Geoscience Reference
In-Depth Information
From the absorption coefficients, spectral slope (
S
) can be calculated using a nonlinear
fit of an exponential function to the absorption spectrum over the range of desired wave-
lengths using an equation such as:
α
g
(
λ
) =
α
g
(
λ
ref
)
e
-s(λ-λ
ref
)
(2.3)
where, in this case,
α
g
(
λ
) is the Naperian absorption coefficient of CDOM at a specified
wavelength,
λ
ref
is a reference wavelength and
S
is the slope fitting parameter (Twardowski
et al.,
2004
). Specific UV absorbance (SUVA) was originally described as the decadal
absorption coefficient in units of cm
-1
at a given wavelength (commonly
λ
= 254 or 280 nm)
divided by DOC concentration in units of mg C L
-1
(Chin et al.,
1994
; Weishaar et al.,
2003
). Currently, it is common to express SUVA in units of L (mg C m)
-1
using the decadal
absorption coefficient. For a discussion about ambiguity in the use of optical concepts,
readers are referred to Hu et al. (
2002
) and the glossary of recommended terms provided
by the IUPAC (Braslavsky,
2007
). Helms et al. (
2008
) provide a discussion about the use of
spectral slopes and slope ratios as indicators of DOM sources and reactivity. In all cases, it
is useful to state whether Naperian or decadal absorption coefficients are being reported.
2.2.2 Fluorescence
Once excited to higher electronic and vibrational states, molecules return to the ground
state by losing energy via a number of competing pathways. The most rapid relaxation
pathway, occurring in 10
-13
to 10
-12
seconds, is by thermal deactivation resulting from the
transfer of energy via collisions of the excited molecule with solvent molecules. Thermal
deactivation includes the processes of vibrational relaxation, which is the loss of vibra-
tional energy in the excited state, and internal conversion, the radiationless transition
from a higher electronic state to a lower one. Many organic molecules that absorb light
return completely to the ground state through thermal deactivation without emitting light
(Schulman,
1985
). In the case of some molecules, however, the transition from the lowest
excited vibrational state to the ground state by thermal deactivation is slow enough to allow
the emission of light by either fluorescence or phosphorescence. These phenomena differ
in the lifetimes of the molecule in the excited state. Fluorescence occurs in a timeframe of
10
-11
to 10
-7
seconds, whereas phosphorescence requires more time (10
-4
to 10 seconds).
Excellent detailed descriptions of these phenomena can be found in a number of sources
(e.g., Schulman,
1985
). Fluorescence specifically refers to the phenomena wherein a mol-
ecule in the first excited singlet state (S
1
) returns to the ground state by the direct emission
of UV or visible light.
The ability of a molecule to fluoresce is, therefore, the result of competing pathways for
relaxation. Molecular structure is an important factor controlling the pathways by which
an excited molecule returns to the ground state, influencing both the intensity and the posi-
tions in the spectrum where molecules fluoresce (Schulman,
1985
). Excited molecules that
have greater degrees of rotational and vibrational freedom, such as alkenes and alicyclic
molecules, relax efficiently by thermal deactivation pathways, and few of these molecules
