An improved method for atmospheric 14CO measurements

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

Automated summary

The study presents a new method to measure atmospheric hydroxyl radical concentration using carbon-14-containing carbon monoxide as a tracer, reducing sample volume requirements while maintaining measurement precision. The method includes accurate characterization of procedural blank and was used to make measurements at the NOAA Mauna Loa Observatory. The results show a seasonal cycle and agreement with prior data from a different site.