What theory is the temperature / salinity correction for SUNA based on? Does this correction only work in natural seawater?
The temperature and salinity correction for the SUNA can be traced to the experiment outlined in Sakamoto et al. 2009, which is the T/S correction our UCI-based SUNA software uses in post-processing only.
Absorption of UV light in seawater is dominated by dissolved nitrate and bromide ions at wavelengths less than 240 nm. To estimate nitrate, it is necessary to remove the absorption due to bromide. The salinity correction addresses the sea salt extinction coefficients due to bromide. In the real ocean, bromide covaries with NaCl. So, during calibration, we can measure the the bromide absorption due to seawater at one salinity and later predict the absorption due to seawater at any salinity.
Artifical seawater surrogates do not necessarily have the correct bromide absorption to be able to validate the the Sakamoto et al. 2009 salinity correction, so the salinity correction may not product accurate results if your data was not collected in natural seawater.