Sea-Bird Scientific SUNA V2, Руководство пользователя

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CDOM absorption properties
Colored Dissolved Organic Matter (CDOM) is one of the main substance classes that
absorbs in the same UV range as nitrate. (Other significant absorbers are seawater,
bromide, bisulfite, and suspended particles). CDOM is a mixture of many chemicals, and
changes across locations and times, usually depending on its origin. Thus, there is no
universal characterization of the UV absorption of CDOM.
CDOM impact on SUNA nitrate concentration
The UV absorption spectra of CDOM changes across the types of CDOM. Some types of
CDOM are closer in their absorption to nitrate than others. Types of CDOM that absorb
similar to nitrate will cause a larger bias than those that absorb dissimilarly. Thus, the
concentration of CDOM alone cannot predict the magnitude of the bias.
Quantification and correction of CDOM impact
Under ideal conditions, CDOM would be isolated from all other absorbers, and its
absorbing properties identified in isolation. A more realistic approach is to independently
find the content (nitrate, salinity, CDOM) of a number of water samples (if CDOM is stable
over time), and calculate the CDOM-induced bias on the nitrate concentration.
FDOM: Fluorescent Dissolved Organic Matter is a subset of CDOM and can be used as a
proxy (if the composition is stable over time). Most FDOM sensors use a 370 nm
excitation source that targets more humic substances, and may not represent absorption
in the nitrate range (~220 nm).
Absorption: The data output of the SUNA V2 is the absorption at 350 nm and 254 nm
(A350 and A254). These wavelengths are outside the nitrate absorption range and can be
used to make an estimate of the impact of CDOM. If absorption is high (>1.3 AU), the
SUNA will not be able to collect adequate light to make a measurement.
RMSE : The root mean square error parameter from the SUNA V2 can be used to make
an estimate of how well the nitrate spectral fit is. This should usually be less than 1E-3. If
it is higher, there is spectral shape (likely due to CDOM) that adversely impacts the nitrate
estimate.
Pathlength: SUNA V2 comes in both a 10 mm and a 5 mm pathlength version. Because
absorbance is directly proportional to pathlength, a halved pathlength gives a tolerance of
approximately twice the background absorption.
7.6 Optional equipment
7.6.1 Anti-fouling guard
Use the anti-fouling guard for moored deployments that do not use a pump. The guard is
attached to the sensor by a plastic clamp. The foul guard is made of perforated copper
that goes around the sample volume to prevent biofouling and lets the sample volume
flush. Operate the sensor horizontally and point the sample volume area downward so
that air bubbles and sediment do not collect.
Reference
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