The Influence Of Dissolved Organic Matter Represented By Urea On The Accuracy Of Seawater Salinity Measurement

The salinity of seawater is one of the most important physical parameters in marine investigation which presents the toal amount of substances dissolved in seawater.It plays an important role in ocean current,biological activities of sea creatures,and air-sea interactions,imposing significant influence on climate change,marine ecology,marine agriculture,and deep-sea exploration.Therefore,the salinity measurement of seawater is of great significance.At present,the measurement of seawater salinity mainly adopts the conductivity temperature depth instrument(CTD)based on the conductivity measurement.This method calculates the salinity of seawater based on empirical formulas obtained by measuring the conductivity,temperature and pressure of seawater,which derives the so-called practical salinity as a dimensionless number.With the development of marine science,the composition of seawater has been investigated with high accuracy,and the relationship between the amount of certain dissolved substance and its influence to seawater density has been established.As an important substance in seawater,dissolved organic matter could bring significant influence on seawater salinity and density,but does not contribute to conductivity.Thus,we could not get any information of dissolved organic matter from conductivity measurents,and there is still no effective calibration method on this point.In theory,the refractive index measurement method could reflect all components dissolved in seawater,including the dissolved organic matter.Among the salinity measurement method base on refractive index measurement of seawater,the sensor based on the V-groove refraction principle has the advantages of simple structure,high sensitivity,high resistance to the perturbation of seawater flow state.It was thus adopted as the sensor to measure the seawater refractive index and study the change of dissolved organic matter content.The main contents of this paper are as follows:(1)Analyze the propagation of optical path of the V-groove sensor,and select the appropriate measurement range and sensitivity based on the measurement requirements and device properties.(2)Combined with the simulation analysis,a refractive index salinity sensor with high sensitivity that meets the measurement requirements is built,and the sensitivity of the refractive index based on Position Sensitive Device(PSD)is 0.96 m/RIU.(3)In order to secure a high stability of refractive index measurement under constant temperature,the temperature of this system is carefully controlled,and a refractive index measurement accuracy within 1.5×10-5 RIU is achieved.(4)We apply this system to study the changes of urea concentration in simulated seawater at different temperatures and different salinities,carrying out a comparative measurement of conductivity and refractive index,and get the relationship between refractive index and urea concentration.The main innovations of this paper are as follows:(1)The influence of dissloved organic matter on the refractive index of seawater was measured and analyzed for the first time,and the regression relationship between the refractive index and urea at different temperatures and salinities was obtained,which verifies that the refractive index method is better than conductivity in effectively reflecting the contribution of dissolved organic matter in seawater.(2)A method is proposed to obtain the urea concentration in seawater with relative high amount of urea by comparing the salinity results measured by the conductivity method and the refractive index method.The results in this paper show that the addition of 1 g/kg of urea to the simulated seawater of 35 PSU at 20℃ has no obvious effect on the practical salinity measured by conductivity method,while leads to an increase of 1.493×10-4 to the index of refraction,bringing a variation of 0.8228 PSU to the salinity measurement by the refractive index method.For seawater containing urea and other dissolved organic matter which has an obvious composition difference with that of standard seawater,the measurement of practical salinity and concentration of dissolved organic matter can be achieved by comparing the electrical conductivity and the refractive index.