Salinity drift prevention experiments in the Korea ocean research stations and suggestions for high quality salinity observation

The importance of salinity has been highlighted to cope with climate changes and disaster prevention. The salinity of accuracy up to 0.005 is normally required in an open ocean to understand various oceanic and climatic phenomena; however, the reliability of salinity measured on the coast and open seas around Korea was low due to the lack of a standardized observation system and post-processing of quality verification. Korea Ocean Research Stations (KORS) has been producing salinity time series since 2003 through the Aanderaa conductivity-temperature (CT) 3919 inductive sensors, which have an advantage of on-site maintenance but tend to drift toward a lower conductivity because of biological attachments to the sensor. This study applied copper taping and UV light exposure techniques to the sensors and then compared its salinity measurements with RBR CTD mooring observations and SeaBird19 CTD profiles to assess a biofouling effect on salinity observations. This experiment shows that the salinity from the CT sensor without biofouling prevention starts to drift in a week, particularly for a surface sensor. This biofouling induced the decrease of salinity up to 10 in a month. The copper taping methodology efficiently suppressed the biological attachment but disturbed an electromagnetic field around the sensor, thus resulting in unrealistic salinity values. When UV light was periodically exposed at a distance of about 5 cm away from the CT sensor, relatively stable salinity could be observed without significant drift at least in two months. Besides, the SBE37 CTD, an electrode-type sensor, seems to be relatively free from biofouling but has difficulties in sensor maintenance and a sensor calibration process. Our results underline a double installation of salinity observation equipment with UV light exposure. In addition, the pre-calibration of a CT(D) sensor and post-verification should be included in a standard procedure for high-quality salinity measurement.