Abstract
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 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 ≈90 L STP while allowing for [14CO] measurement uncertainties that are on par with or better than prior work (≈3 % or better, 1σ). The method also for the first time includes accurate characterization of the overall procedural [14CO] blank associated with individual samples, which is 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 %, 1σ) are achieved for samples that are directly accompanied by procedural blanks and whose mass is increased to ≈50 µgC (micrograms of carbon) prior to the 14C measurement via dilution with a high-CO 14C-depleted gas.
Highlights
1.1 The importance of improving the understanding of OH variabilityAtmospheric hydroxyl radical concentration ([OH]) is arguably the single most important parameter in characterizing the overall chemical state of the atmosphere, because OH serves as the main atmospheric oxidant
The Mauna Loa Observatory (MLO) sample and blank [14 of atmospheric carbon monoxide (14CO)] results are shown in Fig. 3 and listed in Tables S1 and S2. [14CO] at MLO during the year of sampling ranged from 5–13 molecules cm−3 STP
The MLO [14CO] measurements made with this method show good first-order agreement with prior measurements at a different Northern Hemisphere low-latitude site
Summary
Atmospheric hydroxyl radical concentration ([OH]) is arguably the single most important parameter in characterizing the overall chemical state of the atmosphere, because OH serves as the main atmospheric oxidant. This is much shorter than the interhemispheric mixing time of ≈ 1 year, and it means that [14CO] measurements at a given station are sensitive to regional rather than global [OH] (Krol et al, 2008), presenting a challenge for using [14CO] to constrain global mean [OH] abundance and variability. The very large air sample volumes and the need for high-pressure gas cylinders result in relatively complex and expensive logistics and sample processing These challenges have limited the extent of 14CO atmospheric measurements collected to date.
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