Geoscience Reference
In-Depth Information
of DOM fluorescence and incorporated into mixing models for terrigenous DOM transport
(Blough et al.,
1993
; Vodacek et al.,
1997
; Del Castillo et al.,
1999
; Conmy et al.,
2009
).
Photobleaching of DOM fluorescence also occurs in rainwater. Kieber et al. (
2007
)
photobleached coastal rainwater and found losses to EEM peaks A, C, and T similar to
freshwater UDOM. They remark that the origin of the rain event (continental versus marine
precipitation) likely exerts a large control over photobleaching. Freshwater rain can con-
tribute to surface waters DOM and provide an environment for photoreaction in the atmos-
phere (Graber and Rudich,
2006
). In fact, the molecular properties of DOM in atmospheric
water suggest some degree of photobleaching has occurred. Rainwater is often acidic
(pH < 5), which can enhance photobleaching potential within the atmosphere as well as in
surface waters (Gennings et al.,
2001
).
Humic-regions (e.g., the C and A peaks from Coble,
1996
) appear to be most photola-
bile. The degree of susceptibility to photobleaching depends both on exposure duration
and the sunlight spectral quality. An example of these quantitative and qualitative controls
on DOM photobleaching is shown for the Mackenzie River in Arctic Canada by Osburn
et al. (2009b), where marked changes to SF spectra occurred at peaks centered on 350 nm
and 380 nm excitation (Δ
λ
= 14 nm) after sunlight exposure. SF emission intensity at these
wavelengths generally corresponds to highly conjugated humic substances representative of
terrestrial DOM (Senesi,
1990
). In a kinetic experiment (
Figure 7.8A
), more DOM photo-
bleaching occurred for the SF350 than SF380 nm after 72 hours of sunlight exposure. By
contrast,
Figure 7.8B
shows the effect on these peaks when sunlight was modified by cutoff
filters that selectively remove portions of the solar spectrum during the sunlight exposure.
In the 335 nm filter treatment, for example, more photobleaching occurred for the 380 nm
peak than for the 350 nm peak. The 314 nm treatment included nearly all environmentally
relevant UV radiation and produced photobleaching results not statistically different from
the treatment without a cutoff filter. These results would indicate that not all fluorophores
are bleaching in the same manner and thus respond differently to sunlight exposure.
Specific patterns in DOM fluorescence photobleaching should identify molecular
weight changes. Longwave emission should decrease as a function of light exposure as
polyaromatic compounds are broken apart, reducing the extension of the π-electron system
(Senesi and D'Orazio,
2005
). Aromatic rings within HS are opened after photooxidation,
disrupting charge transfer reactions and giving rise to long-wavelength absorption and
fluorescence phenomena (Del Vecchio and Blough,
2004
). Polyaromatic structures may
generate reactive oxygen species (ROS) that photooxidize DOM. O'Sullivan et al. (
2005
)
found a strong correlation between SF350 and hydrogen peroxide production from natural
and simulated sunlight exposures of DOM from river and coastal waters. They inferred that
the destruction of these polyaromatic structures (which have extensive π-electron systems)
decreases the longer wavelength (red shifted) fluorescence) concomitant with a decrease
in the generation of ROS. Singlet oxygen, for example, is known to be causative agent of
DOM oxidation (e.g., Cory et al.,
2010
).
An additional consideration is the degree to which photobleaching can modify terres-
trial DOM fluorescence and the similarity of that modified fluorescent signature to marine
