (the smallest SZA corresponds onlyto the summer months), this behaviour is probably related to seasonality in the long-

term changes of the aerosol loading over the area and its characteristics.

Using the available datasets of irradiances, the RAF due to total ozone was

calculated. The corresponding theoretical RAF was derived from the libRadtran

model using a constant AOD of 0.4 at 350 nm. The resulted RAFs at selected

wavelengths as a function of SZA are shown in figure 3. Comparison of the two

RAFs reveals that the theoretical one is always higher, leading to the conclusion that

the effect of ozone on irradiance is masked by other factors. The RAF calculated fromthe observations at 320 and 324 nm is negative, suggesting that at these wavelengths,

where ozone absorption is very weak, the aerosols dominate the cloud-free UV

variability.

3.2 UV Irradiance and Aerosols

The ratio of the monthly climatological mean irradiance at 305 nm to that at 324 nm is

affected mainly by the seasonal change in total ozone. This relationship was repre-

sented by an exponential regression separately for SZAs 63� and 70�. It was found that

the ratio exhibits a seasonal dependence, with the highest values appearing from

November to February and the lowest from April to August, indicating the involve-

ment of seasonally dependent factors that affect the UV irradiance more at shorter

20 30 40 50 60 70Solar zenith angle (°)

–4.0

0.0

4.0

8.0

12.0

UV

irra

dian

ce c

hang

e (%

/dec

ade)

0.0

1.0

2.0

3.0

4.0

Ozone change (%

/decade)

Wavelength

305 nm

310 nm

315 nm

320 nm

324 nm

Ozone

Figure 2. Long-term changes (% per decade) for total zone and clear-sky spectral UVirradiance at various wavelengths as a function of solar zenith angle (SZA).


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wavelengths. Such behaviour may be attributed to the aerosol loading, and to confirm

this, the AOD at 320 nm was compared with the irradiance reduced to constant totalozone. The ozone effect was removed from the UV irradiance using the model-derived

RAF. Thus, all measurements were normalized to the irradiance corresponding to the

mean total ozone of the time series (322 DU). The derived irradiance at 305 and 324

nm at an SZA of 63� and the corresponding AOD time series are shown in figure 4.

The seasonal amplitude of the irradiance at 305 and 324 nm has decreased by 60% and

25%, respectively. Both time series show a positive long-term change of about 9% per

decade, while the AOD at 320 nm decreases by about 7% per decade. The negative

AOD changes are probably linked to a series of measures that had been taken duringthe 1990s concerning many aspects of the city’s activities (Petrakakis et al. 2005), such

as the use of low-sulfur crude oil in the industry, improvement in fuel quality, and

renewal of the vehicle fleet.

The same analysis was applied to all subsets of the data, and in all cases the spectral

irradiance increases throughout the examined period, while the AOD decreases. The

estimated changes are shown in figure 5. In general, the increase is more evident at

larger SZAs because of the longer optical path traversed by the radiation in the

atmosphere. The AOD changes range from -12% up to –4% per decade. The differ-ences in the estimated changes can be attributed to the different periods in the year

corresponding to the different SZAs and the different optical characteristics of the

aerosols during these periods.

The effect of the aerosols was also estimated by linearly relating the departures of the

irradiance from the long-term mean to those for the AOD. The slope of the derived

relationships is the RAF due to aerosols for the specific wavelengths and SZAs, and is

shown in figure 6 together with the mean AOD at 320 nm at each SZA. The calculated

20 30 40 50 60 70

0.0

1.0

2.0

3.0

4.0

5.0

RA

F o

zone

320

324

328

332

336

340

Total ozone (m

-atm-cm

)

305 nm

310 nm

315 nm

320 nm

324 nm

Modelled calculated

Total ozone

Solar zenith angle (°)

Figure 3. Ozone RAF calculated from the measurements at various wavelengths as a functionof solar zenith angle (SZA) (lines with symbols) and the corresponding model-derived RAF(lines). The line with crosses corresponds to the mean total ozone column for each SZA.


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RAF is higher at the shorter wavelengths and exhibits the smallest values at 324 nm. In

general, the RAF has a tendency to increase with increasing SZA, and then at SZA 70�

decreases abruptly. Minor peaks of different amplitude are shown between 50� and 63�.All these variations in the RAF result from the different time periods covered by each


0.0

4.0

8.0

12.0

16.0

UV

irra

dian

ce c

hang

e (%

/dec

ade)

–16.0

–12.0

–8.0

–4.0

0.0

AO

D change (%

/decade)

Wavelength

305 nm

310 nm

315 nm

320 nm

324 nm

AOD at 320 nm

Figure 5. Long-term changes (% per decade) for AOD and clear-sky spectral UV irradiance atvarious wavelengths reduced to constant total ozone as a function of solar zenith angle (SZA).

1991100

120

140

160

2.5

3.0

3.5

4.0

(a)

(b)

4.5

305 nm+ 9.1%/dec

+ 8.7%/dec

– 7.2%/dec

– 7.2%/dec324 nm

1993

Sol

ar ir

radi

ance

(m

W m

–2 n

m–1

)

1995 1997 1999Year

2001

IrradianceAOD

AO

D at 320

nm

2003 2005 2007

1.00

0.75

0.50

0.25

0.00

0.75

0.50

0.25

0.00

Figure 4. Monthly mean spectral UV irradiance at (a) 305 nm and (b) 324 nm measured atThessaloniki at solar zenith angle (SZA) 63� and reduced to constant total ozone of 322 DU(solid line-circles) and the corresponding monthly mean AOD at 320 nm (broken line-diamonds). The straight lines represent linear regressions on the data.


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subset of data, which are characterized by differences in the AOD (see figure 6) and also

in its optical properties.

Based on the AOD RAF, it was calculated that the observed changes in the aerosols

result in an enhancement of irradiance of about 0.5–2.0% per decade. As the long-

term changes in UV irradiance estimated from the observations are much higher, part

of this increasing trend could be attributed to the improvement in air quality in the

centre of Thessaloniki (Petrakakis et al. 2005), which resulted in a reduction in the

concentration of UV-absorbing air pollutants.

4. Summary and Conclusions

The substances depleting the ozone layer are decreasing because of the implementa-

tion of the Montreal Protocol. Models have been developed to predict the timing of

the recovery, but their accuracy is still questionable, especially for the calculation of

the time of the full recovery. Long-term changes in ozone and the UV can be used to

provide evidence of the success of the Montreal Protocol. Studies have shown that the

UV levels at the ground are dependent not only on the ozone variability but also on

other factors such clouds, aerosols, ground albedo and altitude. Simulations of theUV levels on a global scale are based on ozone recovery scenarios, having as sole input

the predicted (negative) columnar ozone future trends. The tendency for reduced

anthropogenic aerosols in the atmosphere observed in the USA and in Europe during

the past decade would have a large effect on the uncertainty budget of any of the

above simulations. To judge the results of the implementation of the Montreal

Protocol and to improve the model estimations, the effect of other factors affecting

UV irradiance should be determined.


0.05

0.10

0.15

0.20

0.25

RA

F A

OD

0.50

0.55

0.60

0.65

AO

D at 320 nm

305 nm

310 nm

315 nm

320 nm

324 nm

AOD at 320 nm

Figure 6. AOD RAF calculated from the measurements at various wavelengths as a functionof solar zenith angle (SZA) (lines with symbols). The line with crosses corresponds to the meanAOD for each SZA.


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In this work an attempt to establish the effect of the aerosol loading on UV

irradiance is presented. Spectral UV irradiances at specific wavelengths and SZAs

were analysed and the influence of ozone and aerosols studied for all cases.

Summarizing the results of the analysis, the following conclusions can be drawn.

Spectral UV irradiance, total ozone and AOD data measured at Thessaloniki,Greece since the beginning of the 1990s are used for investigating their long- and

short-term variability. Positive, statistically significant changes in UV irradiance

reaching 5% per decade for 305 nm and 9.2% per decade for 324 nm were calculated

from the analysis of the data in the period 1990–2006.

The total ozone column measured during this 16-year period shows a slight increase

of 1.1% per decade. However, the irradiance at all UV-B wavelengths shows a positive

change with the exception of 300 nm, where the changes are negative because of the

positive ozone change. The observed wavelength-dependent positive changes suggestthe influence of other factors, mainly changes in aerosol amount and characteristics.

This is further supported by the difference found between the calculated and model-

derived ozone RAF.

The RAF due to aerosols (AOD) calculated for various SZAs and wavelengths

shows an increasing tendency with SZA. This is because the AOD changes mostly

affect the direct component of solar irradiance, whose contribution to the global

irradiance decreases with increasing SZA.

The UV irradiance reduced to constant total ozone was compared with AOD datato quantify the influence of aerosols. The AOD over Thessaloniki exhibits a decreas-

ing tendency during the past 16 years. The calculated long-term changes in irradiance

increase with wavelength and SZA. These findings are in agreement with similar

studies (Petrakakis et al. 2005, Kazadzis et al. 2007) carried out in a similar, or slightly

larger (Papadimas et al. 2008), area. AOD changes can partly explain the observed

increases in UV irradiance over the area and the remaining part is probably linked to

changes in the aerosol absorption efficiency, as well as to a decrease in air pollutants.

Global changes in aerosol concentrations may indeed have implications for studiesdealing with ozone recovery due to the Montreal Protocol amendments and the

foreseen changes of UV radiation reaching the Earth’s surface. Maintaining existing

observational capabilities and enhancing the integration of information using long-

term measurements of both UV irradiance and AOD is crucial in separating the

effects due to changes in air quality from those due to changes in ozone-depleting

substances. Under clear skies the most important factor affecting UV radiation is

stratospheric ozone, followed by aerosols, so future levels of surface UV radiation will

depend on the evolution of both these factors, which influence the propagation ofsolar UV radiation in the atmosphere.

Acknowledgements

This work was conducted in the framework of the EC Integrated Project SCOUT-O3

(contract 505390-GOCE-CT-2004). S. Kazadzis acknowledges the Marie Curie Intra-European fellowship VAP-OMI, AOR A/119693 PIEF-GA-2008-219908 and the

Research Committee of the Aristotle University of Thessaloniki for the 2008 excel-

lence award.

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