Abstract. Important uncertainties remain in our understanding of the spatial and temporal variability of atmospheric hydroxyl radical concentration ([OH]). Carbon-14-containing carbon monoxide (¹⁴CO) is a useful tracer that can help in the characterization of [OH] variability. Prior measurements of atmospheric ¹⁴CO concentration ([¹⁴CO] are limited in both their spatial and temporal extent, partly due to the very large air sample volumes that have been required for measurements (500–1000 liters at standard temperature and pressure, L STP) and the difficulty and expense associated with the collection, shipment and processing of such samples. Here we present a new method that reduces the air sample volume requirement to ≈ 90 L STP while allowing for [¹⁴CO] measurement uncertainties that are on par with or better than prior work (≈ 3 % or better, 1 σ). The method also for the first time includes accurate characterization of the overall procedural [¹⁴CO] blank associated with individual samples, a key improvement over prior atmospheric ¹⁴CO work. The method was used to make measurements of [¹⁴CO] at the NOAA Mauna Loa Observatory, Hawaii, USA, between November 2017 and November 2018. The measurements show the expected [¹⁴CO] seasonal cycle (lowest in summer) and are in good agreement with prior [¹⁴CO] results from another low-latitude site in the Northern Hemisphere. The lowest overall [¹⁴CO] uncertainties (2.1 %, 1 σ) are achieved for samples that are directly accompanied by procedural blanks and whose mass is increased to ≈ 50 micrograms of carbon (µgC) prior to the ¹⁴C measurement via dilution with a high-CO, ¹⁴C-depleted gas.

中文翻译：

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一种用于大气¹⁴ CO测量的改进方法

摘要。在我们对大气羟基自由基浓度（[OH]）的时空变化的理解中，重要的不确定性仍然存在。含碳14的一氧化碳（¹⁴ CO）是有用的示踪剂，可帮助表征[OH]的变异性。大气¹⁴ CO浓度的先前测量值[[ ¹⁴CO]的空间和时间范围都受到限制，部分原因是测量所需的空气样本量非常大（标准温度和压力下为500–1000升，L STP），以及采集过程中的困难和费用，运输和处理此类样品。在这里，我们提出了一种新方法，可以将空气样本量要求降低到大约 90 L STP，同时允许[ ¹⁴ CO]测量不确定度与之前的工作相当或更好（约3％或更好，1σ）。该方法还首次包括准确表征与单个样品相关的整个程序[ ¹⁴ CO]空白，这是对先前大气¹⁴的关键改进一氧化碳工作。该方法用于在2017年11月至2018年11月之间在美国夏威夷的NOAA莫纳罗阿观测所进行[ ¹⁴ CO]的测量。测量结果显示预期的[ ¹⁴ CO]季节周期（夏季最低），处于良好状态与先前[ ¹⁴ CO]的协议是北半球另一个低纬度地区的结果。对于直接伴随程序空白并且质量增加到大约20％的样品，可实现最低的[ ¹⁴ CO]总体不确定性（2.1％，1σ）。在¹⁴ C测量之前，通过用消耗大量CO的¹⁴ C贫化气体稀释50克碳（µgC）。