Geology Reference
In-Depth Information
Figure 3 UK emission maps (2002) on a 1 1 km grid for (a) NO 2 and (b) SO 2 and in
kg (data from UK NAIE, http://www.naei.org.uk/)
The light source for photochemistry in the atmosphere is the sun. At
the top of the atmosphere there is ca. 1370 W m 2 of energy over a wide
spectral range, from X-rays through the visible to longer wavelength. By
the time the incident light reaches the troposphere much of the more
energetic, shorter wavelength light has been absorbed by molecules such
as oxygen, ozone and water vapour or scattered higher in the atmos-
phere. Typically, in the surface layers, light of wavelengths longer than
290 nm are available (see Figure 4). In the troposphere, the wavelength
at which the intensity of light drops to zero is termed the atmospheric
cut-off. For the troposphere, this wavelength is determined by the
overhead stratospheric ozone column (absorbs ca. l r 310 nm) and
the aerosol loading. In the mid- to upper-stratosphere, the amount of O 3
absorption in the ''window'' region at 200 nm between the O 3 and O 2
absorptions controls the availability of short-wavelength radiation that
can photodissociate molecules that are stable in the troposphere. In the
stratosphere (at 50 km), there is typically no radiation of wavelength
shorter than 183 nm.
The light capable of causing photochemical reactions is termed the
actinic flux, F l (l) (cm 2
s 1 nm 1 ), which is also known as the scalar
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