Variability of solar ultraviolet radiation at the Earth�s surface
Siani Anna Maria Casale R. Giuseppe
Dip.to di Fisica- Sapienza Università di RomaEdificio Fermi di Fisica, stanza 308,
tel 06 49913479; email: [email protected]
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1. Interest on solar UV radiation has increased
after recent findings on stratospheric ozone
decrease.
The importance of UV monitoring
2. The relationship between UV radiation and its
biological effects has been studied for several
years.
The history of UV measurements
A reason to measure solar UV radiation was recognised in the last century and remains a driving force for many of today�s measurements
V sec A.C: Ippocrate di Kos
1822: Poland: Sniadecki advised parents of rachitic children to carry them into the sun;
1890: Palm led to repetition of Polish physician due to the high incidence of rickets in the industrial towns of the British Isles;
1948: Blum sunlight as the major cause of skin cancer in sailors;
1924:Webster et al., described a chemical method of measuring UV radiation using a solution of acetone methylene-blue exposed to sunlightin a silica tube; based on the difference in the colour (before and after), the first �measurements� of UV dose were found.
1927: Hill found a correspondence between the erythema and sun�s rays. Geographical differences were observed.
EARLY TECHINQUES
The history of UV measurements
1931: the spectrophotometer Dobson for the atmospheric community to measure total ozone;1930-1957: no further attempt to measure UV1957: International geophysical year (IGY), the Dobson instrument was adopted as a worldwide standard for O3 monitoring; only ozone record BUT not long-running UV record;
1960-63: Studies of UV radiation at high altitude site of Davos(Switzerland) carried out by Bener analyzing UV measuring made with a double monochromator. He studied the influence of ozone, clouds, SZA, albedo;
1974-1980: Green usedDavos spectral dataset toevaluate the early UV radiative transfer model;
The history of UV measurements
The �70s: Supersonic aircraft, interest from atmospheric sciences about NOx impact on ozone;
1968: Berger et al. published an action spectrum for erythema;1972 -1976: Robertson (Queensland) linked UV, sunburn and skin cancer;Broad �band meter to measure erythemal radiation that became the basis of UV network in Australia. Berger in USA ( not temperature stabilized).
1975-1981: Australia UV network.
1974-1985: USA UV network.
The instruments were not suited to detect small long-term changes in UVB associated with O3 changes.
1976: Davis described a polymer film (polysulphone) with a response similarto that for erythema, then used as dosimeter.
1988: Scotto et al showed a decrease in UVB with USA data, a re-analysis in 1992 showed a slitht increase
The history of UV measurements
1978: TOMS 1985: Farman et al reported severe ozone depletion over the Antarctic based on Dobson data. The chemical mechanism was early postulated by Molina, Rowland and Crutzen in 1974 (Nobel Prize in 1995)
1987: Evidence of the decreasing trend in ozone at mid-latidutes (2-4% per decade); Montreal Protocol;
1990s on: An increase in UV effects research and a request for UV measuremennts. The first UV spectroradiometers in Antaractica
1989: Brewer installed in Toronto, Kerr and McElroy reported high UVB levels in 1992 and 1993 due to low ozone following Pinatubo eruption. Brewer if carefully calibrated can be used for UV monitoring.
Conclusions:
The last decade has seen an increase of the number of UV monitoring stations (broad�band/narrow band radiometers and spectroradiometers) and of atmospheric studies including climatology and trend analysis and research on UV effects: OZONE and UV radiation are international issues.
Future Expectations
WOUDC: UV are included together with O3 data
SUVDAMA (Scientific UV DAta MAnagement) and EDUCE (European Database for Ultraviolet Radiation Climatology and Evaluation) projects funded by EU
COST 713 �UV-B forecasting�and COST n.726 "Long Term Changes and Climatology of UV radiation over Europe".
European FP7 call for proposal was launched in the domain environment. ENV.2008.1.2.1.5: Quantification of changing surface UV radiation levels and its impact on human health.
Ispra-JRC
45.8°N, 8.6°E, 240m
from 1992 to 2007
Now Brewer is located in Aosta
Rome -University �La Sapienza�
41.9°N, 12.5°E, 60m
from 1992 to today
Lampedusa-ENEA
35.5°N, 12.6°E, 50m
from 1998 to today
Solar
spectroradiometry
sites in Italy
The solar spectrum
Hz m ~9% in UV
~ <1% in UVB
~ 6-7% in UVA
UV Variability on the global scale
Factors affecting UV radiation at the earth�s surface
AlbedoAltitude
Surface orientation
geographical
gasesclouds
AerosolsWeather system
atmospheric
Milankovitch cyclesearth-sun distance
solar elevation
astronomical
sun�s activityastrophysical
CAUSEFACTOR
The Extraterrestrial solar spectrum
� UV radiation is controlled by the variation in the Sunemittance which is not costant.
�The 27-day cycle leads to variations less than 1% for ë >250nm , 6-8% in the band 245-250nm.
�The 11-year sunspot cycle determines small changes in irradiance ( less than0.1%) and influences the shortest extra-terrestrial wavelenghts, less than 1% at 300nm
�Changes at UVC band affect the chemical equilibrium of stratospheric ozone;
� INDIRECT EFFECT ON THE TRASMISSION OF UVB TO THE SURFACE (weaker activity result in less ozone , more UVB).
�In the last 70 years solar activity was exceptionally strong
Milankovitch cycles
Eccentricity (~100kyr) obliquity (~ 41kyr), and precession (~ 21kyr) produce changes (30%) in solar irradiance at TOA
Small changes (1%) in UV radiation are expected in the next few century (WMO, 2006)
Variation in the Earth-Sun DistanceYearly cycle
� Rn varies about 3.4% from minimum (perihelion, on about 3 January) to maximum (aphelion, on about July 4).
� The variation in Rn2, and therofore in the intensity of
extraterrestrial radiation, is about 6.9%, and is significantespecially when considering seasonal differences in UV intensities between Southern and Nothern Hemispheres.
The Solar Zenith Angle SZAdaily and annual cycle
� The illumination of the earth varies with time of the day, season, and geographiclocation (latitude and longitude).
� All of this variations may be ascribed to changes in a single parameter, the solarzenith angle èo (the angle between the local vertical direction and the direction of the center of the solar disk).
cos èo = senä senÖ + cosä cosÖcosth
Ö = latitude
ä = solar declination
th= local hour angle
Month
Solar UV Index
0 1 2 3 4 5 6 7 8 9 10 11 120
5
10
15
How solar UV at noon varies with latitude and season
Lat.0°10°N
20°N30°N
40°N50°N
60°N70°N
40°S
(Wester, U. Swedish Radiation Safety Authority)
Atmospheric attenuation
Atmopsheric attenuation is not only due to absorption, but also itdepends on scattering and reflection processes that are wavelengthdependent. UV is the most strongly scattered waveband ( ë-4).
UV radiation (direct beam) is subject to Beer-Lambert law: the grater the amount of absorbing material encountered by radiation on its path, the smaller the amount of radiation reaching the surface.
Total attenuation depends on the radiation pathlength i.e. SZA.When sun is overhead ( SZA=0, noon at the equator at the equinoz)
the attenuation is smallest; a longer pathlength through the atmosphere, leading to more attenuation due to absorption and scattering.
Ozone
O3 strong absorption Hartley band : 200-310 nm; no UV-C (200-280nm) at the ground level;
O3 Huggins band: 310-350m. UV-B (280-315nm) partially absorbed, ~ 1% at the ground level;
~ 80% of exraterrerstrial spectrum is UV-A ( 315-400nm), ~ 6-7% at the ground level.
UV maximum is in July, minimum in January: interaction between O3 cycle and SZA
Ozone and UV irradiance yearly cycle
R om e: tim e series o f the loca l noon U V irradiance m easurem ents (290-325 nm )
from 01 .01 .1992 to 30 .06 .2002 com pared w ith m odel resu lts (ST A R ).
T he upper and low er lim its o f ca lcu lation uncerta inty are show ed.
0
1
2
3
4
5
6
01/0
1/92
01/0
7/92
01/0
1/93
01/0
7/93
01/0
1/94
01/0
7/94
01/0
1/95
01/0
7/95
01/0
1/96
01/0
7/96
01/0
1/97
01/0
7/97
01/0
1/98
01/0
7/98
01/0
1/99
01/0
7/99
01/0
1/00
01/0
7/00
01/0
1/01
01/0
7/01
01/0
1/02
irra
dia
nce (
W/m
2)
Low ozone episode:29 Novembre 2000
Ispra: 208.1 DU
Roma: 221.6 DU
+116+161I305
/I305
+50+65Iw/I
w
+9+24I(290-325)
/I(290-325)
-6-11O3/O
3(>20km)
-64-67O3/O
3(10-
20km)
-29-37O3/O
3(0-10km)
-28-31O3/O
3
RomaIspra(%)
Galliani A., A.M. Siani, G.R. Casale, "An investigation on a low ozone episodes at the end of November 2000 and its effect on ultraviolet radiation", Optical Engineering , 41(12) 3082-3089, 2002.
Clouds
Clouds effects on UV radiation are complex to quantify!!
CloudsCloud Modificaton Factor
clear
cloud
I
ICMF
CMF = [0-0.2 ] overcast
CMF = [0.8-1] clear sky
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0.00 0.20 0.40 0.60 0.80 1.00
CMFg
CMFuv
sza<20
20<sza<30
30<sza<40
40<sza<50
50<sza<60
sza>60
bisettrice
CMFUV>CMFg
Aerosol
Casale G.R, Siani A.M., A.di Sarra, Meloni D. �Caratterizzazione dell'ozono totale, dell'irradianzaultravioletta e dello spessore ottico nelle stazione
spettrofotometriche di Ispra (Va), Roma e Lampedusa.�, XXV Giornata dell�Ambiente, Accademia
dei Lincei, giugno 2007
Lampedusa
Air pollution
� Tropospheric O3, SO2 and NO2 attenuate up ~ 20% UV radiation. In addition many organic species can also absorb UV.
� The UV increases from ozone depletion may bebalanced, at certain degree, by the air pollution ( WMO, 2006).
� O3 affect UV directly, while others (N20) can influence the stratospheric chemistry and hence ozone amounts.
� All greenhouse gases contribute to cooling the stratosphere (ozone chemistry) and to warming the surface and hence changes in snow cover,clouds.
� It is difficult to predict the consequences on UV radiation.
specular isotropic
Surface Albedo á = I↑/I↓ 1<0á<1
� Type of surface � Roughness � Incidence angle� wavelength
Regional albedo over a zone within a radius of around 20km UV-B (Degunther et al,1998), direct local reflections plus back-scattering
Local Albedo: tilted surface (few hundreds meters)
High albedo >> reflectance to the atmosphere
>> back-scattering from the atmosphere
>> increases of UV irradiance
Surface Albedo á = I↑/I↓ 1<0á<1
Regional albedo over a zone within a radius of around 20km UV-B (Degunther et al,1998), direct local reflections plus back-scattering
Local Albedo: tilted surface (few hundreds meters)
Altitude
Early studies: 6-8% per km
New studies: the altitude effect of UV irradiance cannot be described by a single number (i.e not linear) because it depends on a combination of several factors such as reduction of scattering and absorption, clouds effects, tropospheric ozone, albedo (Seckmeyer et al., 1997, WMO 2006) and wavelength.
Decreases in the column ozone lead to the increase in surface UV radiation. At most midlatitude stations in the NH, UV irradiance continued to increase at rates of a few percent per decade.
The significance depends on location and the length of UV record. At least 10-15 years of good UV measurements are need to derive statistically significant trends.
Conclusions
The short lived episodic dynamic changes in the ozone column can produce significant day-to-day UV variations.
New methods to quantify the AO properties studies demonstrated the strong influence of variations in aerosol concentration and composition on long-short term variation in UV radiation at the surface
Clouds influences UV irradiance more strongly than any other parameters including ozone resulting in either a reduction or an increase
The higher the variability of clouds, the longer the record of UV data required for detecting an ozone-induced trend in UV radiation
� Studying UV radiation is a nontrivial task !!!!
Botero, Il Nunzio
