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Dependence of UV radiation on altitude and
aerosol optical depth example Bolivia Monika
Pfeifer, Joachim Reuder, Peter Koepke, Frank
Wagner
Background
Generally, solar UV radiation increases with
increasing altitude due to smaller optical air
masses. This increase in solar radiation is
called the altitude effect (AE) which is given
as an increase in irradiance relative to the
valley side in percent per 1 km. .
Compared with low altitudes, radiation at high
altitudes traverses a shorter path length through
the atmosphere and thus undergoes less scattering
and absorption. The constituents of the
atmosphere in the layer between the mountain and
the valley sites and their interactions with UV
radiation are responsible for the different
irradiances at the two sites. These interactions
are Rayleigh scattering by air molecules,
absorption by tropospheric ozone and other gases,
scattering and absorption by aerosols. All these
effects depend on the solar zenith angle (SZA).
Since aerosols are mainly concentrated in the
boundary layer, the altitude effect will strongly
increase if the mountain station happens to come
out of the aerosol layer. Because of the decrease
of temperature with altitude, the probability of
snow cover at the mountain station increases.
Moreover, there is the possibility of clouds
between the two levels. Both enhance the albedo
of the mountain station and UV radiation is
increased through multiple scattering. Figure 1
shows an overview of all the parameters
contributing to the altitude effect.
Figure 2 Map of Bolivia with the stations used
in this study.
Altitude
SZA
Albedo
Measurements
Erythemally weighted UV radiation was measured
with broadband radiometers in Bolivia in
2000/2001 at four altitudes. In 2002 UV
radiation and aerosol optical depth were measured
in Caranavi, La Paz and Chacaltaya. Caranavi is
located in the tropical part of Bolivia at an
altitude of 605 m. Because of daily precipitation
and wash out of the aerosols, the mean aerosol
optical depth was exceptionally low. La Paz (3420
m) and El Alto (4000 m), the stations of the
plateau, are important sources of aerosol in
altitudes above 3000 m. Mount Chacaltaya (5240 m)
is an example of an isolated mountain peak out
of the boundary layer. See figure 2 for locations.
Clouds
Boundary Layer (O3, Aerosol)
Pressure
Figure 1 Overview of all the parameters
contributing to the altitude effect.
Methods
Results
Figure 4 The altitude of the station against the
averaged UVI for elevations 20 70 . For
clarity, the data has been grouped into boxes
corresponding to the fixed elevations
examined. This elevation is displayed above each
box.
Figure 3 Calibrated UV measurements in Caranavi.
In the upper figure UV radiation varies with
clouds, changes in the total ozone content,
aerosol properties and albedo. In the lower
figure only cloud free cases are considered and
the data is normalized to a fixed total ozone
content.
Figure 4 shows the altitude of the station
against the averaged UVI for an elevation of 20
- 70 . The difference in UVI for an elevation
divided by the difference in altitude of the two
stations gives the altitude effect. The mean
altitude effect measured between Caranavi and La
Paz was 5.5 10.1 / km dependent on the solar
zenith angle. These values are in good agreement
with the values reported by Cabrera (4 10 / km)
and Piazena (8 -10 / km) from the Chilean Andes.
An altitude effect of 9.2-12.7/ km between La
Paz and Chacaltaya and 8.1-12.5 / km between
Caranavi and Chacaltaya was obtained. Because the
Chacaltaya is already located outside the
boundary layer, the differences in UV radiation
between the plateau and the mountain are very
high. Cabrera presented an altitude effect of
only 2 / km for altitudes from 3000 5000 m.
Our effect is more pronounced because of the
important aerosol sources in altitudes of more
than 3500 m. This effect is even stronger at El
Alto. The altitude effect between Caranavi and El
Alto shows relative small values of 4.1 - 6.8 /
km. The altitude effect between El Alto and
Chacaltaya reaches values from 14.3 - 23.0 /
km. In El Alto less UV radiation was measured
than in La Paz, although it lies 580 m higher
than La Paz. A comparision of La Paz and El Alto
would result in negative altitiude effects,
because of the dominating aerosol effect. The
maximal altitude effect for erythemally weighted
UV radiation was measured for an elevation of 20
.
Figure 3 shows the measured UV irradiance at
Caranavi after calibration. In the upper figure,
UV radiation is affected by changes in
cloudiness, total ozone content, aerosol
properties and albedo. In the lower figure, the
irradiance was normalized to a fixed total ozone
content and only cloud free measurements were
taken into account, because variations in cloud
cover between the mountain and valley site would
dominate the altitude effect. At this point the
UV radiation varies only with solar elevation,
atmospheric turbidity and changes in the albedo
of the terrain. The albedo in La Paz, Caranavi
and El Alto can be assumed to be constant during
the measurements, so the remaining variability of
the UVI represents natural changes of atmospheric
turbidity during the measurements. The mean UVI
and standard deviation were calculated for
elevations from 20 70 in steps of 10 and
this data was used to calculate the altitude
effect.
Conclusion
Due to the great natural variability in the
altitude effect, even under cloudless conditions,
it is impossible to describe this effect by a
single number. Moreover, it is necessary to
identify the actual atmospheric conditions and
albedo of the terrain at the measurement sites in
order to interprete the effect correctly and
understand it properly. We propose to seperate
the effects of the alittude, the albedo and the
aerosol optical depth on the altitude effect and
to describe it in the future as the combination
of three different effects. .
Meteorologisches Institut Theresienstr. 37 D
80333 Munich moni_at_meteo.physik.uni-muenchen.de
References Pfeifer, M (2003) Höhenabhängigkeit
der UV Strahlung Untersuchung am Beispiel
Boliviens (Diploma Thesis) Cabrera et al.
(1995) Variations in UV radiation in UV
radiation in Chile. Journal of Photochemistry and
Photobiology, Biology 28137-142 Piazena, H
(1996) The effect of altitude upon the solar
UV-B and UV-A irradiance in the tropical chilean
andes. Solar Energy, 57 133-140
Meteorological Institute of the University of
Munich Theresienstr. 37 80333 Munich
Germany moni_at_meteo.physik.uni-muenchen.de