An improved method for atmospheric <sup>14</sup>CO measurements

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 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.

COSMOGENIC14CO FOR ASSESSING THE OH-BASED SELF-CLEANING CAPACITY OF THE TROPOSPHERE

“…Adding the larger uncertainty of the biogenic fraction estimation of 2%, gives a combined uncertainty that is below the variability of 14 CO in most air masses (remote SH air showed the least variability) during a typical 1-hr air sample collection period using a high-pressure compressor. Recently, Petrenko et al (2021) reported an air sample collection and processing method for much smaller air samples, namely less than 100 L. This avoids the use of high-pressure sample air cylinders, which are more expensive and time consuming to transport from the point of collection to the laboratory.…”

“…14 CO has a source that is probably not affected noticeably by industrial emissions, it is a geophysical tracer. From this point of view is stands to reason to continue 14 CO observations, and indeed recently further improvements were just reported (Petrenko et al 2021). Current atmospheric chemistry models are of great complexity and the reconstructed rate of removal of trace gases by OH remains an essential piece of output.…”

Using ice core measurements from Taylor Glacier, Antarctica to calibrate in situ cosmogenic 14C production rates by muons

“…However, to better characterize the effects from the addition of extraneous C during the graphitization process, we used a linear empirical correction from 10 commensurately-sized 14 C standards and blanks at ANSTO (see Supplementary Materials, Fig. S2A, 265 Table S7) following Petrenko et al (2021). This approach has the benefit of bracketing the effects of extraneous C from graphitization at ANSTO with low and high 14 C standards, similar to the approach for the 14 CH4 samples.…”