Collaborative Research: Improving the accuracy and uncertainty associated with estimated pCO2 from pH sensors on autonomous profiling platforms

了解海洋从大气中吸收了多少人为二氧化碳--以及这种吸收是如何随着时间的推移而变化的--对于理解我们星球的宜居性至关重要。海洋碳汇大小的不确定性仍然很大，主要是因为船只对地表水二氧化碳(PCO2)的测量很少。配备了生物地球化学(BGC)测量传感器的剖面浮标--如可用于估算二氧化碳分压的PH值--有可能极大地改善世界各地海洋表面二氧化碳的观测量，特别是在船舶测量稀缺的地区和一年中的某些时间。通过全球海洋生物地球化学阵列(GO-BGC)和南大洋碳与气候观测与模拟计划(SOCCOM2)等计划，全球部署的BGC浮标数量预计将在未来几年迅速增加。然而，在我们能够充分利用这个新兴的、强大的全球观测网络来改进我们对海洋每年吸收多少人为碳的估计之前，仍然有一些根本问题需要解决。该项目的总体目标是提高浮标数据分析得出的二氧化碳分压估计值的准确性和精确度。该项目将支持一名博士后研究人员和三名本科生暑期实习生。该项目寻求通过解决可能导致这些计算中的系统性偏差的悬而未决的问题和问题，来提高基于浮点数的二氧化碳估计值的准确性和不确定性。具体地说，研究人员的目标是回答以下问题：1)是什么导致了瓶子和浮子之间的pH随压力变化的差异？2)热力学海洋无机碳模型内部不一致的根源是什么，它影响了从pH测量计算的二氧化碳的准确性？3)1500米处酸化速率的全球时空模式是什么，我们如何准确地对这些模式进行建模？该团队将进行一系列实验室研究，以量化关键的热力学常数，测试有机化合物是内部不一致性的主要贡献者的假设，并确定根据pH准确计算二氧化碳的可靠方案。此外，将提供一个开源的全球算法，准确预测特定地区的人为碳估计，这是准确纠正全球浮动pH传感器漂移所必需的。最后，为了评估基于这些努力的基于浮标的二氧化碳估测精度的提高，该团队将使用配备与剖面浮标相同的pH传感器的喷雾水下滑翔机和波浪滑翔机进行一系列现场验证实验，波浪滑翔机是一种自主水面飞行器，配备了经气体标准校准的二氧化碳分析仪，精度为正负两个微大气。在项目过程中，该团队将测试16个不同的pH传感器，以更好地限制基于浮点的二氧化碳估计值的潜在系统偏差。这里概述的活动将导致对浮标二氧化碳的更准确估计，以及更好地约束不确定性，并最终导致从全球BGC剖面浮标网络对海-气二氧化碳通量进行更好的估计。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Understanding how much anthropogenic carbon dioxide the ocean is taking up from the atmosphere - and how this uptake changes over time - is critical for understanding the habitability of our planet. Uncertainties in the size of the ocean carbon sink remain large, mainly because of sparse measurements of surface water carbon dioxide (pCO2) measurements from ships. Profiling floats equipped with sensors for biogeochemical (BGC) measurements such as pH – which can be used to estimate pCO2 – have the potential to greatly improve the amount of surface carbon dioxide observations across the world’s ocean, particularly in regions and times of year where shipboard measurements are scarce. The number of BGC floats deployed worldwide is expected to rapidly increase in the coming years through programs like the Global Ocean Biogeochemistry Array (GO-BGC) and the Southern Ocean Carbon and Climate Observations and Modeling program (SOCCOM2). However, there are still fundamental issues that need to be resolved before we can take full advantage of this emerging, powerful global observational network to improve our estimates of how much anthropogenic carbon the ocean absorbs every year. The overarching aim of this project is to improve the accuracy and precision of pCO2 estimates that are derived from profiling float data. The project will support a postdoctoral researcher and three undergraduate summer interns.This project seeks to improve the accuracy and uncertainty associated with float-based estimates of pCO2 by addressing the outstanding questions and issues that could lead to systematic biases in these calculations. Specifically, the investigators aim to answer the following questions: 1) What causes the pressure-dependent discrepancy between bottle and float pH? 2) What is the source of the internal inconsistency of the thermodynamic marine inorganic carbon model, which affects the accuracy of pCO2 computed from pH measurements? 3) What are the global spatiotemporal patterns of acidification rate at 1500 m, and how do we model the patterns accurately? The team will conduct a series of laboratory studies to quantify key thermodynamic constants, test the hypothesis that organic compounds are a substantial contributor for the internal inconsistency, and determine a robust protocol to accurately calculate pCO2 from pH. Furthermore, an open-source, global algorithm will be made available that accurately predicts region-specific anthropogenic carbon estimates, which is necessary to accurately correct float pH sensor drift globally. Finally, to assess the improvement in accuracy of float based pCO2 estimates based on these efforts, the team will conduct a series of field validation experiments using Spray underwater gliders equipped with the same pH sensor as profiling floats, and a Wave Glider, an autonomous surface vehicle that is equipped with a pCO2 analyzer calibrated with gas standards with an accuracy of plus or minus two microatmospheres. Over the course of the project, the team will test 16 different pH sensors to better constrain the potential systematic biases for the float based pCO2 estimates. The activities outlined here will lead to more accurate estimates of pCO2 from floats, as well as better constrained uncertainties, and ultimately lead to better estimates for air-sea CO2 flux from the global network of BGC profiling floats.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.