Uncertainties associated with in situ high-frequency long-term observations of suspended particulate matter concentration using optical and acoustic sensors

Abstract

Measurement of suspended particulate matter concentration (SPMC) spanning large time and geographical scales have become a matter of growing importance in recent decades. At many places worldwide, complex observation platforms have been installed to capture temporal and spatial variability over scales ranging from cm (turbulent regimes) to whole basins. Long-term in situ measurements of SPMC involve one or more optical and acoustical sensors and, as the ground truth reference, gravimetric measurements of filtered water samples. The estimation of SPMC from optical and acoustical proxies Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. generally results from the combination of a number of independent calibration measurements, as well as regression or inverse models. Direct or indirect measurements of SPMC are inherently associated with a number of uncertainties along the whole operation chain, the autonomous field deployment, to the analyses necessary for converting the observed proxy values of optical and acoustical signals to SPMC. Controlling uncertainties will become an important issue when the observational input comprises systems of sensors spanning large spatial and temporal scales. This will be especially relevant for detecting trends in the data with unambiguous statistical significance, separating anthropogenic impact from natural variations, or evaluating numerical models over a broad ensemble of different conditions using validated field data. The aim of the study is to present and discuss the benefits and limitations of using optical and acoustical backscatter sensors to acquire long-term observations of SPMC. Additionally, this study will formulate recommendations on how to best acquire quality-assured SPMC data sets, based on the challenges and uncertainties associated with those long-term observations. The main sources of error as well as the means to quantify and reduce the uncertainties associated with SPMC measurements are also illustrated. Highlights ► Errors associated with optical and acoustical sensors for SPMC are quantified. ► A strict protocol limits the uncertainties. ► Systematic errors may reach up to ±20% and errors due to biofouling to 100% or more. ► Changes of the inherent particle properties may result in uncertainties up to 200%. ► A model based on the R 2 quantifies the uncertainty of the sensor derived SPMC. ► Hach-turbidities could be a cheap alternative for sample SPMC.

Characterizing the Composition of Sand and Mud Suspensions in Coastal and Estuarine Environments Using Combined Optical and Acoustic Measurements

“…Confidential manuscript submitted to JGR-Oceans els for optical or acoustic sensors are built via regression against laboratory or in situ samples, the latter providing reference gravimetric concentrations [Gray and Elliott, 2009;Fettweis et al, 2019]. Once the calibration for a given instrument has been developed, the calibrated relationship can be applied to the recorded signal from the field (e.g., voltage, NTU, counts, or SNR) and translated into a time series of mass concentration.…”

“…Depending on local and remote pended sediment and their related mass concentration (SSC). Continuous in situ measurements are possible with acoustic or optical instruments [Fettweis et al, 2019]. Optical backscatter sensors have been used successfully to measure suspended sediment in a wide range of environments, from estuaries and embayments [Lunven and Gentien, 2000;Green et al, 2000;Bass et al, 2002;Fugate and Friedrichs, 2002;Bass et al, 2007;Li et al, 2018;Fettweis et al, 2019;Lin et al, 2020] to mud flats and salt marshes [Voulgaris and Meyers, 2004;Guo et al, 2018] to sandy beaches [Downing et al, 1981;Aagaard et al, 2002].…”

“…Continuous in situ measurements are possible with acoustic or optical instruments [Fettweis et al, 2019]. Optical backscatter sensors have been used successfully to measure suspended sediment in a wide range of environments, from estuaries and embayments [Lunven and Gentien, 2000;Green et al, 2000;Bass et al, 2002;Fugate and Friedrichs, 2002;Bass et al, 2007;Li et al, 2018;Fettweis et al, 2019;Lin et al, 2020] to mud flats and salt marshes [Voulgaris and Meyers, 2004;Guo et al, 2018] to sandy beaches [Downing et al, 1981;Aagaard et al, 2002]. Acoustic backscatter sensors have also been successfully used to measure suspended sediment in many different coastal and estuarine settings [Thorne et al, 1993;Green et al, 2000;Fugate and Friedrichs, 2002;Voulgaris and Meyers, 2004;Hoitink and Hoekstra, 2005;Bass et al, 2007;Chanson et al, 2008;Li et al, 2018;Lin et al, 2020] and beyond [Hill et al, 2003;Hawley, 2004].…”

“…The main challenge faced in understanding coastal sediment dynamics and quantifying associated fluxes is to make continuous observations of total (sand and mud) suspended sediment and their related mass concentration (SSC). Continuous in situ measurements are possible with acoustic or optical instruments (Fettweis et al., 2019). Optical backscatter (OBS) sensors have been used successfully to measure suspended sediment in a wide range of environments, from estuaries and embayments (Bass et al., 2002, 2007; Fettweis et al., 2019; Fugate & Friedrichs, 2002; Green et al., 2000; Li et al., 2018; Lin et al., 2020; Lunven & Gentien, 2000) to mud flats and salt marshes (Guo et al., 2018; Voulgaris & Meyers, 2004) to sandy beaches (Aagaard et al., 2002; J. P. Downing et al., 1981).…”

Comparison of Time Resolved Optical Turbidity Measurements for Water Monitoring to Standard Real-Time Techniques

“…More investigations are being conducted to optimize TROT. We are also trying to determine how TROT would react to known optical method biases such as the presence of biofilm or the presence of micro-bubbles [ 31 ]. Nevertheless, the application of TROT to the survey of particle load in water showed satisfactory results.…”