A comparison between Erythemal Daily Dose ... - PMOD/WRC

A comparison between Erythemal Daily Dose retrievals from YES
broadband radiometer and OMI data at Rome station
I. Ialongo, A. M. Siani, G. R. Casale and M. Cacciani
Sapienza - University of Rome, Dept. of Physics G-MET, P.le A.Moro 2, I-00185 Rome, Italy
Abstract. Erythemal Daily Doses (EDDs) have been
determined by a broad-band radiometer (model YES
UVB-1) operational since 2000 at Rome station. EDDs
were compared with OMI (Ozone Monitoring Instrument)
satellite UV data, corrected by the cloud/aerosol effect.
The results show that OMI overestimates EDD data by
about 22%. Furthermore daily total ozone values measured
by Brewer spectrophotometer 067, situated nearby the
radiometer, were compared with OMI-TOMS ozone data
showing a good agreement.
Introduction
The amount of solar ultraviolet (UV) radiation
(200-400 nm) reaching the Earth’s surface is affected
mainly by atmospheric ozone absorption, cloudiness and
aerosol.
The erythemal dose (the incident weighted irradiance
on a given surface over a specific period of time) provides
the degree of effectiveness of UV radiation in producing
the reddening of the skin. This quantity is obtained by
weighting the incoming solar radiation on a flat surface
with erythema action spectrum (Diffey and McKinlay,
1987) and taking into account a specific integration time
(usually over one day). Both broadband radiometers,
which have quite an erythemal response and
spectroradiometers yield measures of erythemal UV
irradiance.
Satellite instruments provide UV data on global scale
with a better geographical distribution of surface UV
irradiance especially for regions not covered by UV
ground-based instruments. On the other hand, long-term
series of reliable UV measurements from ground networks
are needed for validation strategy of the satellite
monitoring system. Recently UV estimates have become
available from the Ozone Monitoring Instrument (OMI) on
NASA's Aura satellite, launched on 15 July 2004
(Tanskanen et al., 2006).
In this study the Erythemal Daily Doses (EDDs),
derived by the broad-band radiometer (model UVB-1,
Yankee Environmental System, MA, USA) operational at
Rome (Italy) since 2000, are presented. EDDs and total
ozone amounts measured, respectively, by the YES
radiometer and the Brewer 067 (located nearby the
radiometer) are compared with OMI satellite data.
Instrumentation
The YES broad band radiometer has a spectral response
similar to that of skin erythema and it measures the
erythemal dose rate between 280 and 400 nm. The YES
radiometer of the Solar Radiometry Observatory,
University of Rome Sapienza (41.9°N, 12.5°E, 75 m a.s.l.)
is installed on the roof of a building of the University
Campus (in the centre of Rome) and it has been working
reliably since 2000. The Solar Radiometry Observatory is
one of the stations that regularly measures UV irradiance
in Italy, and it also has a Brewer spectrophotometer
(operational since 1992) for measurements of UV spectral
irradiance and total ozone amounts (Casale et al., 2000).
The radiometer was calibrated at the European
Reference Centre for Ultraviolet Radiation Measurements
(Joint Research Centre, Ispra, Italy) in 2004 and it
participated into the broadband radiometer
inter-comparison at PMOD/WRC at Davos (Switzerland)
in August 2006 (Webb et al., 2006).
The Ozone Monitoring Instrument (OMI) onboard the
NASA EOS Aura spacecraft is a nadir viewing
spectrometer that measures solar reflected and
backscattered light in a selected range of the ultraviolet
and visible spectrum. The Aura satellite describes a
sun-synchronous polar orbit, crossing the equator at 13:38
local time. OMI measurements of ozone columns and
profiles, aerosols, clouds, surface UV irradiance and the
trace gases (NO 2 , SO 2 , HCHO, BrO, and OClO) are
available (Levelt et al., 2006).
OMI surface ultraviolet retrievals are determined by
means of a radiative transfer model using as inputs satellite
ozone column measurements and surface albedo
(Tanskanen et al., 2006). Afterward a cloud/aerosol
transmittance factor derived from OMI data is applied to
clear sky UV retrievals to obtain more accurate surface UV
irradiance and then the erythemal dose rate and the daily
doses.
Method and results
An algorithm was developed to compute the erythemal
dose rate E CIE (Wm -2 ) from the radiometer signal, by
means of the following formula (Webb et al., 2006):
E
CIE
= U ⋅C
⋅ f ( Θ,
TO3)
⋅Coscor(
Θ)
n
where U is the raw signal of the instrument (V); C is the
calibration coefficient (C=0.1104 Wm -2 V -1 ); f n (Θ,TO3) is a
function of the solar zenith angle Θ and the total column
ozone TO3; Coscor(Θ) is the cosine correction function.
The calibration factor and f n (Θ,TO3) were obtained during
the broadband radiometer inter-comparison at Davos
(Webb et al., 2006) and all YES data were then
reprocessed.
Erythemal Daily Doses were computed under all sky
conditions. The values of f n were obtained taking into
account the daily mean total ozone from Brewer 067.
Figure 1 shows the EDD time series at Rome station since
2000. It is clearly visible the seasonal behavior with a
maximum in summer and a minimum in winter.
OMI provides total ozone amounts obtained by means
of TOMS algorithm (Bhartia and Wellemeyer, 2002). Daily
values of Brewer direct sun total ozone measurements
were compared with OMI-TOMS ozone data for the period
2004-2006 (Figure 2) obtaining a correlation coefficient
(1)

(r 2 ) of 0.96.
6
0.93. In addition OMI-TOMS ozone data were used to
retrieve EDDs: in this case, the average difference is
21.8% and the correlation coefficient r 2 is 0.95.
Erythemal Daily Dose (kJ/m 2 )
5
4
3
2
1
0
2000 2001 2002 2003 2004 2005 2006
time (year)
OMI EDD no clear sky (kJ/m 2 )
7
6
5
4
3
2
1
Figure 1. Erythemal daily dose (kJ/m 2 ) time series from YES
UVB-1 at Rome calculated taking into account Brewer total
ozone.
0
0 1 2 3 4 5 6 7
YES EDD (kJ/m 2 )
Figure 3. OMI vs YES radiometer erythemal daily dose (kJ/m 2 )
calculated taking into account Brewer total ozone at Rome
(period 2004-2006). Dotted line indicates the bisectrix.
500
Conclusions
O 3
OMI_TO3 (DU)
450
400
350
300
250
Ground-based well calibrated broadband instruments
can provide reliable measurements of the surface solar
erythemal UV radiation and thus they can be used in the
satellite-derived UV data validation. The good agreement
observed between OMI and Brewer daily total ozone data
allows to use satellite ozone data in the EDDs retrieval.
The results of OMI-YES EDDs comparison show on
average, OMI overestimates ground-derived UV data of
about 22%, probably because of the effect of aerosols in an
urban site such as Rome.
200
200 250 300 350 400 450 500
O 3
Brewer (DU)
Figure 2. Brewer vs OMI-TOMS total ozone column (DU) with
linear regression (black line). Dotted line indicates the bisectrix.
In order to quantify the agreement between OMI (y i )
and ground-based (x i ) data, the averaged percentage
difference was computed by means of the following
formula:
1
Average difference % =
n
n
i
∑ (
i= 1 ( xi
+
y − xi
y )
i
)*100
2
Concerning ozone data, it was found that the averaged
percentage difference is -0.66% (n=400). The comparison
shows that OMI total ozone data are in good agreement
with ground-based measurements. From these results, the
f n values in the formula (1) can be determined also by
means of OMI-TOMS total ozone.
EDDs from YES radiometer, taking into account
Brewer total ozone, were compared with OMI UV data,
corrected for the effect of clouds and aerosol using OMI
no-clear sky algorithm, and the results are reported in
Figure 3. The average difference between OMI and YES
data is about 21.4% and the correlation coefficient r 2 is
(2)
Acknowledgments The authors are grateful to Aura
Validation Data Center (NASA) for providing OMI data.
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