2. UV solar radiation – a basic review
Solar radiation includes ultraviolet (UV) radiation, visible radiation (light), and infrared (IR) radiation. The radiation is often characterised by its wavelength, usually expressed in nano-meters (1nm=10-9m). When describing biological effects ultraviolet radiation is often subdivided into three spectral bands: UV-C radiation (100-280nm), UV-B radiation (280-315nm) and UV-A radiation (315-400nm). UV radiation can be measured as an irradiance – the power incident upon a surface unit area – in units of W/m2, or as a radiant exposure, or dose – the energy incident upon a surface unit area during a specified period of time – in units of J/m2. The most important factors affecting the UV radiation reaching the Earth’s surface are described below.
Atmospheric
ozone
UV radiation is absorbed and
scattered in the atmosphere. UV-C radiation is completely
absorbed in the upper atmosphere by oxygen and ozone molecules.
Most of the UV-B radiation is absorbed in the stratosphere by
ozone molecules and only a few percent reach the surface of the
Earth. Therefore, at the surface of the Earth the solar UV
radiation is composed of a large amount of UV-A radiation and
only a very small amount of UV-B radiation. UV-B radiation is
known to be biologically damaging, whereas UV-A radiation is much
less damaging but is known for its ability to tan the human skin.
As ozone is the main absorber of UV-B radiation the UV-B
intensity at the Earth’s surface depends strongly on the
total amount of ozone in the atmosphere, and thus on the
thickness of the ozone layer. A factor, which describes the
relation between the sensitivity of the UV-B intensity to changes
in total ozone, is the so-called Radiation Amplification Factor
(RAF). For small changes in the ozone layer thickness the RAF
represents the percent change in UV-B intensity for a 1-percent
change in the total column ozone. For CIE-weighted irradiance,
i.e., for the erythemally effective UV radiation, and for varying
solar elevations and ozone, the RAF’s are in the range of
1.1-1.3.
Solar
elevation
Solar elevation is the angle between
the horizon and the direction to the sun. The solar zenith angle
(SZA) is often used in place of the solar elevation: it is the
angle between the zenith and the direction to the sun. For high
solar elevations the UV radiation is more intense because the
rays from the sun have a shorter path through the atmosphere and
therefore pass through a smaller amount of absorbers. As the UV
irradiance depends strongly on the solar elevation it changes
with latitude, season and time, being highest in the tropics, in
summer and at noon.
Altitude
The UV irradiance increases with
altitude because the amount of absorbers in the overlying
atmosphere decreases with altitude. Measurements show that the UV
irradiance increases by 6-8% per 1000 m increase in altitude.
Atmospheric
scattering
At the surface of the Earth solar
radiation is composed of direct and scattered (diffuse)
components. Solar radiation is scattered on air molecules and on
particles such as aerosols and water droplets. The direct
component consists of the rays from the sun that has passed
directly through the atmosphere without being scattered or
absorbed. The diffuse component consists of rays that have been
scattered at least once before reaching the ground. Scattering
depends strongly on wavelength. The sky looks blue because blue
radiation is scattered more than the other visible
wavelengths. UV radiation is scattered even more and at the
surface of the Earth the UV-B is roughly composed of a 1:1
mixture of direct and diffuse radiation.
Clouds
and haze
The UV irradiance is higher when the
sky is cloudless. Clouds generally reduce the UV irradiance but
the attenuation by clouds depends on both the thickness and the
type of cloud (optical depth of clouds). Thin or scattered clouds
have only a little effect on UV at the ground. In certain
conditions and for short times a small amount of cloud may even
enhance the UV irradiance compared to fully clear skies. In hazy
conditions UV radiation is absorbed and scattered by water
droplets and aerosols and this leads to decrease in the UV
irradiance.
Ground
reflection
Part of the UV radiation that
reaches the ground is absorbed by the Earth’s surface and
part of it is reflected back to space. The amount of reflected
radiation depends on the properties of the surface. Most natural
surfaces such as grass, soil and water reflect less than about
10% of the incident UV radiation. Fresh snow, on the other hand,
may reflect up to about 80% of the incident UV radiation. During
spring and with a cloud-free sky the reflection of snow may
increase the UV irradiance on inclined surfaces to summer values.
This is important at higher altitudes and at higher latitudes.
Sand may reflect about 25% of the UV radiation and can increase
the UV exposure at the beach. Up to 95% of the UV radiation
penetrates into the water and up to 50% penetrates to a
depth of about 3 m (in clear ocean water).