Measurements of the UV-B erythemal dose, based on solar spectra acquired with a Brewer spectrophotometer at Thessaloniki, Greece, are compared to measurements performed with the recently introduced, by the Yankee Environmental Systems, (Robertson type) broad band solar UV-B detector. The spectral response function of this detector, when applied to the Brewer spectral UV-B measurements, results in remarkably comparable estimates of the erythemal UV-B dose. The two instruments provide similar information on the UV-B dose when they are cross-examined under a variety of meteorological and atmospheric conditions and over the a large range of solar zenith angles and total ozone

Bais, Alkiviadis F.

Meleti, Charicleia

Zerefos, Christos S.

Ziomas, Ioannis C.

English

NASA Technical Reports Server

N95- 11091
COMPARISON OF UV-B MEASUREMENTS PERFORMED WITH A BREWER
SPECTROPHOTOMETER AND A NEW UVB-I BROAD BAND DETECTOR
Alkdviadis F. Bais, Christos S. Zerefos, Charicleia Meleti, loannis C. Ziornas
Laboratory of Atmospheric Physics, University of Thessaloniki
ABSTRACI _
Measurements of the UV-B erythemal dose,
based on solar spectra acquired with a Brewer spec-
trophotometer at Thessaloniki, Greece, are compa-
red to measurements performed with the recently
introduced, by the Yankee Environmental Systems,
(Robertson type) broad band solar UV-B detector.
The spectral response function of this detector,
when applied to the Brewer spectral UV-B measure-
ments, results to remarkably comparable estimates of
the erythemat UV-B dose. The two instruments pro-
vide similar information on the UV-B dose when they
are cross-examined under a variety of meteorological
and atmospheric conditions and over a large range of
solar zenith angles and total ozone.
I. 1NTRODUC*I'ION
Although spectral UV-B measurements are
highly important for studying the penetration of solar
ultraviolet radiation through the atmosphere, its da-
maging effects on the human beings and the ecosys-
tems are mostly associated with the total dose re-
ceived during a given period of exposure (Setlow,
1974; McKenzie et al., 1991).To obtain the total dose,
spectral measurements are convolved with a suitable
action spectrum and the result is integrated over the
UV-B spectral region. Action spectra are varying ac-
cording to the type of the biological damage referred
(ACGIIt, 1978; Parrish, 1982; McKinlay and Diffey,
1987). In some cases the differences are significant,
as it will be shown in the following. The advantage of
spectral UV-B measurements is that they are com-
pletely independent of the action spectra, so that they
can be easily used in many cases, provided that the
corresponding action spectra are available.
However, UV-B broadband detectors, operat-
ing continuously and capable in providing erythem-
ally weighted UV-B doses, may become useful supple-
ments to the detailed spectral measurements. Then it
would be feasible to obtain accurate dose estimates
on a daily basis and to perform spectral measure-
ments several times per day, in order to monitor and
control the performance and stability of the UV-B
detectors. Of course detailed comparison is required
between the two methods of measurements, to prove
the capability of the UV-B broadband detectors to
accurately supplement the spectral measurements.
Such a comparison, between a Brewer spectropho-
tometer and a UV-B (Robertson-type) pyranometer,
is presented in this study.
2. INSTRUMENTATION
The Brewer spectrophotometer #005, operat-
ing in Thessaloniki, Greece, since 1982 is used during
the last three years for regular spectral global UV-B
measurements, which are performed on a daily basis
at 50 ° and 63 ° of solar zenith angle (both in the
morning and evening) as well as at local noon. These
spectral scans cover the range from 290 nm to 325 nm
with a resolution of 0.5 nm. The spectrophotometer's
calibration is checked regularly with a 40 Watts halo-
gen lamp traceable to NIST (National Institute of
Standards and Technology) standard of spectral irra-
diance.
Since July 1991 a UV-B pyranometer (Yankee
Environmental Systems, model UVB-1), designed to
measure the biologically effective solar UV-B radia-
tion, was installed at Thessaloniki and operates con-
tinuously at the same location with the Brewer spec-
trophotometer. The spectral response of the pyrano-
meter approximates the erythemal action spectrum
(Parrish, 1982), but a correction factor must be ap-
plied on the output in order to match the real erythe-
mal spectrum. This factor is obtained from model
estimations (Green, 1974), which takes into account
the total ozone, the cloud cover and the aerosol thick-
ness at the time of measurements. The first two pa-
rameters are measured in Thessaloniki and thus they
can be fed into the model, whereas no data are avail-
able on the aerosol thickness and default values are
used instead.
786
https://ntrs.nasa.gov/search.jsp?R=19950004678 2020-06-16T11:30:08+00:00Z
1.2
1.0
Z
O
0_0.8
bo
LD
rw
0.6
LD
F--
.,_j 0.4
k.i
0.2
GO
280
em .o ,p.c,um
,_, ACGIH-NIOSH oclion spectrum
,i_ -- UVB-1 response funclion
/ ' \
?
WAVELENGTH (nm)
Fig. 1. Relative response of the ACGIH-NIOSH (dotted
line) and the erythemal (dashed line) spectra, and
the UVB-1 response function (solid line).
The pyranometer's measurements are integra-
ted over ten minutes intervals and fed automatically
to a computer. Almost eleven months of collocated
measurements from both instruments were collected
and used in this study. In addition data from two days
of measurements with both instruments in July 1991
and May 1992 are also used. During these days spec-
tral measurements with the Brewer spectrophotome-
ter were obtained several times per day (in some cases
every 15 minutes).
3. RESULTS AND DISCUSSION
As mentioned previously, the UVB-1 pyrano-
meter's output is similar to the erythemal action spec-
trum. The Brewer uses for the calculation of the UVB
dose the action spectrum recommended by the Amer-
ican Conference of Governmental Industrial Hygien-
ists, ACGIH-NIOSH curve (1978), which is different
from the one of Parrish (1982). Both action spectra
and the response function of the pyranometer are
shown in Figure 1. It appears from this figure that
there are slight differences between the response
function of the UVB-1 and the erythemal spectrum,
which become significant when the solar spectrum
convolves with them, since they appear in the longer
wavelengths where the solar spectrum is more in-
tense. Therefore differences from 25% to 60% are
expected especially in the low zenith angles, based on
the Green's (1974) model estimations. On the other
hand, the ACGIH-NIOSH curve, which is used in the
Brewer's spectra, is extremely different from the ery-
themal action spectrum and it is expected that the
UV-B dose based on the Brewer measurements will
be much lower than the one from the UVB-1 pyrano-
meter.
300
i THESSALONIKI 40°N
1 10 MAY 1992
i
E
200
E
LJ
(D
Z
__ lOO
1::2,
<
MODEL
UVB-1
: ........ BREWEP SIMULATED
• .°.- BREWER DUV
i
2 4 6 8 10 12 14 16 18 20 22 24
TIME (GMT)
300
i
E
200
E
Z
100
C_
Fig. 2.
THESSALONIKI 40°N MODEL
11 JULY 1991 UVB-1
_ " BREWER %IMULATFD
•'f' • .... BREWER DUV
: !,i
.f" . ,:,
2 4 6 8 10 12 14 16 18 20 22
TIME (GMT)
24
Examples of the daily variation of the UV-B dose
as measured by the Brewer (DUV) and the UVB-I
during a cloud-free (upper) and a partly cloudy
(lower) day. Solid line represents the model esti-
mates for clear days (Green, 1074).
This is seen in the examples of Figures 2a and
2b, in which measurements made respectively on July
11, 1991 (a partly cloudy day) and on May 10, 1002 (a
cloudless day) are presented for both instruments,
along with the model estimation for these days (asum-
ing no clouds) and a simulated Brewer dose weighted
with the erythemal action spectrum of the pyrano-
meter. Apparently, only the later dose is comparable
with the UVB-1 output since they are both weighted
with the same action spectrum. Small differences still
observed between them, might be attributed to the
fact that the pyranometer's sensitivity extends further
to shorter and longer than the Brewer wavelengths
(290 nm to 320 nm), and consequently it is receiving
more energy than the Brewer.
The comparison of the two instruments for the
common period of operation August 1091 through
April 1992 is shown in Figure 3. In this figure the
787
200
i
E
E
"---"IO0
F'F"
I,I
Ld
Crl
ERYTIIEHAL DOSE
12 AUG 1991 30 APR 1992
N = 931, r = 0.991
/i/
0 I O0
roW.m-2)UVB- I (
20O
6OO
I
E 400
E
I,I
,,_:J200
nn
o
o
UVB-I RESPONSE FUNCTION
12 AUG 1991 - 50 APR 1992
N = 931, • = 0.997
%o
o 8_ °
200 400 600
UVB- 1 (mW.m -2)
Fig. 3. Comparison of the UV-B erythemal dose as de-
rived from the Brewer and the UVB-I instruments.
Solid curve represents a power-low best fit of the
data pairs.
Fig. 4. Comparison of the UV-B radiation as derived from
the Brewer (weighted with the UVB-1 response
function) and the UVB-1 instruments. Solid line
represents a linear best fit of the data pairs.
Brewer dose was computed from spectra weighted
with the erythemal action spectrum and it appears
that the output of the two instruments differ by about
25%. The regression describing the relation between
the two data sets seems to be different than the ex-
pected linear model. Better results are achieved when
using a power law fit instead, with a highly significant
(at the 99.9% confidence limit) correlation coeffi-
cient (r=0.995, n=931 pairs). This deviation from
the linear regression could be attributed either to the
differences in the cosine response of the two detec-
tors, or to the model estimated correction factor ap-
plied to the pyranometer. To eliminate any possible
effects induced by the correction factor we compared
the uncorrected output of the UVB-I with the inte-
grated output of the Brewer, weighted with the spec-
tral response function of the pyranometer. It is ex-
pected that these two quantities should be similar
since they are affected only by the slightly different
spectral ranges of the two instruments. This can seen
in the scatter diagram of Figure 4, in which the rela-
tion between the Brewer and the UVB-1 now be-
comes linear, again with a highly significant correla-
tion coefficient r = 0.997.
It is believed, therefore, that the model correc-
tion factor is responsible for the non-linear relation
between the erythemal dose measured by the two
788
instruments, rather than the suspected differences in
their cosine response. This could be due to the default
aerosol thickness used in the model and it needs fur-
ther work to check the sensitivity of this model on the
various aerosol loads.
4. CONCLUSIONS
From the comparison of the Brewer global
spectral measurements, integrated over the UV-B re-
gion, with the global UV-B radiation as measured by
the UVB-1 pyranometer, under a variety of atmo-
spheric conditions it appears that there is a linear
relation between them with a highly significant corre-
lation coefficient (r = .997) and a relatively low error
estimate (s =+7). In addition, there is no sign of
significant drifts between the two instruments, at
least during the 9 months period of study. The above
suggest that the UVB-1 pyranometer is capable in
supplementing the Brewer spectral measurements
throughout the year. Slight differences between the
erythemal doses, as they are derived from both instru-
ments, are attributed to our insufficient information
concerning the aerosol thickness over Thessaloniki,
because it enters in the Green's model used to obtain
the erythemal in the UVB-1 sensor.
ACKNOWLEDGMENTS
This work was granted by the STEP-CT90-0076
project of the Commission of the European Commu-
nities.
REFERENCES
ACGIH, 1978: Threshold Limit Value for Chemical
Substances and Physical Agents in the Work-
room Environment with Intended Changes for
1978, American Conference of Governmental
Industrial Hygienists, P.O. Box 1937, Cincin-
nati OHIO 45201, USA.
Green, A.E.S., T. Sawada and E.P. Shettle, 1974: The
Middle Ultraviolet Reaching the Ground, Pho-
tochem. Photobiol., 19, 251-259.
McKenzie, R.L., W.A. Matthews and P.V. Johnston,
1991: The Relationship between Erythemal
UV and Ozone, derived from Spectral Irradi-
ance Measurements, Geophys. Res. Lett., 18,
12, 2269-2272.
McKinlay, A.F. and B.L. Diffey, 1987: A Reference
Action Spectrum for Ultraviolet Induced Ery-
thema in Human Skin, in Human Exposure to
Ultraviolet Radiation: Risks and Regulations
(Eds. W.R. Passchler and B.F.M. Bosnajako-
vic), Elsevier, Amsterdam, 83-87.
Parrish, J.A., K.F. Jaenicke and R.R. Anderson, 1982:
Erythema and Melanogenesis Action Spectra
of Normal Human Skin, Photochem. Photo-
biol., 36, 187-191.
Setlow, R.B., 1974: The Wavelength in Sunlight Ef-
fective in Producing Skin Cancer: A Theoreti-
cal Analysis, Proc. Nat. Acad. Sci. USA, 71,
3363-3366.
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Comparison of UV-B measurements performed with a Brewer spectrophotometer and a new UVB-1 broad band detector

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