OZONE AND SOLAR UV-B ERYTHEMAL IRRADIANCE

ABSTRACT
Results of the total column ozone and ultraviolet (UV-B) erythemally weighted irradiance
measurements at the ground-based solar monitoring station at the Kishinev (Moldova) are
presented. Diffuse and global components of solar UV-B erythemal irradiance on horizontal
plane were continuously measured with sensors UV-S-B-C (of broadband 280-315 nm),
Kipp&Zonen. Monthly totals of global and diffuse components of solar UV-B erythemal
radiation reveal distinct seasonal variation with respective minimum in winter and maximum in
summer. Typical values for these components in limiting cases are presented. A simple
polynomial relationship between the global and diffuse components of solar UV-B erythemal
radiation measured for cloudless days was derived. It was shown that coefficients of the
polynomial depend on daily mean value of aerosol optical thickness (AOT). Collocated
measurements of AOT have been carried out with the sunphotometer Cimel CE-318 within
the framework of the Aerosol Robotic Network (AERONET) program, managed by
NASA/GSFC.
Total column ozone content was retrieved from direct solar ultraviolet radiation measurements
at 3 discrete wavelengths centered at 305.5, 312.5, and 320 nm within the UV-B range.
Ozone measurements were regularly carried out with the hand-held MICROTOPS II
Ozonemeter, Solar Light Co. Monthly average values of total column ozone content
measured with the MICROTOPS II at the Kishinev are in close agreement with those ones
retrieved from the multiyear (1978-2004) database statistics acquired from satellite platforms
measurements with the Total Ozone Mapping Spectrometer (TOMS). It was shown the
existence of seasonal variability of the total column ozone content with respective minimum
values observed at the end of autumn and winter, and maximum values observed at the end
of winter and in spring. The maximum and minimum of daily mean values of total column
ozone ever measured with TOMS at the satellite platforms overpassed Kishinev site,
amounted of ~540 DU (on February 19, 1985) and ~204 DU (on December 1, 1999). Yearly
mean value of total column ozone measured at the Kishinev was ~ 338 DU. Total column
ozone measurements carried out with MICROTOPS at the Kishinev site from September
2003 to August 2004, gave maximum and minimum values of ozone daily means at ~ 489 DU
(on February 12, 2004) and ~259 DU (on December 3, 2003). The estimation of total column
ozone trend derived from the TOMS multi-year statistics was ~ -10 DU/decade.
KEYWORDS: column ozone content, UV-B erythemal radiation, aerosol optical thickness.
1. INTRODUCTION
Ozone and aerosol particles in atmosphere modify the intensity and spectral composition of
the solar ultraviolet radiation at the Earth’s surface. Each of components has a specific
influence upon the radiation exchange and interaction processes that finally define of solar
UV-B radiation reaching the surface. Aerosols produced by both human activities and natural
processes affect the UV-B radiation within the whole atmosphere through the scattering and
TOTAL COLUMN OZONE AND SOLAR UN-B ERYTHEMAL IRRADIANCE 205
absorption processes. The absorption of the UV-B radiation is largely controlled by ozone in
the stratosphere at altitudes between 25 and 100 km. There are, however, other effects that
influence the UV-B radiation transfer: cloud cover, tropospheric ozone, other gaseous
pollutions, surface albedo. To investigate complex relationships between various phenomena
taking place in the atmosphere it is necessary to have reliable total column ozone and solar
UV-B radiation data obtained from the ground-based measurements. As a consequence of
ozone and aerosols can change the UV-B exposure both locally and globally, ground-based
solar radiation monitoring stations become of a particular of interest to obtain an exhaustive
and reliable continuous flow of information about the resulting UV-B erythemal radiation field
on the Earth’s surface and the total column ozone content at the sites of observation.
This paper discusses results of the total column ozone and solar broadband ultraviolet (UV-B)
erythemal weighted irradiance measurements carried out at the ground-based solar
monitoring station at the Kishinev, Moldova.
2. MEASUREMENT APPROACH
For the first time in Moldova it was established ground-based station for continuous solar
radiation monitoring. Station was equipped with the solar radiation sensors for broadband
measurements of radiation from UV-B to IR, data logger CR10X, and active solar tracker unit
2AP BD, (Kipp&Zonen). These instruments were assembled into the multifunctional
radiometric complex. Additional instruments, such as an automatic weather station MiniMet,
ozonemeter MICROTOPS II and sunphotometer Cimel CE-318, are used at the station. Data
sets collected from weather station MiniMet (Sky Instruments Ltd.), which is arranged at a
distance of 30 meters apart from the radiometric measuring complex, supplement the solar
radiation measurements. Radiometric complex is placed in an urban environment at the
Kishinev site (see Fig. 1) with coordinates: ϕ=47.00130N, λo=28.81560E, h=205 m a.s.l. All
instrumentation was mounted on the roof of the building of the Institute of Applied Physics,
Academy of Sciences of Moldova.
Radiometric complex is used to carry out long-term continuous monitoring of solar radiation at
the Earth’s surface. Measurements are made with 1 sec resolution and 1 minute averaging
interval. Solar radiation sensors used at the station allows for covering wavelength range from
UV-B to IR and to make broadband measurements of global, diffuse and direct components
of solar radiation. Diffuse and global components of the solar UV-B irradiance are measured
with two sensors UV-S-B-C (of broadband 280-315 nm) installed at the moving platform of the
active solar tracker 2AP BD unit and mounted at the stationary platform, respectively. Solar
UV-B erythemal weighted irradiance is re-calculated by using the specific adjustment tables
for each sensors taking into account the solar zenith angle and total column ozone content at
the site of observation.
Typical values of measured global and diffuse components of monthly totals of solar UV-B
erythemal radiation are presented. Total column ozone content is regularly measured at the
station by hand-held narrowband filter MICROTOPS II Ozonemeter, (Solar Light Co) [1]. This
instrument is equipped with the highest grade and long stability filters with ion-beam assisted
deposition and centered at λ= 305.5, 312.5 320, 936 & 1020 nm. MICROTOPS II Ozonemeter
gives an accuracy < mquvb ="8.5" mduvb ="7.4" mquvb="144.5" mduvb =" 104.7" y=" C"> measured with the sunphotometer Cimel CE-318 at
λ=500 nm (see Figure 4). Coefficient A has a little dependence on <τa(500)> and mean value
of is ~ 1.1, whereas coefficient B has strong dependence on <τa(500)>. Coefficient B
decreases with the increase of AOT <τa(500)> . For the set of clear free days these
scattergrams have the analogous dependence, but with specific A and B coefficients. For
overcast days coefficient A tends to be equal to the ratio QUVB/ DUVB ~1.07 and coefficient B
becomes equal to 0.0.
Figure 3. Scattergram of minute average
values of global vs. diffuse components of the
solar UV-B erythemal irradiances for cloud free
day on September 6, 2004.
Figure 4. Variation of coefficients A and
B versus daily means of aerosol optical
thickness <τa(500)> at λ=500 nm for
selected cloud free days from April to
September 2004.
Figure 5 shows monthly average values of total column ozone content retrieved from
multiyear (1978-2004) statistics retrieved from TOMS measurements and measured with
hand-held MICROTOPS II ozonemeter at the Kishinev site during September 2003 – August
2004. It is clear seen the existence of seasonal variability of total column ozone content with
minimum observed at the end of autumn and in winter, and maximum observed at the end of
winter and in spring. Ozone values obtained from measurements made with MICROTOPS
208 ACULININ
and retrieved from TOMS measurements are in good agreement with each other. The most
scatter in the data measured with MICROTOPS was observed from December to March. The
error bars show one standard deviation. Figure 6 shows multiyear (1978-2004) statistics of
the yearly average values of total column ozone content retrieved for the Kishinev site from
the measurements made at the satellite platforms with the TOMS.
Figure 5. Monthly average values of total
column ozone content retrieved from
multiyear (1978-2004) statistics from TOMS
measurements and measured with hand-held
MICROTOPS II at the Kishinev site during
September 2003 – August 2004.
Figure 6. Multiyear (1978-2004)
statistics of the yearly average values
of total column ozone content retrieved
from measurements made with TOMS
at the satellite platforms overpassed
Kishinev site.
The maximum and minimum of daily mean values of total column ozone ever measured with
TOMS from satellite platform overpassed Kishinev site, amounted of ~540 DU (on February
19, 1985) and ~204 DU (on December 1, 1999), respectively. The last extreme value of
column ozone content is attributed to the mini ozone holes evolution over the West and
Central Europe in December 1999. The estimation of total column ozone trend gives the
value of ~ -10 DU/decade. Total column ozone measurements carried out with MICROTOPS
at the Kishinev site from September 2003 to August 2004, gave maximum and minimum
values of ozone daily means at ~ 489 DU (on February 12, 2004) and ~259 DU (on
December 3, 2003), respectively. Yearly mean values of the total column ozone content
derived from ground-based measurements at the Kishinev site and retrieved from the multiyear
statistics of the TOMS measurements were very close to each other and gave the
values of ~ 338 DU and ~334 DU, respectively.
4. CONCLUSIONS
Continuous solar radiation measurements from UV-B to IR have been carrying out at the solar
radiation monitoring station established in an urban environment of Kishinev. Period of
observation was chosen from October 2003 to September 2004. Results of measurements of
the monthly totals of global and diffuse components of solar UV-B erythemal radiation (with
UV-S-B-C sensors of broadband 280-315 nm) on horizontal plane are presented. It was
shown seasonal variation of these components with the presence of minimum (for winter
season) and maximum (for summer) of their values. It was shown the influence of the number
of overcast days upon the variation of the monthly totals of sunshine duration. The regression
relationship of the scattergram for global QUVB and diffuse DUVB components of solar UV-B
erythemal radiation measured for cloud free days may be represented by second order
polynomial regression curve. The regression coefficient at term of order two has strong
dependence on aerosol optical thickness <τa(500)> measured with the sunphotometer Cimel
CE-318 at λ=500 nm: this coefficient decreases with increasing of the aerosol optical
thickness. Coefficient at term of order one is practically independent on <τa(500)>, and free
term is negligible one.
TOTAL COLUMN OZONE AND SOLAR UN-B ERYTHEMAL IRRADIANCE 209
Total column ozone trend gives the value of ~ -10 DU/decade. This value was derived from
the multiyear (1978-2004) statistics of the TOMS measurements. Measurements of total
column ozone made with MICROTOPS at the Kishinev site from September 2003 to August
2004, gave maximum and minimum values of daily means of ozone with ~489 DU (on
February 12, 2004) and ~259 DU (on December 3, 2003), respectively. Yearly mean value of
total column ozone measured with ozonometer at the Kishinev site was ~ 338 DU.
ACKNOWLEDGEMENTS
I thank Dr. Brent Holben, the Principal Investigator of the AERONET program (NASA/GSFC) and
his staff in supplying sunphotometer Cimel-318 used in this investigation at the Kishinev site and
data processing. The work was funded by the U.S. Civilian Research &Development Foundation
(CRDF) and the Moldovan Research and Development Association (MRDA) through grant #ME2-
3033.
REFERENCES
1. Morys M., Mims III F.M., Hagerup S., Anderson S.E., A. Baker, J. Kia, T. Wallkup (2001) J.
Geophys. Res., 106, 14573-14582.
1. Holben B.N. et al. (1998), Rem. Sens. Environ., 66, 1-16.
2. Eck T.F., Holben B.N., Reid J.S., Dubovik O., Smirnov A., O’Neill N.T., Slutsker I. and Kinne
S. (1999) J. Geophys. Res., 104, 31333-31350.
3. Smirnov A., Holben B.N., Eck T.F., Dubovik O. and Slutsker I. (2000) Rem. Sens. Env., 73,
337-349.