Water vapor isotopic composition during TRACER IOP

The TRACER-Iso project took place from June 1 to September 30, 2022 as part of the intensive observation period (IOP) of the Tracking Aerosol Convection Interactions (TRACER) project at the main project site at the La Porte, Texas, municipal airport. The project sought to use measurements of the stable isotopic composition of water vapor to better understand cloud-aerosol interactions in deep convection in a polluted urban setting. Water vapor isotopologue and humidity measurements were determined using a Picarro L2130 Water Vapor Isotope Analyzer. The instrument was deployed in the guest van and consists of three main components: a water vapor isotope analyzer, and a Los Gatos Research Water Vapor Isotope Standard Source (WVISS), and a Los Gatos Research Dry Air Source. Ambient air samples were collected through an inlet located about one meter above roof of the shipping container. Samples were then delivered to the analyzer through teflon tubing by use of an external pump to minimize the transport time between the inlet and analyzer. The inlet tubing was surrounded by a Briskheat heat trace and insulating material to ensure ambient air samples did not fall below the dew point and result in condensation. The analyzer uses cavity ringdown spectroscopy to report isotopic ratios of ambient air samples at a frequency of 1 Hz. The calibration unit of the instrument was used in conjunction with the dry air source to periodically measure the δ values of standard waters. It uses a nebulizer to push small water droplets into a hot chamber that vaporizes the water without fractionation. This vapor was then transported to the analyzer using a built-in compressor and the dry air source, which allows each standard with known δ values to be measured at a wide range of humidity values for post-measurement calibration of ambient air samples. There are multiple sources of uncertainty introduced at different stages during the data collection and processing. These include (1) instrument precision, (2) uncertainty in the secondary standards, (3) humidity-correction uncertainty, and (4) VSMOW-SLAP calibration uncertainty. Uncertainty from each step is propagated in quadrature to calculate a total uncertainty of each isotopologue. Uncertainty was determined to be 2.3‰ for δD and 0.4‰ for δ18O.

TRACER-Iso项目于2022年6月1日至9月30日实施，作为追踪气溶胶与对流相互作用（Tracking Aerosol Convection Interactions, TRACER）项目在得克萨斯州拉波特市政机场主项目场地的强化观测期（intensive observation period, IOP）的组成部分。本项目旨在通过测定水蒸气的稳定同位素组成，深化对污染城市环境中深对流过程内云-气溶胶相互作用机制的认知。研究采用Picarro L2130水蒸气同位素分析仪开展水蒸气同位素体（water vapor isotopologue）与湿度观测。该仪器部署于客用厢式货车内，包含三大核心组件：水蒸气同位素分析仪、Los Gatos Research公司生产的水蒸气同位素标准源（Water Vapor Isotope Standard Source, WVISS）以及Los Gatos Research公司生产的干燥空气源。环境空气样品通过位于集装箱屋顶上方约1米处的进气口采集，随后经由特氟龙（Teflon）管材输送至分析仪，过程中借助外置泵以尽可能缩短进气口与分析仪之间的样品传输时间。进气管材外围包裹有Briskheat加热伴热带与隔热材料，确保环境空气样品的温度不低于露点，避免发生水汽冷凝。该分析仪采用腔环衰荡光谱（cavity ringdown spectroscopy）技术，以1赫兹的频率输出环境空气样品的同位素比值。仪器的校准单元与干燥空气源协同工作，可定期测定标准水样的δ值。其校准流程为：通过雾化器将细小水滴送入高温腔室，在不发生同位素分馏的前提下使水汽化，随后通过内置压缩机与干燥空气源将该水汽输送至分析仪，从而可在宽湿度范围内对已知δ值的各标准样品进行测定，用于环境空气样品的测量后校准。数据采集与处理的不同阶段均会引入多种不确定性来源，具体包括：(1) 仪器精度误差；(2) 二级标准物质的不确定性；(3) 湿度校正不确定性；(4) VSMOW-SLAP校准不确定性。通过将各步骤的不确定性以正交方式传播，可计算得到每种同位素体的总不确定度。经测定，δD的不确定度为2.3‰，δ18O的不确定度为0.4‰。