An improved method for atmospheric 14CO measurements

Petrenko, Vasilii V.; Smith, Andrew M.; Crosier, Edward M.; Kazemi, Roxana; Place, Philip; Colton, Aidan; Yang, Bin; Hua, Quan; Murray, Lee T.

Important uncertainties remain in our understanding of the spatial and temporal variability of atmospheric hydroxyl radical concentration ([OH]). Carbon-14-containing carbon monoxide (inline-formula14CO) is a useful tracer that can help in the characterization of [OH] variability. Prior measurements of atmospheric inline-formula14CO concentration ([inline-formula14CO] 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 L 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 inline-formula≈90 L STP while allowing for [inline-formula14CO] measurement uncertainties that are on par with or better than prior work (inline-formula≈3 % or better, 1inline-formulaσ). The method also for the first time includes accurate characterization of the overall procedural [inline-formula14CO] blank associated with individual samples, which is a key improvement over prior atmospheric inline-formula14CO work. The method was used to make measurements of [inline-formula14CO] at the NOAA Mauna Loa Observatory, Hawaii, USA, between November 2017 and November 2018. The measurements show the expected [inline-formula14CO] seasonal cycle (lowest in summer) and are in good agreement with prior [inline-formula14CO] results from another low-latitude site in the Northern Hemisphere. The lowest overall [inline-formula14CO] uncertainties (2.1 %, 1inline-formulaσ) are achieved for samples that are directly accompanied by procedural blanks and whose mass is increased to inline-formula≈50inline-formulaµgC (micrograms of carbon) prior to the inline-formula14C measurement via dilution with a high-CO inline-formula14C-depleted gas.

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Petrenko, Vasilii V. / Smith, Andrew M. / Crosier, Edward M. / et al: An improved method for atmospheric 14CO measurements. 2021. Copernicus Publications.

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