Deep SUNA Manual
For SUNA running firmware version 2.4 or later
3. The SUNA Sensor
TiO2 Titanium Dioxide
Turbidity
Sample
NTU
per mg/l
Absorbance at
225 nm (10 mm)
per mg/l
NO3 shift μM
in freshwater
per mg/l
NO3 shift μM
in seawater
per mg/l
ARD
1.25
0.0016
<-0.002
0.01
Kaolin
1.5
0.0085
<0.001
0.02
TiO2
15.0
0.0090
<0.001
<0.001
The following samples, obtained from the International Humic Substances Society, were
used as proxies for CDOM:
PLFA Pony Lake Fulvic Acid – Reference (1R109F)
SRFA Suwannee River Fulvic Acid – Standard (1S101F)
PPHA Pahokee Peat Humic Acid – Reference (1R103H-2)
CDOM
Sample
QSD
per mg/l
Absorbance at
225 nm (10 mm)
per mg/l
NO3 shift μM
in freshwater
per mg/l
NO3 shift μM
in seawater
per mg/l
PLFA
N/A
0.017
0.4
0.6
SRFA
N/A
0.027
<0.1
<0.1
PPHA
42
0.003
<0.01
<0.1
An interfering species generates a spurious nitrate concentration when the spectral
characteristics of the interfering species resembles that of nitrate. Typically, an RMSE
value that is more than a few times the RMSE of a pure nitrate sample should be taken
as an indication that interfering species are impacting the measurement. The RMSE
value is the square root of the mean of the sum of the squared differences between the
measured and the fitted absorbance; it provides a measure for the quality of the fit.
Independent measurements of turbidity and CDOM, as well as an analysis of the
absorption spectrum, can refine the impact analysis.
3.3 Operating Principles
3.3.1 Absorbance Spectroscopy
The SUNA measures the concentration of dissolved nitrate in water. The sensor
illuminates the water sample with its deuterium UV light source, and measures the
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